Ligand functional selectivity and quantitative pharmacology at G protein-coupled receptors

β2-Adrenoceptor-mediated regulation of glucose uptake in skeletal muscle--ligand-directed signalling or a reflection of system complexity?

The capacity of G protein-coupled receptors (GPCRs) to activate multiple G protein isoforms and additional effectors such as β-arrestins has become a well-established paradigm and provides the basis for developing drugs that preferentially activate beneficial signalling pathways. There are many published examples of ligand-directed signalling, and recent studies have provided direct evidence that different agonists stabilise distinct GPCR conformations. This field is rapidly evolving, but a key question is whether signalling bias observed in heterologous cell expression systems can be translated to physiological systems of therapeutic relevance. The paper by Ngala et al. in this issue of the journal addresses the capacity of agonists acting at the β2-adrenoceptor to engender signalling bias in relation to glucose uptake in isolated skeletal muscle, an area of considerable potential interest in targeting insulin-independent pathways for the treatment of type 2 diabetes. The authors show that clenbuterol and BRL37344 have opposite effects on glucose uptake, despite both having agonist actions at β2-adrenoceptors. This study underlines some of the obstacles associated with studies in a complex physiological system but nonetheless highlights the need to consider signalling bias in the relevant target tissue when developing novel drugs.

Autocrine signaling based selection of combinatorial antibodies that transdifferentiate human stem cells

We report here the generation of antibody agonists from intracellular combinatorial libraries that transdifferentiate human stem cells. Antibodies that are agonists for the granulocyte colony stimulating factor receptor were selected from intracellular libraries on the basis of their ability to activate signaling pathways in reporter cells. We used a specialized "near neighbor" approach in which the entire antibody library and its target receptor are cointegrated into the plasma membranes of a population of reporter cells. This format favors unusual interactions between receptors and their protein ligands and ensures that the antibody acts in an autocrine manner on the cells that produce it. Unlike the natural granulocyte-colony stimulating factor that activates cells to differentiate along a predetermined pathway, the isolated agonist antibodies transdifferentiated human myeloid lineage CD34+ bone marrow cells into neural progenitors. This transdifferentiation by agonist antibodies is different from more commonly used methods because initiation is agenetic. Antibodies that act at the plasma membrane may have therapeutic potential as agents that transdifferentiate autologous cells.

Regulation of G protein-coupled receptors by allosteric ligands

Topographically distinct, druggable, allosteric sites may be present on all G protein-coupled receptors (GPCRs). As such, targeting these sites with synthetic small molecules offers an attractive approach to develop receptor-subtype selective chemical leads for the development of novel therapies. A crucial part of drug development is to understand the acute and chronic effects of such allosteric modulators at their corresponding GPCR target. Key regulatory processes including cell-surface delivery, endocytosis, recycling, and down-regulation tightly control the number of receptors at the surface of the cell. As many GPCR therapeutics will be administered chronically, understanding how such ligands modulate these regulatory pathways forms an essential part of the characterization of novel GPCR ligands. This is true for both orthosteric and allosteric ligands. In this Review, we summarize our current understanding of GPCR regulatory processes with a particular focus on the effects and implications of allosteric targeting of GPCRs.

Cooperativity of allosteric receptors

Cooperativity of ligand binding to allosteric receptors can be quantified using the Hill coefficient (nH) to measure the sigmoidal character of the binding curve. However, for measurements of the transition between conformational states, nH values can be misleading due to ambiguity of the reference state. For cooperative ligand binding, the reference state is a hyperbolic curve for a monomer with a single binding site characterized by nH=1. Therefore, binding curves with nH>1 provide a direct measure of cooperativity. For the dependence of the conformational state on ligand concentration, curves with nH>1 are observed, but in virtually all cases, the equivalent allosteric monomer has a value of nH<1. The ratio of the two nH values defines the effective cooperativity and always corresponds to nH=N (the number of protomers in the oligomer) for concerted transitions as specified by the Monod-Wyman-Changeux model. Dose-response curves for homopentameric α7 nicotinic receptors illustrate this relationship for both wild-type and mutant forms. For functional allosteric monomers such as G-protein-coupled receptors, normalization stretches the dose-response curve along the y-axis, thereby masking the "allosteric range" and increasing the apparent cooperativity to a limit for monomers of nH =1. The concepts of equivalent monomer and allosteric range were originally proposed in 1965 by Crick and Wyman in a manuscript circulated among the proponents of allostery, but only now published for the first time in this special issue.

Differentiation of opioid drug effects by hierarchical multi-site phosphorylation

Differences in the ability of opioid drugs to promote regulated endocytosis of μ-opioid receptors are related to their tendency to produce drug tolerance and dependence. Here we show that drug-specific differences in receptor internalization are determined by a conserved, 10-residue sequence in the receptor's carboxyl-terminal cytoplasmic tail. Diverse opioids induce receptor phosphorylation at serine (S)375, present in the middle of this sequence, but opioids differ markedly in their ability to drive higher-order phosphorylation on flanking residues [threonine (T)370, T376, and T379]. Multi-phosphorylation is required for the endocytosis-promoting activity of this sequence and occurs both sequentially and hierarchically, with S375 representing the initiating site. Higher-order phosphorylation involving T370, T376, and T379 specifically requires GRK2/3 isoforms, and the same sequence controls opioid receptor internalization in neurons. These results reveal a biochemical mechanism differentiating the endocytic activity of opioid drugs.

Allosteric modulation of a chemogenetically modified G protein-coupled receptor

Designer receptors exclusively activated by designer drugs (DREADDs) are chemogenetically modified muscarinic acetylcholine receptors (mAChRs) that have minimal responsiveness to acetylcholine (ACh) but are potently and efficaciously activated by an otherwise inert synthetic ligand, clozapine-N-oxide (CNO). DREADDs have been used as tools for selectively modulating signal transduction pathways in vitro and in vivo. Recent comprehensive studies have validated how the pharmacology of a CNO-bound DREADD mirrors that of an ACh-bound wild-type (WT) mAChR. However, nothing is known about whether this equivalence extends to the allosteric modulation of DREADDs by small molecules. To address this, we investigated the actions at an M(1) DREADD of benzyl quinolone carboxylic acid (BQCA), a positive allosteric modulator of ACh binding and function that is known to behave according to a simple two-state mechanism at the WT receptor. We found that allosteric modulation of the CNO-bound DREADD receptor is not equivalent to the corresponding modulation of the ACh-bound WT receptor. We also found that BQCA engenders stimulus bias at the M(1) DREADD, having differential types of cooperativity depending on the signaling pathway. Furthermore, the modulation of ACh itself by BQCA at the DREADD is not compatible with the two-state model that we previously applied to the M(1) WT receptor.

Synthesis and characterization of novel 2-amino-3-benzoylthiophene derivatives as biased allosteric agonists and modulators of the adenosine A(1) receptor

A series of novel 2-amino-3-benzoylthiophenes (2A3BTs) were screened using a functional assay of A(1)R mediated phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in intact CHO cells to identify potential agonistic effects as well as the ability to allosterically modulate the activity of the orthosteric agonist, R-PIA. Two derivatives, 8h and 8i, differing only in terms of the absence or presence of an electron-withdrawing group on the benzoyl moiety of the 2A3BT scaffold, were identified as biased allosteric agonists and positive allosteric modulators of agonist function at the adenosine A(1) receptor (A(1)R) in two different functional assays. Our findings indicate that subtle structural variations can promote functionally distinct receptor conformational states.

Impedance responses reveal β₂-adrenergic receptor signaling pluridimensionality and allow classification of ligands with distinct signaling profiles

The discovery that drugs targeting a single G protein-coupled receptor (GPCR) can differentially modulate distinct subsets of the receptor signaling repertoire has created a challenge for drug discovery at these important therapeutic targets. Here, we demonstrate that a single label-free assay based on cellular impedance provides a real-time integration of multiple signaling events engaged upon GPCR activation. Stimulation of the β₂-adrenergic receptor (β₂AR) in living cells with the prototypical agonist isoproterenol generated a complex, multi-featured impedance response over time. Selective pharmacological inhibition of specific arms of the β₂AR signaling network revealed the differential contribution of G_s-, G_i- and Gβγ-dependent signaling events, including activation of the canonical cAMP and ERK1/2 pathways, to specific components of the impedance response. Further dissection revealed the essential role of intracellular Ca²⁺ in the impedance response and led to the discovery of a novel β₂AR-promoted Ca²⁺ mobilization event. Recognizing that impedance responses provide an integrative assessment of ligand activity, we screened a collection of β-adrenergic ligands to determine if differences in the signaling repertoire engaged by compounds would lead to distinct impedance signatures. An unsupervised clustering analysis of the impedance responses revealed the existence of 5 distinct compound classes, revealing a richer signaling texture than previously recognized for this receptor. Taken together, these data indicate that the pluridimensionality of GPCR signaling can be captured using integrative approaches to provide a comprehensive readout of drug activity.

A Monod-Wyman-Changeux mechanism can explain G protein-coupled receptor (GPCR) allosteric modulation

The Monod-Wyman-Changeux (MWC) model was initially proposed to describe the allosteric properties of regulatory enzymes and subsequently extended to receptors. Yet despite GPCRs representing the largest family of receptors and drug targets, no study has systematically evaluated the MWC mechanism as it applies to GPCR allosteric ligands. We reveal how the recently described allosteric modulator, benzyl quinolone carboxylic acid (BQCA), behaves according to a strict, two-state MWC mechanism at the M1 muscarinic acetylcholine receptor (mAChR). Despite having a low affinity for the M1 mAChR, BQCA demonstrated state dependence, exhibiting high positive cooperativity with orthosteric agonists in a manner that correlated with efficacy but negative cooperativity with inverse agonists. The activity of BQCA was significantly increased at a constitutively active M1 mAChR but abolished at an inactive mutant. Interestingly, BQCA possessed intrinsic signaling efficacy, ranging from near-quiescence to full agonism depending on the coupling efficiency of the chosen intracellular pathway. This latter cellular property also determined the difference in magnitude of positive cooperativity between BQCA and the orthosteric agonist, carbachol, across pathways. The lack of additional, pathway-biased, allosteric modulation by BQCA was confirmed in genetically engineered yeast strains expressing different chimeras between the endogenous yeast G(pa1) protein and human Gα subunits. These findings define a chemical biological framework that can be applied to the study and classification of allosteric modulators across different GPCR families.

Allostery in GPCRs: 'MWC' revisited

G protein-coupled receptors (GPCRs) constitute the largest family of receptors in the genome and are the targets for at least 30% of current medicines. In recent years, there has been a dramatic increase in the discovery of allosteric modulators of GPCR activity and a growing appreciation of the diverse modes by which GPCRs can be regulated by both orthosteric and allosteric ligands. Interestingly, some of the contemporary views of GPCR function reflect characteristics that are shared by prototypical allosteric proteins, as encompassed in the classic Monod-Wyman-Changeux (MWC) model initially proposed for enzymes and subsequently extended to other protein families. In this review, we revisit the MWC model in the context of emerging structural, functional and operational data on GPCR allostery.