Identification of the cardiac beta-adrenergic receptor protein: solubilization and purification by affinity chromatography

G Protein-Coupled Receptor Systems and Their Role in Cellular Senescence

Aging is a complex biological process that is inevitable for nearly all organisms. Aging is the strongest risk factor for development of multiple neurodegenerative disorders, cancer and cardiovascular disorders. Age-related disease conditions are mainly caused by the progressive degradation of the integrity of communication systems within and between organs. This is in part mediated by, i) decreased efficiency of receptor signaling systems and ii) an increasing inability to cope with stress leading to apoptosis and cellular senescence. Cellular senescence is a natural process during embryonic development, more recently it has been shown to be also involved in the development of aging disorders and is now considered one of the major hallmarks of aging. G-protein-coupled receptors (GPCRs) comprise a superfamily of integral membrane receptors that are responsible for cell signaling events involved in nearly every physiological process. Recent advances in the molecular understanding of GPCR signaling complexity have expanded their therapeutic capacity tremendously. Emerging data now suggests the involvement of GPCRs and their associated proteins in the development of cellular senescence. With the proven efficacy of therapeutic GPCR targeting, it is reasonable to now consider GPCRs as potential platforms to control cellular senescence and the consequently, age-related disorders.

Small molecule target identification using photo-affinity chromatography

Identification of the protein targets of bioactive small molecules is a routine challenge in chemical biology and phenotype-based drug discovery. Recent years have seen an explosion of approaches to meeting this challenge, but the traditional method of affinity pulldowns remains a practical choice in many contexts. This technique can be used as long as an affinity probe can be synthesized, usually with a crosslinking moiety to enable photo-affinity pulldowns. It can be applied to varied tissue types and can be performed with minimal specialized equipment. Here, we provide our protocol for photo-affinity pulldown experiments, with notes on making this method generally applicable to varied target identification challenges.

A Pharmacological Perspective on the Study of Taste

The study of taste has been guided throughout much of its history by the conceptual framework of psychophysics, where the focus was on quantification of the subjective experience of the taste sensations. By the mid-20th century, data from physiologic studies had accumulated sufficiently to assemble a model for the function of receptors that must mediate the initial stimulus of tastant molecules in contact with the tongue. But the study of taste as a receptor-mediated event did not gain momentum until decades later when the actual receptor proteins and attendant signaling mechanisms were identified and localized to the highly specialized taste-responsive cells of the tongue. With those discoveries a new opportunity to examine taste as a function of receptor activity has come into focus. Pharmacology is the science designed specifically for the experimental interrogation and quantitative characterization of receptor function at all levels of inquiry from molecules to behavior. This review covers the history of some of the major concepts that have shaped thinking and experimental approaches to taste, the seminal discoveries that have led to elucidation of receptors for taste, and how applying principles of receptor pharmacology can enhance understanding of the mechanisms of taste physiology and perception.

New and Emerging Therapies and Targets: Beta-3 Agonists

While crucial for the acute physiologic response to stress, the adrenergic system may become maladaptive upon prolonged stimulation in the course of development of heart failure. This has been the basis for the development of beta-blocking therapies, targeting mainly beta1-2 adrenoreceptors (B1-2AR). The third isotype, B3AR, was more recently identified in cardiac myocytes and endothelial cells from human (and many other animal species), where its distinctive coupling to nitric oxide and antioxidant pathways suggested potential protective properties that were unexploited so far. The observation of beneficial effects of B3AR expression/activation on myocardial remodeling and the availability of specific agonists for clinical use now open the way for directly testing the hypothesis in heart failure patients. We will briefly review the specificities of B3AR signaling in the context of the cardiovascular adrenergic system, the evidence supporting its beneficial effects and outline an ongoing clinical trial using the B3AR agonist, mirabegron in patients with/at risk of developing heart failure with preserved ejection fraction (HFpEF).

Phenotypic Assessment and the Discovery of Topiramate

The role of phenotypic assessment in drug discovery is discussed, along with the discovery and development of TOPAMAX (topiramate), a billion-dollar molecule for the treatment of epilepsy and migraine.

Identification of target proteins of mangiferin in mice with acute lung injury using functionalized magnetic microspheres based on click chemistry

Prevention of the occurrence and development of inflammation is a vital therapeutic strategy for treating acute lung injury (ALI). Increasing evidence has shown that a wealth of ingredients from natural foods and plants have potential anti-inflammatory activity. In the present study, mangiferin, a natural C-glucosyl xanthone that is primarily obtained from the peels and kernels of mango fruits and the bark of the Mangifera indica L. tree, alleviated the inflammatory responses in lipopolysaccharide (LPS)-induced ALI mice. Mangiferin-modified magnetic microspheres (MMs) were developed on the basis of click chemistry to capture the target proteins of mangiferin. Mass spectrometry and molecular docking identified 70 kDa heat-shock protein 5 (Hspa5) and tyrosine 3-monooxygenase (Ywhae) as mangiferin-binding proteins. Furthermore, an enzyme-linked immunosorbent assay (ELISA) indicated that mangiferin exerted its anti-inflammatory effect by binding Hspa5 and Ywhae to suppress downstream mitogen-activated protein kinase (MAPK) signaling pathways. Thoroughly revealing the mechanism and function of mangiferin will contribute to the development and utilization of agricultural resources from M. indica L.

The fall and rise of pharmacology--(re-)defining the discipline?

Pharmacology is an integrative discipline that originated from activities, now nearly 7000 years old, to identify therapeutics from natural product sources. Research in the 19th Century that focused on the Law of Mass Action (LMA) demonstrated that compound effects were dose-/concentration-dependent eventually leading to the receptor concept, now a century old, that remains the key to understanding disease causality and drug action. As pharmacology evolved in the 20th Century through successive biochemical, molecular and genomic eras, the precision in understanding receptor function at the molecular level increased and while providing important insights, led to an overtly reductionistic emphasis. This resulted in the generation of data lacking physiological context that ignored the LMA and was not integrated at the tissue/whole organism level. As reductionism became a primary focus in biomedical research, it led to the fall of pharmacology. However, concerns regarding the disconnect between basic research efforts and the approval of new drugs to treat 21st Century disease tsunamis, e.g., neurodegeneration, metabolic syndrome, etc. has led to the reemergence of pharmacology, its rise, often in the semantic guise of systems biology. Against a background of limited training in pharmacology, this has resulted in issues in experimental replication with a bioinformatics emphasis that often has a limited relationship to reality. The integration of newer technologies within a pharmacological context where research is driven by testable hypotheses rather than technology, together with renewed efforts in teaching pharmacology, is anticipated to improve the focus and relevance of biomedical research and lead to novel therapeutics that will contain health care costs.

Identification and validation of protein targets of bioactive small molecules

Identification and validation of protein targets of bioactive small molecules is an important problem in chemical biology and drug discovery. Currently, no single method is satisfactory for this task. Here, we provide an overview of common methods for target identification and validation that historically were most successful. We have classified for the first time the existing methods into two distinct and complementary types, the 'top-down' and 'bottom-up' approaches. In a typical top-down approach, the cellular phenotype is used as a starting point and the molecular target is approached through systematic narrowing down of possibilities by taking advantage of the detailed existing knowledge of cellular pathways and processes. In contrast, the bottom-up approach entails the direct detection and identification of the molecular targets using affinity-based or genetic methods. A special emphasis is placed on target validation, including correlation analysis and genetic methods, as this area is often ignored despite its importance.

Biochemical target isolation for novices: affinity-based strategies

Although a number of genomic and biochemical technologies are now used to elucidate the mechanisms of action of bioactive small molecules, affinity-based isolation of molecular targets is a classic, but still powerful, approach. This review highlights recent cases where biochemical isolation of target proteins of bioactive small molecules highlighted general strategies for a successful isolation and identification of molecular targets. This review is intended to be both an update on the most recent findings for those already active in the field of forward chemical genetics and a guide for scientists entering this burgeoning field.

Antitumor antibiotic fostriecin covalently binds to cysteine-269 residue of protein phosphatase 2A catalytic subunit in mammalian cells

Fostriecin is a phosphate monoester with excellent antitumor activity against mouse leukemia, and it is a potent inhibitor of protein phosphatase (PP) 2A. This compound has been predicted to covalently bind to the Cys269 residue of the PP2A catalytic subunit (PP2Ac) at the alpha,beta-unsaturated lactone via a conjugate addition reaction. However, this binding has not yet been experimentally proven. To confirm such binding, we synthesized biotin-labeled fostriecin (bio-Fos), which has an inhibitory activity against the proliferation of mouse leukemia cells. We showed that fostriecin directly binds to PP2Ac in HeLa S3 cells by pull-down assays using bio-Fos. Moreover, we directly demonstrated that fostriecin covalently binds to the Cys269 residue of PP2Ac by matrix assisted laser desorption/ionization time-of-flight mass spectrometry analysis. From these results, the inhibitory mechanism of fostriecin on PP2A activity is discussed.

Isolation and identification of eukaryotic initiation factor 4A as a molecular target for the marine natural product Pateamine A

Natural products continue to demonstrate their utility both as therapeutics and as molecular probes for the discovery and mechanistic deconvolution of various cellular processes. However, this utility is dampened by the inherent difficulties involved in isolating and characterizing new bioactive natural products, in obtaining sufficient quantities of purified compound for further biological studies, and in developing bioactive probes. Key to characterizing the biological activity of natural products is the identification of the molecular target(s) within the cell. The marine sponge-derived natural product Pateamine A (PatA) has been found to be an inhibitor of eukaryotic translation initiation. Herein, we describe the methods utilized for identification of the eukaryotic translation initiation factor 4A (eIF4A) as one of the primary protein targets of PatA. We begin by describing the synthesis of an active biotin conjugate of PatA (B-PatA), made possible by total synthesis, followed by its use for affinity purification of PatA binding proteins from cellular lysates. We have attempted to present the methodology as a general technique for the identification of protein targets for small molecules including natural products.

G-Proteins and GPCRs: From the Beginning

From the point of view of a participant observer, I tell the discovery stories of trimeric G-proteins and GPCRs, beginning in the 1970s. As in most such stories, formidable obstacles, confusion, and mistakes make eventual triumphs even more exciting. Because these pivotally important signaling molecules were discovered before the recombinant DNA revolution, today's well-trained molecular biologist may find it amazing that we learned anything at all.

Polyproline-Rod Approach to Isolating Protein Targets of Bioactive Small Molecules: Isolation of a New Target of Indomethacin

Identification of protein targets of bioactive small molecules has been a technical hurdle of chemical genetics. Here we report a polyproline-rod approach to isolating protein targets of small molecules from cell lysates. The results indicate that insertion of a long, rigid polyproline helix between a small-molecule bait and a biotin tag boosts the capacity of affinity purification and thereby permits isolation of low-abundance or low-affinity proteins. In the course of the proof-of-concept experiments, we isolated glyoxalase 1 (GLO1) as a new target of indomethacin, a widely used antiinflammatory drug. Molecular biological experiments suggest that inhibition of GLO1 enzyme activity is related to the clinically recognized beneficial side effects of the indomethacin family of nonsteroidal antiinflammatory drugs.

Trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) play an integral role in the signal transduction of an enormous array of biological phenomena, thereby serving to modulate at a molecular level almost all components of human biology. This role is nowhere more evident than in cardiovascular biology, where GPCRs regulate such core measures of cardiovascular function as heart rate, contractility, and vascular tone. GPCR/ligand interaction initiates signal transduction cascades, and requires the presence of the receptor at the plasma membrane. Plasma membrane localization is in turn a function of the delivery of a receptor to and removal from the cell surface, a concept defined most broadly as receptor trafficking. This review illuminates our current view of GPCR trafficking, particularly within the cardiovascular system, as well as highlights the recent and provocative finding that components of the GPCR trafficking machinery can facilitate GPCR signaling independent of G protein activation.

Oligomerization of G protein-coupled receptors: past, present, and future

G protein-coupled receptor (GPCR)-mediated signal transduction has been studied for more than a century. Despite the intense focus on this class of proteins, a molecular understanding of what constitutes the functional form of the receptor is still uncertain. GPCRs have traditionally been conceptualized as monomeric proteins, and this view has changed little over the years until relatively recently. Recent biochemical and biophysical studies have challenged this traditional concept, and point instead to a mechanistic view of signal transduction wherein the receptor functions as an oligomer. Cooperative interactions within such an oligomeric array may be critical for the propagation of an external signal across the cell membrane and to the G protein, and may therefore underlie the mechanistic basis of signaling.

Solubilization of a thromboxane A2/prostaglandin H2 antagonist binding site from human platelets

A binding site for 9,11-dimethylmethano-11,12-methano-16-(3-[125I]iodo-4-hydroxyph eny l)-13,14-dihydro-13-aza-15 alpha beta-omega-tetranorthromboxane A2 ([125I]-PTA-OH), a thromboxane A2/prostaglandin H2 antagonist, was solubilized into the 200,000 X g supernatant from human platelet membranes by using the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. Binding to the solubilized site was saturable, displaceable, and reversible. Displaceable binding was not affected by sodium, potassium, or phosphate concentrations up to 50 mM or by magnesium to 5 mM but was increased 14% (P less than 0.05) by 5 mM calcium. A pH optimum for displaceable binding occurred between pH 7.0 and 7.5. Scatchard analysis of [125I]-PTA-OH binding to the solubilized binding site revealed a single class of sites, having a dissociation constant (Kd) of 66 +/- 16 nM (n = 3) and a Bmax of 750 +/- 80 fmol/mg of protein. The Kd for the membranes prior to solubilization was 47 +/- 11 nM (n = 3) and the Bmax was 700 +/- 90 fmol sites per mg of protein. The association rate constant, k1, was 1.57 X 10(7) M-1 X min-1 and the dissociation rate constant, k-1, was 0.61 +/- 0.04 min-1 (n = 4), yielding a Kd (k-1/k1) of 39 nM. Several thromboxane A2/prostaglandin H2 agonists and antagonists displaced bound [125I]-PTA-OH at concentrations similar to those at which they affect platelet aggregation. Collectively, these observations suggest that the solubilized protein is the thromboxane A2/prostaglandin H2 binding site that mediates platelet aggregation.

Purification from rat sarcolemma of the saxitoxin-binding component of the excitable membrane sodium channel

The saxitoxin-binding component (SBC) of the excitable membrane sodium channel has been solubilized and purified from rat skeletal muscle sarcolemma. Phospholipid was required in mixed micelles with detergent for stability of the mammalian SBC. Even at optimal detergent-to-phospholipid ratio, the solubilized SBC showed significant temperature-dependent loss of specific toxin binding with time, necessitating maintenance of low temperatures during purification. Characteristics of saxitoxin binding to the solubilized material closely resembled those seen in intact membranes. A weak anion-exchange column was synthesized; it provided rapid 10- to 20-fold purification of the solubilized SBC. Additional necessary purification was obtained by chromatography on immobilized wheat germ agglutinin. Specific saxitoxin-binding activity of the purified material averaged approximately 1500 pmol of saxitoxin bound per mg of protein. Three bands were present in this material on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The purified material sedimented on a sucrose gradient with an apparent s20,w of 9.9 S.

Purification of the tetrodotoxin-binding component associated with the voltage-sensitive sodium channel from Electrophorus electricus electroplax membranes

The tetrodotoxin-binding component associated with the voltage-sensitive sodium channel from electroplax membranes of Electrophorus electricus has been purified. The toxin-binding site could be efficiently solubilized with Lubrol-PX, resulting in an extract of high initial specific activity. Purification was facilitated by the development of a rapid, quantitative binding assay. The binding component was stabilized during purification by the use of mixed lipid/detergent micelles of defined composition, and by the saturation of the site with tetrodotoxin. The purification was achieved by means of a highly selective adsorption of the toxin-binding component to DEASE-Sephadex A-25, followed by desorption at high ionic strength and chromatography over Sepharose 6B. Final peak specific activities were at least 50% of the specific activity expected for a pure, undenatured toxin-binding componenet of 230,000 molecular weight. The purified material exhibited a sedimentation coefficient of approximately 8 S and an unusual Stokes radius of 95 A. Purified material showed a relatively simple pattern on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, being comprised of only three polypeptides.

Specific radiolabeling of a cell surface receptor for epidermal growth factor

A photoreactive derivative of epidermal growth factor (EGF) has been used to identify and specifically label a membrane receptor for EGF on mouse 3T3 cells. Photoactivable EGF, labeled with 125I, was incubated with 3T3 cells and then photolyzed in situ to generate a nitrene capable of reacting with a wide variety of chemical bonds. Analysis of the system by sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed, besides the band of EGF, only one other major radioactive band, at a position indicating an apparent molecular weight of 190,000. This band was absent when a nonresponsive and nonbinding variant of 3T3 was used. A direct proportionality between binding activity and crosslinked complex formation was demonstrated using a variety of binding conditions. "Down regulated" cells, in which EGF binding activity was greatly reduced by prolonged incubation with an appropriate concentration of EGF, also had a decrease in covalent complex formation proportional to the decrease in EGF binding activity.

Purification of histamine receptor. (IV) Specificity of binding of various drugs to the histamine receptor-rich fraction and to solubilized binding sites

Studies were made on tritiated histamine binding to the receptor-rich membrane fraction and solubilized sites and its displacement by various drugs. H1-Agonists and antagonists displaced histamine most effectively. A H2-agonist and atropine were less effective and propranolol, phentolamine and imidazole acetic acid had little effect. The solubilized binding sites showed the same specificity of binding as the membrane fraction. Membrane fragments had two binding constants, whereas solubilized sites had only one. Solubilized sites bound similar amounts of histamine and dibenamine: the latter was applied to intact tissue under conditions which would presumably cause specific binding to histamine receptors. These binding characteristics show that the method used was adequate for purification of histamine receptors from smooth muscle of cat small intestine.

Epinephrine-binding plasma-membrane antigens in rat liver

Detergent extracts of isolated rat liver plasma membranes were analysed in two-dimensional immunoelectrophoresis against antiserum to plasma membranes. Enzyme staining of the immunoprecipitates revealed the presence of about ten antigens with nucleoside di- and triphosphatase activity. Most of these were earlier shown also to be NADH-neotetrazolium reductase active. In addition, two of these antigens exhibited L-leucyl-beta-naphthylamidase activity. As judged from autoradiography these plasma membrane antigens earlier characterized as multienzyme complexes bound [14C]epinephrine, and the same antigens were labelled regardless of whether membranes or membrane extracts were incubated with the radioactive hormone. The specificity of this binding was established in displacement experiments with unlabelled hormones or their analogues. Another hormone-binding antigen, also identified in the plasma membrane extract did not exhibit any known enzyme activity while three antigens with different enzyme activities had no epinephrine-binding capacity. [14C]Epinephrine-labelled plasma membrane extracts were chromatographed on Sepharose 4B and the fractions obtained were analysed in two-dimensional immunoelectrophoresis combined with autoradiography. Nucleoside di- and triphosphatases of high molecular weights (5000000) were associated with L-leucyl-beta-naphthylamidase activity, while no such associations were detected in a lower molecular weight region (70000). Further immunological studies on the various fractionated antigens provided evidence that at least two of them occurred in both low and high molecular weight fractions. Hormone-binding membrane components in varying concentrations were found throughout the eluted extract.

Identification of cardiac beta-adrenergic receptors by (minus) [3H]alprenolol binding

(Minus) [3-H] alprenolol, a potent beta-adrenergic antagonist, was used to identify binding sites in a fraction of canine cyocardium. Beta adrenergic agonists and antagonists compete for these binding sites in a manner which directly parallels their known affinity for the cardiac beta-adrenergic receptor. Thus, binding was highly stereo-specific, with the (minus) isomers of beta-adrenergic agonists or antagonists being at least two orders of magnitude more potent than were the (plus) isomers in competing for these sites. The order of potency for inhibition of binding by beta-adrenergic agonists was (minus) isoproterenol greater than (minus) epinephrine greater than (minus) norepinephrine. The dissociation constant (KD) of (minus) alprenolol for the beta-adrenergic receptors was 7-11 nM as determined independently by direct binding studies or by inhibition of isoproterenol-stimulated adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. The beta-adrenergic antagonist (minus) propranolol also had high affinity for the binding sites (KD equals 12 nM). The physiologically inactive catechol-containing compounds pyrocatechol and (plus or minus) dihydroxymandelic acid, as well as the metabolite (plus or minus) normetanephrine, and the alpha-adrenergic antagonist phentolamine did not compete for the binding sites at a concentration of 160 muM. Binding was rapid (t1/2 less than 30 sec) and was rapidly reversible (t1/2 less than 15 sec). The binding sites were saturable and bound 0.35 pmol of (minus) [3-H] alprenolol per mg of membrane protein. These characteristics suggest that these binding sites represent the cardiac beta-adrenergic receptors.

Beta-adrenergic receptor: stereospecific interaction of iodinated beta-blocking agent with high affinity site

An iodine-labeled beta-adrenergic inhibitor ((125)l-hydroxybenzylpindolol) binds specifically to a site on turkey erythrocyte membranes. A series of beta-adrenergic agonists and inhibitors compete for this binding site, with apparent affinities paralleling biological effectiveness as activators or inhibitors of catecholaminestimulated adenylate cyclase. The activity of d-(+) agonists or inhibitors was 1 percent (or less) than that of the corresponding l-(-) isomers in competing for binding of the iodinated blocker as well as in affecting catecholamine-stimulated adenylate cyclase. 1-(-)-Norepinephrine was about one-tenth as active as l-(-)-isoproterenol in competing for the beta-blocking agent site. The stereospecificity of the interaction with the iodinated beta-blocking agent and the correspondence between affinity for site and biological potency of analogs suggested that this interaction is involved in function of the beta-adrenergic receptor.

The binding characteristics and number of beta-adrenergic receptors on the turkey erythrocyte

Turkey erythrocyte ghosts (empty membranes) possess a class of receptors that can bind both L-[(3)H]isoproterenol and DL-[(3)H]propranolol. The binding of [(3)H]isoproterenol to these receptors occurs with a dissociation constant of 0.15 muM and can be fully inhibited by 1 muM propranolol. The binding of [(3)H]propranolol occurs with a dissociation constant of 2.5 nM and can be fully inhibited by 0.2 mM DL-isoproterenol. Ligand binding is sensitive to sonication, boiling, and 8 M urea. The cells possess 500 to 1000 beta-adrenergic receptors per cell. Binding of propranolol to the beta-receptor was found to be stereospecific for the L stereoisomer. If one assumed a 1:1 relationship between beta-adrenergic receptors and adenylate cyclase, the turnover number of this adenylate cyclase would be close to 100 min(-1).

Studies on the mode of action of cholera toxin. Effects on solubilized adenylate cyclase

To gain further insight into the mechanism of action of cholera toxin, solubilized preparations of adenylate cyclase from control and toxin-treated rat livers were studied. Adenylate cyclase activity was measured in both particulate and solubilized form in rat liver under control conditions and after intravenous injection of cholera toxin. Cholera toxin caused a 3.3-fold activation of adenylate cyclase in the particulate preparation and a 5.8-fold increase in the solubilized preparation. Thus, the ability of cholera toxin to stimulate adenylate cyclase is present even when the enzyme membrane environment is disrupted. Furthermore, the solubilized enzyme, after treatment with cholera toxin, retained its ability to respond to catecholamines, but not to glucagon. In contrast, the control enzyme lost its responsiveness to catecholamines and glucagon after solubilization.