New insights into GPCR function: implications for HTS

The in vivo specificity of synaptic Gβ and Gγ subunits to the α2a adrenergic receptor at CNS synapses

G proteins are major transducers of signals from G-protein coupled receptors (GPCRs). They are made up of α, β, and γ subunits, with 16 Gα, 5 Gβ and 12 Gγ subunits. Though much is known about the specificity of Gα subunits, the specificity of Gβγs activated by a given GPCR and that activate each effector in vivo is not known. Here, we examined the in vivo Gβγ specificity of presynaptic α_2a-adrenergic receptors (α_2aARs) in both adrenergic (auto-α_2aARs) and non-adrenergic neurons (hetero-α_2aARs) for the first time. With a quantitative MRM proteomic analysis of neuronal Gβ and Gγ subunits, and co-immunoprecipitation of tagged α_2aARs from mouse models including transgenic FLAG-α_2aARs and knock-in HA-α_2aARs, we investigated the in vivo specificity of Gβ and Gγ subunits to auto-α_2aARs and hetero-α_2aARs activated with epinephrine to understand the role of Gβγ specificity in diverse physiological functions such as anesthetic sparing, and working memory enhancement. We detected Gβ₂, Gγ₂, Gγ₃, and Gγ₄ with activated auto α_2aARs, whereas we found Gβ₄ and Gγ₁₂ preferentially interacted with activated hetero-α_2aARs. Further understanding of in vivo Gβγ specificity to various GPCRs offers new insights into the multiplicity of genes for Gβ and Gγ, and the mechanisms underlying GPCR signaling through Gβγ subunits.

GPCR regulation of secretion

Modulation of neurotransmitter exocytosis by activated Gi/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.

Label-free drug screening assay multiplexed with an orthogonal time-resolved fluorescence labeled assay

Cell-based assays against cell surface receptor targets are essential in vitro models of target-based drug discovery. At the lead generation phase large-scale functional screening assays monitoring individual cellular readouts detect interactions between the compounds and the predefined pathways but might lack sufficient sensitivity owing to the complexity of downstream signaling pathways. Cellular label-free assays offer advantages over labeled detection approaches as they reflect whole-cell responses without the prerequisite of detecting only a single cellular analyte and introducing additional genetic manipulations in favor of the chosen detection method. The combination of a label-free assay and labeled assays might integrate the advantageous characteristics of both approaches with regards to added pharmacological information and a bigger pool of chemical starting material. Here we report multiplexing of dynamic mass redistribution label-free technology with HTRF-based cAMP detection on an alpha2c adrenergic receptor expressing cell line. Besides describing the challenging assay development work associated with the set goal, a pilot screening campaign on ca. 1600 compounds is also presented. The combined assay demonstrated the ability to detect relevant activities in both readouts. Interpretation of the results as well as an outlook for further possible opportunities and applications are also discussed.

Free Fatty Acid Receptor 4 Mediates the Beneficial Effects of n-3 Fatty Acids on Body Composition in Mice

As populations continue to age worldwide, sarcopenic obesity has heightened interest due to its medical importance. Although much evidence now indicates that n-3 fatty acids (FAs) may have beneficial effects on body composition including fat and muscle, their exact mechanisms have not yet been elucidated. Because free FA receptor 4 (FFA4) has been reported to be a receptor for n-3 FAs, we hypothesized that the protective role of n-3 FAs on body composition could be mediated by FFA4. To test this possibility, we generated mice overexpressing n-3 FAs but lacking FFA4 by crossing fat-1 transgenic (fat-1 ^Tg+) and FFA4 knockout (Ffar4 ^-/-) mice. Because fat-1 ^Tg+ mice, in which n-6 is endogenously converted into n-3 FAs, contain high n-3 FA levels, they could be a good animal model for studying the effects of n-3 FAs in vivo. Male and female littermates were included in high-fat-diet- (HFD) and ovariectomy-induced models, respectively. In the HFD model, male fat-1 ^Tg+ mice had a lower percentage of fat mass and a higher percentage of lean mass than their wild-type littermates only when they had the Ffar4 ^+/+ not the Ffar4 ^-/- background. Female fat-1 ^Tg+ mice showed less increase of fat mass percentage and less decrease of lean mass percentage after ovariectomy than wild-type littermates. However, these effects on body composition were attenuated in the Ffar4 ^-/- background. Taken together, our results indicate that the beneficial effects of n-3 FAs on body composition were mediated by FFA4 and thus suggest that FFA4 may be a potential therapeutic target for modulating sarcopenic obesity.

Positive Detection of GPCR Antagonists Using a System for Inverted Expression of a Fluorescent Reporter Gene

The yeast Saccharomyces cerevisiae is a useful eukaryotic host organism for studying GPCRs as monomolecular models. Fluorescent reporter gene assays for GPCRs provide a convenient assay for measuring receptor activity using fluorometric instruments. Generally, these assays detect receptor activation by agonistic ligands as the induction of fluorescent reporter expression, whereas antagonistic activities are detected by competition with agonistic ligands, resulting in decreases in fluorescence intensity. In the current study, we established a system for inverted expression of a fluorescent reporter by incorporating a PEST-tag and finding out a promoter inhibited by activation of the GPCR signaling pathway from yeast endogenous promoters. Because agonists prevent fluorescent reporter expression in this system, antagonists compete with agonists and yield increased fluorescence intensity. We used the yeast endogenous pheromone receptor as a model GPCR to demonstrate the feasibility of our system for positive detection targeted at antagonists. Compared to results when only agonists were added to yeast cells, more than 10-fold higher fluorescence intensity was observed when antagonists were added in combination with agonists. The approach described here has the potential to markedly accelerate the identification of GPCR antagonists by providing rapid and straightforward responses.

Opportunities for therapeutic antibodies directed at G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) are activated by a diverse range of ligands, from large proteins and proteases to small peptides, metabolites, neurotransmitters and ions. They are expressed on all cells in the body and have key roles in physiology and homeostasis. As such, GPCRs are one of the most important target classes for therapeutic drug discovery. The development of drugs targeting GPCRs has therapeutic value across a wide range of diseases, including cancer, immune and inflammatory disorders as well as neurological and metabolic diseases. The progress made by targeting GPCRs with antibody-based therapeutics, as well as technical hurdles to overcome, are presented and discussed in this Review. Antibody therapeutics targeting C-C chemokine receptor type 4 (CCR4), CCR5 and calcitonin gene-related peptide (CGRP) are used as illustrative clinical case studies.

Free Fatty Acid Receptor 4 (GPR120) Stimulates Bone Formation and Suppresses Bone Resorption in the Presence of Elevated n-3 Fatty Acid Levels

Free fatty acid receptor 4 (FFA4) has been reported to be a receptor for n-3 fatty acids (FAs). Although n-3 FAs are beneficial for bone health, a role of FFA4 in bone metabolism has been rarely investigated. We noted that FFA4 was more abundantly expressed in both mature osteoclasts and osteoblasts than their respective precursors and that it was activated by docosahexaenoic acid. FFA4 knockout (Ffar4(-/-)) and wild-type mice exhibited similar bone masses when fed a normal diet. Because fat-1 transgenic (fat-1(Tg+)) mice endogenously converting n-6 to n-3 FAs contain high n-3 FA levels, we crossed Ffar4(-/-) and fat-1(Tg+) mice over two generations to generate four genotypes of mice littermates: Ffar4(+/+);fat-1(Tg-), Ffar4(+/+);fat-1(Tg+), Ffar4(-/-);fat-1(Tg-), and Ffar4(-/-);fat-1(Tg+). Female and male littermates were included in ovariectomy- and high-fat diet-induced bone loss models, respectively. Female fat-1(Tg+) mice decreased bone loss after ovariectomy both by promoting osteoblastic bone formation and inhibiting osteoclastic bone resorption than their wild-type littermates, only when they had the Ffar4(+/+) background, but not the Ffar4(-/-) background. In a high-fat diet-fed model, male fat-1(Tg+) mice had higher bone mass resulting from stimulated bone formation and reduced bone resorption than their wild-type littermates, only when they had the Ffar4(+/+) background, but not the Ffar4(-/-) background. In vitro studies supported the role of FFA4 as n-3 FA receptor in bone metabolism. In conclusion, FFA4 is a dual-acting factor that increases osteoblastic bone formation and decreases osteoclastic bone resorption, suggesting that it may be an ideal target for modulating metabolic bone diseases.

Opioid doses required for pain management in lung cancer patients with different cholesterol levels: negative correlation between opioid doses and cholesterol levels

Background

Pain management has been considered as significant contributor to broad quality-of-life improvement for cancer patients. Modulating serum cholesterol levels affects analgesia abilities of opioids, important pain killer for cancer patients, in mice system. Thus the correlation between opioids usages and cholesterol levels were investigated in human patients with lung cancer.

Methods

Medical records of 282 patients were selected with following criteria, 1) signed inform consent, 2) full medical records on total serum cholesterol levels and opioid administration, 3) opioid-naïve, 4) not received/receiving cancer-related or cholesterol lowering treatment, 5) pain level at level 5–8. The patients were divided into different groups basing on their gender and cholesterol levels. Since different opioids, morphine, oxycodone, and fentanyl, were all administrated at fixed low dose initially and increased gradually only if pain was not controlled, the percentages of patients in each group who did not respond to the initial doses of opioids and required higher doses for pain management were determined and compared.

Results

Patients with relative low cholesterol levels have larger percentage (11 out of 28 in female and 31 out of 71 in male) to not respond to the initial dose of opioids than those with high cholesterol levels (0 out of 258 in female and 8 out of 74 in male). Similar differences were obtained when patients with different opioids were analyzed separately. After converting the doses of different opioids to equivalent doses of oxycodone, significant correlation between opioid usages and cholesterol levels was also observed.

Conclusions

Therefore, more attention should be taken to those cancer patients with low cholesterol levels because they may require higher doses of opioids as pain killer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12944-016-0212-9) contains supplementary material, which is available to authorized users.

Multidimensional GPCR profiling and screening using impedance-based label-free and real-time assay

GPCRs constitute one of the most sought-after targets in drug discovery because they are associated with conditions ranging from cardiovascular diseases, autoimmune diseases, inflammation, cancer, and diseases of the nervous system. Moreover, they are one of the most amenable targets for drug discovery because they can be modulated by small molecules, peptides, proteins, and antibodies. Therefore it may not come as a surprise that close to 40 % of the drugs that are currently on the market are targeting GPCRs. It has become evident that GPCR signaling is highly complex and may involve multiple or a subset of pathways depending on the interaction of a GPCR with an agonist or antagonist. It is imperative that any functional screening for GPCR activity integrates this complexity. In this assay protocol, we describe how the xCELLigence RTCA HT impedance-based platform which can be used for functional cell-based GPCR assays can be utilized for GPCR screening.

Recent developments in cell-based assays and stem cell technologies for botulinum neurotoxin research and drug discovery

Botulinum neurotoxins (BoNTs) are exceptionally potent inhibitors of neurotransmission, causing muscle paralysis and respiratory failure associated with the disease botulism. Currently, no drugs are available to counter intracellular BoNT poisoning. To develop effective medical treatments, cell-based assays provide a valuable system to identify novel inhibitors in a time- and cost-efficient manner. Consequently, cell-based systems including immortalized cells, primary neurons and stem cell-derived neurons have been established. Stem cell-derived neurons are highly sensitive to BoNT intoxication and represent an ideal model to study the biological effects of BoNTs. Robust immunoassays are used to quantify BoNT activity and play a central role during inhibitor screening. In this review, we examine recent progress in physiologically relevant cell-based assays and high-throughput screening approaches for the identification of both direct and indirect BoNT inhibitors.

G protein-coupled receptors revisited: therapeutic applications inspired by synthetic biology

G protein-coupled receptors (GPCRs) mediate the majority of cellular responses to hormones and neurotransmitters within the human body. They have much potential in the emerging field of synthetic biology, which is the rational, systematic design of biological systems with desired functionality. The responsiveness of GPCRs to a plethora of endogenous and exogenous ligands and stimuli make them ideal sensory receptor modules of synthetic gene networks. Such networks can activate target gene expression in response to a specific stimulus. Additionally, because GPCRs are important pharmacological targets of various human diseases, genes encoding their protein/peptide ligands can also be incorporated as target genes of the response output elements of synthetic gene networks. This review aims to critically examine the potential role of GPCRs in constructing therapeutic synthetic gene networks and to discuss various challenges in utilizing GPCRs for synthetic biology applications.

G protein-coupled receptor deorphanizations

G protein-coupled receptors (GPCRs) are major regulators of intercellular interactions. They initiate these actions by being activated by a wide variety of natural ligands. Historically, ligands were discovered first, but the advent of molecular biology reversed this trend. Most GPCRs are identified on the basis of their DNA sequences and thus are initially unmatched to known natural ligands. They are termed orphan GPCRs. Discovering their ligands-i.e., "deorphanizing" the GPCRs-gave birth to the field of reverse pharmacology. This review discusses the present status of GPCR deorphanization, presents a few examples of successes and surprises, and highlights difficulties encountered in these efforts.

GPCR mediated regulation of synaptic transmission

Synaptic transmission is a finely regulated mechanism of neuronal communication. The release of neurotransmitter at the synapse is not only the reflection of membrane depolarization events, but rather, is the summation of interactions between ion channels, G protein coupled receptors, second messengers, and the exocytotic machinery itself which exposes the components within a synaptic vesicle to the synaptic cleft. The focus of this review is to explore the role of G protein signaling as it relates to neurotransmission, as well as to discuss the recently determined inhibitory mechanism of Gβγ dimers acting directly on the exocytotic machinery proteins to inhibit neurotransmitter release.

Fungal mating pheromones: choreographing the dating game

Pheromones are ubiquitous from bacteria to mammals - a testament to their importance in regulating inter-cellular communication. In fungal species, they play a critical role in choreographing interactions between mating partners during the program of sexual reproduction. Here, we describe how fungal pheromones are synthesized, their interactions with G protein-coupled receptors, and the signals propagated by this interaction, using Saccharomyces cerevisiae as a reference point. Divergence from this model system is compared amongst the ascomycetes and basidiomycetes, which reveals the wealth of information that has been gleaned from studying pheromone-driven processes across a wide spectrum of the fungal kingdom.

Primary cells and stem cells in drug discovery: emerging tools for high-throughput screening

Many drug discovery screening programs employ immortalized cells, recombinantly engineered to express a defined molecular target. Several technologies are now emerging that render it feasible to employ more physiologically, and clinically relevant, cell phenotypes. Consequently, numerous approaches use primary cells, which retain many functions seen in vivo, as well as endogenously expressing the target of interest. Furthermore, stem cells, of either embryonic or adult origin, as well as those derived from differentiated cells, are now finding a place in drug discovery. Collectively, these cells are expanding the utility of authentic human cells, either as screening tools or as therapeutics, as well as providing cells derived directly from patients. Nonetheless, the growing use of phenotypically relevant cells (including primary cells or stem cells) is not without technical difficulties, particularly when their envisioned use lies in high-throughput screening (HTS) protocols. In particular, the limited availability of homogeneous primary or stem cell populations for HTS mandates that novel technologies be developed to accelerate their adoption. These technologies include detection of responses with very few cells as well as protocols to generate cell lines in abundant, homogeneous populations. In parallel, the growing use of changes in cell phenotype as the assay readout is driving greater use of high-throughput imaging techniques in screening. Taken together, the greater availability of novel primary and stem cell phenotypes as well as new detection technologies is heralding a new era of cellular screening. This convergence offers unique opportunities to identify drug candidates for disorders at which few therapeutics are presently available.

Identification of residue-to-residue contact between a peptide ligand and its G protein-coupled receptor using periodate-mediated dihydroxyphenylalanine cross-linking and mass spectrometry

Fundamental knowledge about how G protein-coupled receptors and their ligands interact is important for understanding receptor-ligand binding and the development of new drug discovery strategies. We have used cross-linking and tandem mass spectrometry analyses to investigate the interaction of the N terminus of the Saccharomyces cerevisiae tridecapeptide pheromone, α-factor (WHWLQLKPGQPMY), and Ste2p, its cognate G protein-coupled receptor. The Trp(1) residue of α-factor was replaced by 3,4-dihydroxyphenylalanine (DOPA) for periodate-mediated chemical cross-linking, and biotin was conjugated to Lys(7) for detection purposes to create the peptide [DOPA(1),Lys(7)(BioACA),Nle(12)]α-factor, called Bio-DOPA(1)-α-factor. This ligand analog was a potent agonist and bound to Ste2p with ∼65 nanomolar affinity. Immunoblot analysis of purified Ste2p samples that were treated with Bio-DOPA(1)-α-factor showed that the peptide analog cross-linked efficiently to Ste2p. The cross-linking was inhibited by the presence of either native α-factor or an α-factor antagonist. MALDI-TOF and immunoblot analyses revealed that Bio-DOPA(1)-α-factor cross-linked to a fragment of Ste2p encompassing residues Ser(251)-Met(294). Fragmentation of the cross-linked fragment and Ste2p using tandem mass spectrometry pinpointed the cross-link point of the DOPA(1) of the α-factor analog to the Ste2p Lys(269) side chain near the extracellular surface of the TM6-TM7 bundle. This conclusion was confirmed by a greatly diminished cross-linking of Bio-DOPA(1)-α-factor into a Ste2p(K269A) mutant. Based on these and previously obtained binding contact data, a mechanism of α-factor binding to Ste2p is proposed. The model for bound α-factor shows how ligand binding leads to conformational changes resulting in receptor activation of the signal transduction pathway.

Human kinome drug discovery and the emerging importance of atypical allosteric inhibitors

IMPORTANCE OF THE FIELD

Protein kinases are important targets for drug discovery because they possess critical roles in many human diseases. Several protein kinase inhibitors have entered clinical development with others having already been approved for treating a host of diseases. However, many kinase inhibitors suffer from non-selectivity because they interact with the ATP binding region which has similar structures amongst the protein kinases and this non-selectivity sometimes can cause side effects. As a consequence, there is much interest in developing drugs that inhibit kinases through non-classical mechanisms with the hope of avoiding the side effects of previous kinase drugs.

AREAS COVERED IN THIS REVIEW

This review covers emerging information on kinase biology and discusses new approaches to design selective inhibitors that do not compete with ATP.

WHAT THE READER WILL GAIN

The reader will gain a better understanding of the importance of the field of allosteric inhibitor drug discovery and how this has required the adoption of a new generation of high-throughput screening techniques.

TAKE HOME MESSAGE

Discovery and development of allosteric modulators will result in a family of novel kinase therapies with greater selectivity and more varied ways to control activity of disease causing kinase targets.