Regulation of G protein-coupled receptor signaling by scaffold proteins

What is the role of beta-adrenergic signaling in heart failure?

This review addresses open questions about the role of beta-adrenergic receptors in cardiac function and failure. Cardiomyocytes express all three beta-adrenergic receptor subtypes-beta1, beta2, and, at least in some species, beta3. The beta1 subtype is the most prominent one and is mainly responsible for positive chronotropic and inotropic effects of catecholamines. The beta2 subtype also increases cardiac function, but its ability to activate nonclassical signaling pathways suggests a function distinct from the beta1 subtype. In heart failure, the sympathetic system is activated, cardiac beta-receptor number and function are decreased, and downstream mechanisms are altered. However, in spite of a wealth of data, we still do not know whether and to what extent these alterations are adaptive/protective or detrimental, or both. Clinically, beta-adrenergic antagonists represent the most important advance in heart failure therapy, but it is still debated whether they act by blocking or by resensitizing the beta-adrenergic receptor system. Newer experimental therapeutic strategies aim at the receptor desensitization machinery and at downstream signaling steps.

Regulation of cardiac excitation-contraction coupling: a cellular update

The primary purpose of this paper is to present a basic overview of some "relatively" new ideas related to the regulation of cardiac performance and underlying excitation-contraction (EC) coupling that have yet to be incorporated to textbooks currently used for introductory graduate-level physiology courses. Within the context of cardiac EC coupling, this review incorporates information on microdomains and local control theory, with particular emphasis on the role of Ca(2+) sparks as a key regulatory component of ventricular myocyte contraction dynamics. Recent information pertaining to Ca(2+) release mechanisms specific to the sarcoplasmic reticulum is also presented, as well as the idea of the ryanodine receptor as a macromolecular signaling complex. Because of the potential relationship to maladaptive functional responses under conditions of cardiovascular pathology, the regulatory role of cardiac adrenergic and additional G protein-coupled receptors known to regulate cardiac function is included, and fundamental concepts related to intracellular signaling are discussed. Finally, information on the roles of vascular and cardiac nitric oxide as an important regulator of cardiac performance is included to allow students to begin to think about the ubiquitous role of nitric oxide in the regulation of the cardiovascular system. An important point of emphasis is that whole organ cardiac dynamics can be traced back to the cellular events regulating intracellular Ca(2+) homeostasis and as such provides an important conceptual framework from which the students can begin to think about whole organ physiology in health and disease.

The angiotensin II type I receptor-associated protein, ATRAP, is a transmembrane protein and a modulator of angiotensin II signaling

Our group identified angiotensin II type 1 (AT1) receptor-associated protein (ATRAP) in a yeast two-hybrid screen for proteins that bind to the carboxyl-terminal cytoplasmic domain of the AT1. In this work, we characterize ATRAP as a transmembrane protein localized in intracellular trafficking vesicles and plasma membrane that functions as a modulator of angiotensin II-induced signal transduction. ATRAP contains three hydrophobic domains at the amino-terminal end of the protein, encompassing the amino acid residues 14-36, 55-77, and 88-108 and a hydrophilic cytoplasmic carboxyl-terminal tail from residues 109-161. Endogenous and transfected ATRAP cDNA shows a particulate distribution; electron microscopy reveals the presence of ATRAP in prominent perinuclear vesicular membranes; and colocalization analysis by immunofluorescence shows that ATRAP colocalizes in an intracellular vesicular compartment corresponding to endoplasmic reticulum, Golgi, and endocytic vesicles. Real-time tracking of ATRAP vesicles shows constitutive translocation toward the plasma membrane. Using epitope-tagged forms of ATRAP at either the amino or carboxyl end of the molecule, we determined the orientation of the amino end as being outside the cell. Mutant forms of ATRAP lacking the carboxyl end are unable to bind to the AT1 receptor, leading to the formation of prominent perinuclear vesicle clusters. Functional analysis of the effects of ATRAP on angiotensin II-induced AT1 receptor signaling reveals a moderate decrease in the generation of inositol lipids, a marked decrease in the angiotensin II-stimulated transcriptional activity of the c-fos promoter luciferase reporter gene, and a decrease in cell proliferation.

The unique amino-terminal region of the PDE4D5 cAMP phosphodiesterase isoform confers preferential interaction with beta-arrestins

Isoproterenol challenge of Hek-B2 cells causes a transient recruitment of the endogenous PDE4D isoforms found in these cells, namely PDE4D3 and PDE4D5, to the membrane fraction. PDE4D5 provides around 80% of the total PDE4D protein so recruited, although it only comprises about 40% of the total PDE4D protein in Hek-B2 cells. PDE4D5 provides about 80% of the total PDE4D protein found associated with beta-arrestins immunopurified from Hek-B2, COS1, and A549 cells as well as cardiac myocytes, whereas its overall level in these cells is between 15 and 50% of the total PDE4D protein. Truncation analyses indicate that two sites in PDE4D5 are involved in mediating its interaction with beta-arrestins, one associated with the common PDE4 catalytic region and the other located within its unique amino-terminal region. Truncation analyses indicate that two sites in beta-arrestin 2 are involved in mediating its interaction with PDE4D5, one associated with its extreme amino-terminal region and the other located within the carboxyl-terminal domain of the protein. We suggest that the unique amino-terminal region of PDE4D5 allows it to preferentially interact with beta-arrestins. This specificity appears likely to account for the preferential recruitment of PDE4D5, compared with PDE4D3, to membranes of Hek-B2 cells and cardiac myocytes upon challenge with isoproterenol.

Independent beta-arrestin 2 and G protein-mediated pathways for angiotensin II activation of extracellular signal-regulated kinases 1 and 2

Stimulation of a mutant angiotensin type 1A receptor (DRY/AAY) with angiotensin II (Ang II) or of a wild-type receptor with an Ang II analog ([sarcosine1,Ile4,Ile8]Ang II) fails to activate classical heterotrimeric G protein signaling but does lead to recruitment of beta-arrestin 2-GFP and activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) (maximum stimulation approximately 50% of wild type). This G protein-independent activation of mitogen-activated protein kinase is abolished by depletion of cellular beta-arrestin 2 but is unaffected by the PKC inhibitor Ro-31-8425. In parallel, stimulation of the wild-type angiotensin type 1A receptor with Ang II robustly stimulates ERK1/2 activation with approximately 60% of the response blocked by the PKC inhibitor (G protein dependent) and the rest of the response blocked by depletion of cellular beta-arrestin 2 by small interfering RNA (beta-arrestin dependent). These findings imply the existence of independent G protein- and beta-arrestin 2-mediated pathways leading to ERK1/2 activation and the existence of distinct "active" conformations of a seven-membrane-spanning receptor coupled to each.

The PDZ-binding motif of the beta2-adrenoceptor is essential for physiologic signaling and trafficking in cardiac myocytes

beta1- and beta2-adrenergic receptors (AR) regulate cardiac myocyte function through distinct signaling pathways. In addition to regulating cardiac rate and contractility, beta1AR and beta2AR may play different roles in the pathogenesis of heart failure. Studies on neonatal cardiac myocytes from beta1AR and beta2AR knockout mice suggest that subtype-specific signaling is determined by subtype-specific membrane targeting and trafficking. Stimulation of beta2ARs has a biphasic effect on contraction rate, with an initial increase followed by a sustained Gi-dependent decrease. Recent studies show that a PDZ domain-binding motif at the carboxyl terminus of human beta2AR interacts with ezrin-binding protein 50/sodium-hydrogen exchanger regulatory factor, a PDZ-domain-containing protein. The human beta2AR carboxyl terminus also binds to N-ethylmaleimide-sensitive factor, which does not contain a PDZ domain. We found that mutation of the three carboxyl-terminal amino acids in the mouse beta2AR (beta2AR-AAA) disrupts recycling of the receptor after agonist-induced internalization in cardiac myocytes. Nevertheless, stimulation of the beta2AR-AAA produced a greater contraction rate increase than that of the wild-type beta2AR. This enhanced stimulation of contraction rate can be attributed in part to the failure of the beta2AR-AAA to couple to Gi. We also observed that coupling of endogenous, wild-type beta2AR to Gi in beta1AR knockout myocytes is inhibited by treatment with a membrane-permeable peptide representing the beta2AR carboxyl terminus. These studies demonstrate that association of the carboxyl terminus of the beta2AR with ezrin-binding protein 50/sodium-hydrogen exchanger regulatory factor, N-ethylmaleimide-sensitive factor, or some related proteins dictates physiologic signaling specificity and trafficking in cardiac myocytes.

C5a receptor oligomerization. II. Fluorescence resonance energy transfer studies of a human G protein-coupled receptor expressed in yeast

Recent studies demonstrate that members of the superfamily of G protein-coupled receptors (GPCRs) form oligomers both in vitro and in vivo. The mechanisms by which GPCRs oligomerize and the roles of accessory proteins in this process are not well understood. We used disulfide-trapping experiments to show that C5a receptors, expressed in mammalian cells, reside in membranes as oligomers (Klco, J. M., Lassere, T. B., and Baranski, T. J. (2003) J. Biol. Chem. 278, 35345-35353). To begin to address how C5a receptors form oligomers, we now use fluorescence resonance energy transfer experiments on human C5a receptors expressed in the lower eukaryote Saccharomyces cerevisiae. C5a receptors tagged with variants of the green fluorescent protein display energy transfer in intact yeast, demonstrating that mammalian accessory proteins are not required for C5a receptor oligomerization. In both intact yeast cells and membrane preparations, agonist does not affect FRET efficiency, and little energy transfer is observed between the C5a receptor and a co-expressed yeast pheromone receptor (encoded by STE2), indicating that C5a receptor oligomerization is both receptor-specific and constitutive. FRET studies performed on fractionated membranes demonstrate similar levels of energy transfer between tagged C5a receptors in endoplasmic reticulum compared with plasma membrane, and urea washing of membranes has little effect on the extent of energy transfer. The oligomerization of C5a receptors expressed in yeast displays characteristics similar to those observed for other GPCRs studied in mammalian cells. This model system should prove useful for further studies to define mechanisms of oligomerization of mammalian GPCRs.

Protein kinase A and G protein-coupled receptor kinase phosphorylation mediates beta-1 adrenergic receptor endocytosis through different pathways

Agonist-induced phosphorylation of beta-adrenergic receptors (beta ARs) by G protein-coupled receptor kinases (GRKs) results in their desensitization followed by internalization. Whether protein kinase A (PKA)-mediated phosphorylation of beta ARs, particularly the beta 1AR subtype, can also trigger internalization is currently not known. To test this, we cloned the mouse wild type beta 1AR (WT beta 1AR) and created 3 mutants lacking, respectively: the putative PKA phosphorylation sites (PKA-beta 1AR), the putative GRK phosphorylation sites (GRK-beta 1AR), and both sets of phosphorylation sites (PKA-/GRK-beta 1AR). Following agonist stimulation, both PKA-beta 1AR and GRK-beta 1AR mutants showed comparable increases in phosphorylation and desensitization. Saturating concentrations of agonist induced only 50% internalization of either mutant compared with wild type, suggesting that both PKA and GRK phosphorylation of the receptor contributed to receptor sequestration in an additive manner. Moreover, in contrast to the WT beta 1AR and PKA-beta 1AR, sequestration of the GRK-beta 1AR and PKA-/GRK-beta 1AR was independent of beta-arrestin recruitment. Importantly, clathrin inhibitors abolished agonist-dependent internalization for both the WT beta 1AR and PKA-beta 1AR, whereas caveolae inhibitors prevented internalization only of the GRK-beta 1AR mutant. Taken together, these data demonstrate that: 1) PKA-mediated phosphorylation can trigger agonist-induced internalization of the beta 1AR and 2) the pathway selected for beta 1AR internalization is primarily determined by the kinase that phosphorylates the receptor, i.e. PKA-mediated phosphorylation directs internalization via a caveolae pathway, whereas GRK-mediated phosphorylation directs it through clathrin-coated pits.

Beta-agonists and asthma: too much of a good thing?

In an unusual paradox, asthmatics who are chronically treated with bronchodilating beta-agonists sometimes experience a worsening of their condition. A new study describes one possible mechanism and reveals a potential new therapeutic target in the treatment of asthma.

Light adaptation through phosphoinositide-regulated translocation of Drosophila visual arrestin

Photoreceptor cells adapt to bright or continuous light, although the molecular mechanisms underlying this phenomenon are incompletely understood. Here, we report a mechanism of light adaptation in Drosophila, which is regulated by phosphoinositides (PIs). We found that light-dependent translocation of arrestin was defective in mutants that disrupt PI metabolism or trafficking. Arrestin bound to PIP(3) in vitro, and mutation of this site delayed arrestin shuttling and resulted in defects in the termination of the light response, which is normally accelerated by prior exposure to light. Disruption of the arrestin/PI interaction also suppressed retinal degeneration caused by excessive endocytosis of rhodopsin/arrestin complexes. These findings indicate that light-dependent trafficking of arrestin is regulated by direct interaction with PIs and is required for light adaptation. Since phospholipase C activity is required for activation of Drosophila phototransduction, these data point to a dual role of PIs in phototransduction.

The 'magic tail' of G protein-coupled receptors: an anchorage for functional protein networks

All cell types express a great variety of G protein-coupled receptors (GPCRs) that are coupled to only a limited set of G proteins. This disposition favors cross-talk between transduction pathways. However, GPCRs are organized into functional units. They promote specificity and thus avoid unsuitable cross-talk. New methodologies (mostly yeast two-hybrid screens and proteomics) have been used to discover more than 50 GPCR-associated proteins that are involved in building these units. In addition, these protein networks participate in the trafficking, targeting, signaling, fine-tuning and allosteric regulation of GPCRs. To date, proteins that interact with the GPCR C-terminus are the most abundant and are the focus of this review.