Mining the receptorome

Screening the receptorome: an efficient approach for drug discovery and target validation

The receptorome, comprising at least 5% of the human genome, encodes receptors that mediate the physiological, pathological and therapeutic responses to a vast number of exogenous and endogenous ligands. Not surprisingly, the majority of approved medications target members of the receptorome. Several in silico and physical screening approaches have been devised to mine the receptorome efficiently for the discovery and validation of molecular targets for therapeutic drug discovery. Receptorome screening has also been used to discover, and thereby avoid, the molecular targets responsible for serious and unforeseen drug side effects.

Screening the receptorome

The term 'receptorome' is now being used to describe receptors, ion channels and transporters in the human genome that are potential drug targets. These proteins comprise a considerable fraction of the human genome, and include the G protein-coupled receptors, which are the targets for many medications. In this review, we summarize recent advances in the field, including the concept that the ultimate goal of drug discovery may not be the development of highly selective single-target drugs, the idea that potential side-effects can also be the goal of multi-target drug screening, and a discussion of the application of computational screening and public domain databases available to interested investigators.

p90 ribosomal S6 kinase 2 exerts a tonic brake on G protein-coupled receptor signaling

G protein-coupled receptors (GPCRs) are essential for normal central CNS function and represent the proximal site(s) of action for most neurotransmitters and many therapeutic drugs, including typical and atypical antipsychotic drugs. Similarly, protein kinases mediate many of the downstream actions for both ionotropic and metabotropic receptors. We report here that genetic deletion of p90 ribosomal S6 kinase 2 (RSK2) potentiates GPCR signaling. Initial studies of 5-hydroxytryptamine (5-HT)(2A) receptor signaling in fibroblasts obtained from RSK2 wild-type (+/+) and knockout (-/-) mice showed that 5-HT(2A) receptor-mediated phosphoinositide hydrolysis and both basal and 5-HT-stimulated extracellular signal-regulated kinase 1/2 phosphorylation are augmented in RSK2 knockout fibroblasts. Endogenous signaling by other GPCRs, including P2Y-purinergic, PAR-1-thrombinergic, beta1-adrenergic, and bradykinin-B receptors, was also potentiated in RSK2-deficient fibroblasts. Importantly, reintroduction of RSK2 into RSK2-/- fibroblasts normalized signaling, thus demonstrating that RSK2 apparently modulates GPCR signaling by exerting a "tonic brake" on GPCR signal transduction. Our results imply the existence of a novel pathway regulating GPCR signaling, modulated by downstream members of the extracellular signal-related kinase/mitogen-activated protein kinase cascade. The loss of RSK2 activity in humans leads to Coffin-Lowry syndrome, which is manifested by mental retardation, growth deficits, skeletal deformations, and psychosis. Because RSK2-inactivating mutations in humans lead to Coffin-Lowry syndrome, our results imply that alterations in GPCR signaling may account for some of its clinical manifestations.

Finding new tricks for old drugs: an efficient route for public-sector drug discovery

With the annotation of the human genome approaching completion, public-sector researchers - spurred in part by various National Institutes of Health Roadmap Initiatives - have become increasingly engaged in drug discovery and development efforts. Although large and diverse chemical libraries of 'drug-like' compounds can be readily screened to yield chemically novel scaffolds, transforming these 'chemical probes' into drugs is a daunting endeavour. A more efficient approach involves screening libraries of approved and off-patent medications; both phenotypic- and molecular target-based screening of 'old drugs' can readily yield compounds that could be immediately used in clinical trials. Using case studies, we describe how this approach has rapidly identified candidate medications suitable for clinical trials in disorders such as progressive multifocal leukoencephalopathy and amyotrophic lateral sclerosis. This approach has also led to the discovery of the molecular targets responsible for serious drug side effects, thereby allowing efficient 'counter-screening' to avoid these side effects.

Receptor systems: will mining the receptorome yield novel targets for pharmacotherapy?

We have recently defined the receptorome as 'that part of the proteome encoding receptors'. In this article, I provide a general overview of the members of the receptorome as well as methods used to screen the receptorome-both in silico and physically. Case histories of receptorome-based discovery efforts are then highlighted and the relevance of this approach to the discovery and validation of molecular targets for drug abuse treatment is emphasized.

Caveolae and lipid rafts: G protein-coupled receptor signaling microdomains in cardiac myocytes

A growing body of data indicates that multiple signal transduction events in the heart occur via plasma membrane receptors located in signaling microdomains. Lipid rafts, enriched in cholesterol and sphingolipids, form one such microdomain along with a subset of lipid rafts, caveolae, enriched in the protein caveolin. In the heart, a key caveolin is caveolin-3, whose scaffolding domain is thought to serve as an anchor for other proteins. In spite of the original morphologic definition of caveolae ("little caves"), most work related to their role in compartmenting signal transduction molecules has involved subcellular fractionation or immunoprecipitation with anti-caveolin antibodies. Use of such approaches has documented that several G protein-coupled receptors (GPCR), and their cognate heterotrimeric G proteins and effectors, localize to lipid rafts/caveolae in neonatal cardiac myocytes. Our recent findings support the view that adult cardiac myocytes appear to have different patterns of localization of such components compared to neonatal myocytes and cardiac fibroblasts. Such results imply the existence of multiple subcellular microdomains for GPCR-mediated signal transduction in cardiac myocytes, in particular adult myocytes, and raise a major unanswered question: what are the precise mechanism(s) that determine co-localization of GPCR and post-receptor components with lipid rafts/caveolae in cardiac myocytes and other cell types?

Screening the receptorome reveals molecular targets responsible for drug-induced side effects: focus on ‘fen–phen’

The in vitro pharmacological profiling of drugs using a large panel of cloned receptors (e.g., G protein-coupled receptors, ligand-gated ion channels, Na(+)-dependent monoamine transporters), an approach that has come to be known as 'receptorome screening', has unveiled novel molecular mechanisms responsible for the actions and/or side effects of certain drugs. For instance, receptorome screening has been employed to uncover novel molecular targets involved in the actions of antipsychotic medications and the hallucinogenic mint extract salvinorin A. This review highlights the recent application of receptorome screening to discover why the anorexigen fenfluramine causes serious cardiopulmonary side effects. Receptorome screening has implicated N-deethylation of fenfluramine and serotonin 5-hydroxy-t-ryptamine 2B receptors in the adverse effects of the drug; subsequent studies corroborated this finding. The results discussed highlight the utility of determining the potential activity of drugs -- and, importantly, of their in vivo metabolites -- at as many molecular targets as possible in order to reliably predict side effect profiles. Receptorome screening represents one of the most effective methods for identifying potentially serious drug-related side effects at the preclinical stage, thereby avoiding significant economic and human health consequences.

Membrane Protein Solubilization: Recent Advances and Challenges in Solubilization of Serotonin1A Receptors

Solubilization of integral membrane proteins is a process in which the proteins and lipids that are held together in native membranes are suitably dissociated in a buffered detergent solution. The controlled dissociation of the membrane results in formation of small protein and lipid clusters that remain dissolved in the aqueous solution. Effective solubilization and purification of membrane proteins, especially heterologously-expressed proteins in mammalian cells in culture, in functionally active forms represent important steps in understanding structure-function relationship of membrane proteins. In this review, critical factors determining functional solubilization of membrane proteins are highlighted with the solubilization of the serotonin 1A receptor taken as a specific example.