Showing papers by "Robert J. Lefkowitz published in 1996"

Desensitization of G protein-coupled receptors.

Abstract: Waning responsiveness to continuous or repeated stimulation constitutes the phenomenon of desensitization, which pervades biological systems. Over the last several years, molecular mechanisms for desensitization of cellular signaling through G protein-coupled receptors have been delineated, particularly at the level of the receptors themselves. This review focuses on those aspects of G protein-coupled receptor desensitization which occur within minutes of agonist exposure. Agonist-dependent desensitization of these receptors can reduce their signaling responsiveness to maximum stimulation by up to 70-80%; indeed, in some receptor systems, the process of receptor desensitization appears to effect the termination of the cellular signaling response. Agonist-induced desensitization involves phosphorylation of G protein-coupled receptors by two currently recognized classes of serine/threonine protein kinases. Second messenger-dependent kinases, phosphorylating a variety of proteins, mediate a generalized cellular hyporesponsiveness termed heterologous desensitization. G protein-coupled receptor kinases (GRKs) phosphorylate specifically agonist-occupied, or activated, receptors, and thereby initiate receptor-specific, or homologous, desensitization. GRK-mediated receptor phosphorylation facilitates the binding of an inhibitory arrestin protein to the phosphorylated receptor, an event which substantially impairs receptor signaling. The GRK family comprises six, and the arrestin family comprises four known members. Each GRK demonstrates the ability to phosphorylate a limited number of model receptor substrates, but the widespread expression of several GRKs and the two somatic arrestins suggest that GRK-initiated homologous desensitization is of general importance to a wide range of G protein-coupled receptors. Exploration of the substrate specificity for GRKs and the ligand specificity for arrestins remains in its early stages. Currently, GRKs can most readily be differentiated by the mechanisms each employs to associate with the plasma membrane. Accumulating data from human disease states and transgenic mice attest to the physiologic significance of GRK-initiated receptor desensitization.

Mitogenic Signaling via G Protein-Coupled Receptors

Abstract: I. Introduction RECEPTORS coupled to heterotrimeric GTP-binding proteins (G proteins) comprise the largest known family of cell surface receptors and mediate cellular responses to a diverse array of signaling molecules, including peptide and glycopeptide hormones, neurotransmitters, phospholipids, odorants, and photons. The basic unit of G protein-coupled receptor (GPCR) signaling is comprised of three parts; receptor, which detects ligand in the extracellular milieu; heterotrimeric G protein, which is dissociated into active Gα-GTP and Gβγ-subunits after interaction with the liganded receptor; and effector, which interacts with dissociated Gα-GTP and Gβγ-subunits to mediate the intracellular effects of ligand binding. In a given cell type, the responsiveness to stimulus, as well as the nature of the response, is dictated by the available complement of receptor, G protein, and effector. Despite the diverse array of ligands with which they interact, GPCRs share a conserved predicted tertiary structure char...

Essential role of beta-adrenergic receptor kinase 1 in cardiac development and function.

Abstract: The beta-adrenergic receptor kinase 1 (beta ARK1) is a member of the G protein-coupled receptor kinase (GRK) family that mediates the agonist-dependent phosphorylation and desensitization of G protein-coupled receptors. We have cloned and disrupted the beta ARK1 gene in mice by homologous recombination. No homozygote beta ARK1-/- embryos survive beyond gestational day 15.5. Prior to gestational day 15.5, beta ARK1-/- embryos display pronounced hypoplasia of the ventricular myocardium essentially identical to the "thin myocardium syndrome" observed upon gene inactivation of several transcription factors (RXR alpha, N-myc, TEF-1, WT-1). Lethality in beta ARK1-/- embryos is likely due to heart failure as they exhibit a > 70% decrease in cardiac ejection fraction determined by direct in utero intravital microscopy. These results along with the virtual absence of endogenous GRK activity in beta ARK1-/- embryos demonstrate that beta ARK1 appears to be the predominant GRK in early embryogenesis and that it plays a fundamental role in cardiac development.

Receptor-specific in vivo desensitization by the G protein-coupled receptor kinase-5 in transgenic mice

Abstract: Transgenic mice were generated with cardiac-specific overexpression of the G protein-coupled receptor kinase-5 (GRK5), a serine/threonine kinase most abundantly expressed in the heart compared with other tissues. Animals overexpressing GRK5 showed marked beta-adrenergic receptor desensitization in both the anesthetized and conscious state compared with nontransgenic control mice, while the contractile response to angiotensin II receptor stimulation was unchanged. In contrast, the angiotensin II-induced rise in contractility was significantly attenuated in transgenic mice overexpressing the beta-adrenergic receptor kinase-1, another member of the GRK family. These data suggest that myocardial overexpression of GRK5 results in selective uncoupling of G protein-coupled receptors and demonstrate that receptor specificity of the GRKs may be important in determining the physiological phenotype.

Ras-dependent activation of fibroblast mitogen-activated protein kinase by 5-HT1A receptor via a G protein beta gamma-subunit-initiated pathway.

Abstract: Serotonin (5-HT) is a potent mitogen in many cells types, an action which is frequently mediated through pertussis toxin-sensitive G proteins. In the current study, we used pharmacological inhibitors and dominant negative signaling constructs to delineate elements which participate in the activation of MAPK, a growth-associated mitogen-activated protein kinase, by human G protein-coupled 5-HT1A receptor transfected into CHO-K1 cells in a stable manner. The activation pathway does not directly involve phorbol ester-sensitive protein kinase C types, but does require (i) pertussis toxin-sensitive G protein beta gamma-subunits, (ii) a staurosporine- and genistein-sensitive protein kinase, (iii) phosphoinositide-3'-kinase activity, (iv) activation of Sos in a multimolecular complex that contains p46Shc, and p52Shc, and Grb2, (v) the GTPase p21Ras, and (vi) the protein kinase p74Raf-1. These data demonstrate that the 5-HT1A receptor mediates MAPK activity by convergence upon a common activation pathway that is shared with receptor tyrosine kinases.

Monoclonal antibodies reveal receptor specificity among G-protein-coupled receptor kinases.

Abstract: Guanine nucleotide-binding regulatory protein (G protein)-coupled receptor kinases (GRKs) constitute a family of serine/threonine kinases that play a major role in the agonist-induced phosphorylation and desensitization of G-protein-coupled receptors. Herein we describe the generation of monoclonal antibodies (mAbs) that specifically react with GRK2 and GRK3 or with GRK4, GRK5, and GRK6. They are used in several different receptor systems to identify the kinases that are responsible for receptor phosphorylation and desensitization. The ability of these reagents to inhibit GRK- mediated receptor phosphorylation is demonstrated in permeabilized 293 cells that overexpress individual GRKs and the type 1A angiotensin II receptor. We also use this approach to identify the endogenous GRKs that are responsible for the agonist-induced phosphorylation of epitope-tagged beta2- adrenergic receptors (beta2ARs) overexpressed in rabbit ventricular myocytes that are infected with a recombinant adenovirus. In these myocytes, anti-GRK2/3 mAbs inhibit isoproterenol-induced receptor phosphorylation by 77%, while GRK4-6-specific mAbs have no effect. Consistent with the operation of a betaAR kinase-mediated mechanism, GRK2 is identified by immunoblot analysis as well as in a functional assay as the predominant GRK expressed in these cells. Microinjection of GRK2/3-specific mAbs into chicken sensory neurons, which have been shown to express a GRK3-like protein, abolishes desensitization of the alpha2AR-mediated calcium current inhibition. The intracellular inhibition of endogenous GRKs by mAbs represents a novel approach to the study of receptor specificities among GRKs that should be widely applicable to many G-protein-coupled receptors.

Members of the G protein-coupled receptor kinase family that phosphorylate the beta2-adrenergic receptor facilitate sequestration.

Abstract: We recently reported that a beta2-adrenergic receptor (beta2AR) mutant, Y326A, defective in its ability to sequester in response to agonist stimulation was a poor substrate for G protein-coupled receptor kinase (GRK)-mediated phosphorylation; however, its ability to be phosphorylated and sequestered could be restored by overexpressing GRK2 [Ferguson et al. (1995) J. Biol. Chem. 270, 24782]. In the present report, we tested the ability of each of the known GRKs (GRK1-6) to phosphorylate and rescue the sequestration of the Y326A mutant in HEK-293 cells. We demonstrate that in addition to GRK2, GRK3-6 can phosphorylate the Y326A mutant and rescue its sequestration; however, GRK1 was totally ineffective in rescuing either the phosphorylation or the sequestration of the mutant receptor. We found that the agonist-dependent rescue of Y326A mutant phosphorylation by GRK2, -3, and -5 was associated with the agonist-dependent rescue of sequestration. In contrast, overexpression of GRK4 and -6 led mainly to agonist-independent phosphorylation of the Y326A mutant accompanied by increased basal receptor sequestration. Our results demonstrate that phosphorylation per se, but not the interaction with a specific GRK, is required to facilitate beta2AR sequestration.

Enhanced myocardial relaxation in vivo in transgenic mice overexpressing the beta2-adrenergic receptor is associated with reduced phospholamban protein.

Abstract: To assess the effect of targeted myocardial beta-adrenergic receptor (AR) stimulation on relaxation and phospholamban regulation, we studied the physiological and biochemical alterations associated with overexpression of the human beta2-AR gene in transgenic mice These mice have an approximately 200-fold increase in beta-AR density and a 2-fold increase in basal adenylyl cyclase activity relative to negative littermate controls Mice were catheterized with a high fidelity micromanometer and hemodynamic recordings were obtained in vivo Overexpression of the beta2-AR altered parameters of relaxation At baseline, LV dP/dt(min) and the time constant of LV pressure isovolumic decay (Tau) in the transgenic mice were significantly shorter compared with controls, indicating markedly enhanced myocardial relaxation Isoproterenol stimulation resulted in shortening of relaxation velocity in control mice but not in the transgenic mice, indicating maximal relaxation in these animals Immunoblotting analysis revealed a selective decrease in the amount of phospholamban protein, without a significant change in the content for either sarcoplasmic reticulum Ca2+ ATPase or calsequestrin, in the transgenic hearts compared with controls This study indicates that myocardial relaxation is both markedly enhanced and maximal in these mice and that conditions associated with chronic beta-AR stimulation can result in a selective reduction of phospholamban protein

G Protein–Coupled Receptors and Receptor Kinases: From Molecular Biology to Potential Therapeutic Applications

Abstract: Understanding the intimate connection between activation and desensitization in seven-membrane-spanning receptors may lead to new therapies for heart disease

Myocardial β-adrenergic receptor signaling in vivo: insights from transgenic mice

Abstract: Heart failure is a problem of increasing importance in cardiovascular medicine. An important characteristic of heart failure is reduced agonist-stimulated adenylyl cyclase activity (receptor desensitization) due to both diminished receptor number (receptor downregulation) and impaired receptor function (receptor uncoupling). These changes in the §-adrenergic receptor (§ AR) system may in part account for some of the abnormalities of contractile function in this disease. Myocardial contraction is closely regulated by G protein coupled β-adrenergic receptors through the action of the second messenger cAMP. The β-adrenergic receptors themselves are regulated by a set of specific kinases, termed the G protein-coupled receptor kinases. The study of this complex system in vivo has recently been advanced by the development of transgenic and gene targeted (“knock out”) mouse models. Combining transgenic technology with sophisticated physiological measurements of cardiac hemodynamics is an extremely powerful strategy to study the regulation of myocardial contractility in the normal and failing heart.

Transgenic Manipulation of Myocardial G Protein–Coupled Receptors and Receptor Kinases

Abstract: The ability to maintain and manipulate mouse embryos in vitro, perfected over the last decade, has launched the expanding field of transgenic experimentation. With the successful insertion of foreign genes into the mouse genome, important transgenic models have emerged in several venues of biomedical research. Transgenic mice permit investigation of the consequences of a protein’s overexpression in a particular tissue. In addition, the loss of function of a protein or enzyme can be examined by tissue-targeted overexpression of an inhibitor peptide or a dominant-negative mutant. Elimination of a protein from all tissues can also be achieved by “knockout” technology, where a gene is disrupted by homologous recombination. These approaches are well suited to study the physiological roles of cellular proteins. Transgenic models geared toward the study of cardiovascular regulation have recently been described and provide powerful tools to study normal and compromised cardiac physiology. Some of these transgenic models address changes in blood pressure and apolipoprotein levels as well as the consequences of overexpression of specific nuclear factors.1 Most recently, transgenic mice have been developed in which sarcolemmal G protein–coupled receptor signaling has been altered; these animals provide new information regarding the role of signal transduction in cardiac function. Manipulation of various components of the myocardial AR system has led to novel agonist-independent approaches to enhance signaling and augment cardiac function. This mini review summarizes these recent transgenic studies, which have provided unique experimental models for the study of receptor signaling in both normal and diseased myocardium. Probably the most important receptors involved in cardiac regulation are the ARs. These receptors modulate cardiomyocyte function by coupling to and activating G proteins, which give rise to increases in intracellular second messengers like cAMP and diacylglycerol. In the human heart, ARs are critical regulators of function under both normal and diseased …

Production of adrenergic receptors in yeast.

Abstract: Using a recombinant yeast strain expressing human beta 2 adrenergic receptors under a galactose-inducible promoter, we established conditions for receptor production in 1-15 liter fermenter culture. Crucial factors contributing to consistent high-level expression included the use of selective glucose-free medium, the maintenance of the pH of the culture at 7.2-7.5 and the presence of an antagonist. The expression strategy and production conditions used with the beta 2 adrenergic receptor were then employed to express the human alpha 2-C2 adrenergic receptor in Saccharomyces cerevisiae. Galactose-induced yeast cells displayed specific, high-affinity [3H]rauwolscine binding and contained a 50-kDa species recognized by an alpha 2-C2 receptor specific antiserum. In fermenter culture, up to 10(5) high-affinity [3H]rauwolscine binding sites per cell (corresponding to 30-60 pmol/mg of protein) were obtained. The high expression level combined with relative ease and low cost of scaling-up make yeast a promising alternative to mammalian cells for the production of adrenergic and other G-protein-coupled receptors for structural studies.