Homogeneous GTP binding assay employing QRET technology

Homogeneous Dual-Parametric-Coupled Assay for Simultaneous Nucleotide Exchange and KRAS/RAF-RBD Interaction Monitoring

We have developed a rapid and sensitive single-well dual-parametric method introduced in linked RAS nucleotide exchange and RAS/RAF-RBD interaction assays. RAS mutations are frequent drivers of multiple different human cancers, but the development of therapeutic strategies has been challenging. Traditionally, efforts to disrupt the RAS function have focused on nucleotide exchange inhibitors, GTP-RAS interaction inhibitors, and activators increasing GTPase activity of mutant RAS proteins. As the amount of biological knowledge grows, targeted biochemical assays enabling high-throughput screening have become increasingly interesting. We have previously introduced a homogeneous quenching resonance energy transfer (QRET) assay for nucleotide binding studies with RAS and heterotrimeric G proteins. Here, we introduce a novel homogeneous signaling technique called QTR-FRET, which combine QRET technology and time-resolved Förster resonance energy transfer (TR-FRET). The dual-parametric QTR-FRET technique enables the linking of guanine nucleotide exchange factor-induced Eu3+-GTP association to RAS, monitored at 615 nm, and subsequent Eu3+-GTP-loaded RAS interaction with RAF-RBD-Alexa680 monitored at 730 nm. Both reactions were monitored in a single-well assay applicable for inhibitor screening and real-time reaction monitoring. This homogeneous assay enables separable detection of both nucleotide exchange and RAS/RAF interaction inhibitors using low nanomolar protein concentrations. To demonstrate a wider applicability as a screening and real-time reaction monitoring method, the QTR-FRET technique was also applied for G(i)α GTP-loading and pertussis toxin-catalyzed ADP-ribosylation of G(i)α, for which we synthesized a novel γ-GTP-Eu3+ molecule. The study indicates that the QTR-FRET detection technique presented here can be readily applied to dual-parametric assays for various targets.

Application of lanthanide luminescence in probing enzyme activity

Enzymes play critical roles in the regulation of cellular function and are implicated in numerous disease conditions. Reliable and practicable assays are required to study enzyme activity, to facilitate the discovery of inhibitors and activators of enzymes related to disease. In recent years, a variety of enzyme assays have been devised that utilise luminescent lanthanide(iii) complexes, taking advantage of their high detection sensitivities, long luminescence lifetimes, and line-like emission spectra that permit ratiometric and time-resolved analyses. In this Feature article, we focus on recent progress in the development of enzyme activity assays based on lanthanide(iii) luminescence, covering a variety of strategies including Ln(iii)-labelled antibodies and proteins, Ln(iii) ion encapsulation within defined peptide sequences, reactivity-based Ln(iii) probes, and discrete Ln(iii) complexes. Emerging approaches for monitoring enzyme activity are discussed, including the use of anion responsive lanthanide(iii) complexes, capable of molecular recognition and luminescence signalling of polyphosphate anions.

Homogeneous single-label tyrosine kinase activity assay for high throughput screening

Protein post-translational modifications (PTMs) are regulatory mechanisms carried out by different enzymes in a cell. Kinase catalyzed phosphorylation is one of the most important PTM affecting the protein activity and function. We have developed a single-label quenching resonance energy transfer (QRET) assay to monitor tyrosine phosphorylation in a homogeneous high throughput compatible format. Epidermal growth factor receptor (EGFR) induced phosphorylation was monitored using Eu(3+)-chelate labeled peptide and label-free phosphotyrosine specific antibody in presence of a soluble quencher molecule. In the QRET kinase assay, antibody binding to phosphorylated Eu(3+)-peptide protects the Eu(3+)-chelate from luminescence quenching, monitoring high time-resolved luminescence (TRL) signals. In the presence of specific kinase inhibitor, antibody recognition and Eu(3+)-chelate protection is prevented, allowing an efficient luminescence quenching. The assay functionality was demonstrated with a panel of EGFR inhibitors (AG-1478, compound 56, erlotinib, PD174265, and staurosporine). The monitored IC50 values ranged from 0.08 to 155.3 nM and were comparable to those found in the literature. EGFR activity and inhibition assays were performed using low nanomolar enzyme and antibody concentration in a 384-well plate format, demonstrating its compatibility for high throughput screening (HTS).

Quenching resonance energy transfer (QRET): a single-label technique for inhibitor screening and interaction studies

The increased number of therapeutic targets has led to a growing need for screening methods enabling possible inhibitor compound selection. Information for new therapeutic targets has been found mostly from sequencing of the human genome but this knowledge cannot be directly converted into clinically relevant drug molecules. After target identification, the multistep drug development process takes many years and hundreds of millions of dollars are spent without certainty of the outcome. The first and the most critical step in the drug development process is hit selection. The optimal high throughput screening method should provide the highest possible number of true positive hits for further studies and lead discovery. The result should be achieved with low material consumption in a rapid and automated process. Radioactive label based methods are sensitive, but due to the problems arising from the radioactivity, luminescence-based methods have become increasingly popular in screening. In this review, the time-resolved luminescence based quenching resonance energy transfer (QRET) technique is discussed for primary screening.

Multiparametric homogeneous method for identification of ligand binding to G protein-coupled receptors: receptor-ligand binding and β-arrestin assay

Two homogeneous assay systems have been combined to provide a new cell-based functional assay. The assay can be used to identify ligand binding to β(2)-adrenergic receptors, but also the downstream response can be determined in the same assay. Both the quenching resonance energy transfer (QRET) and the DiscoveRx PathHunter assay formats allow the use of intact cells. The homogeneous QRET technique is a single-label approach based on nonspecific quenching of the time-resolved luminescence, enabling agonist and antagonist receptor binding measurements. The commercial PathHunter assay is in turn based on enzyme fragment complementation, which can be detected on the basis of chemiluminescence signal. In the PathHunter technology the enzyme complementation is recorded immediately downstream of agonist-induced receptor activation. The new multiparametric detection technology combines these two assay methods enabling the identification of agonist, and antagonist binding to the receptor, and the agonist-induced response. Using the QRET and the PathHunter methods a panel of β(2)-adrenergic receptor ligands (epinephrine, terbutaline, metaproterenol, salmeterol, propranolol, alprenolol, bisoprolol, ICI 118,551, and bucindolol) was tested to prove the assay performance. The signal-to-background ratio for tested ligands ranged from 5 to 11 and from 6 to 18 with QRET and PathHunter, respectively. Combined homogeneous assay technique can provide an informative method for screening purposes and an efficient way to monitor receptor-ligand interaction, thus separating agonist from antagonist.

Homogeneous single-label biochemical Ras activation assay using time-resolved luminescence

Mutations of the small GTP-binding protein Ras have been commonly found in tumors, and Ras oncogenes have been established to be involved in the early steps of cancerogenesis. The detection of Ras activity is critical in the determination of the cell signaling events controlling cell growth and differentiation. Therefore, development of improved methods for primary screening of novel potential drugs that target small GTPase or their regulators and their signaling pathways is important. Several assays have been developed for small GTPases studies, but all these methods have limitations for a high-throughput screening (HTS) use. Multiple steps including separation, use of radioactive labels or time-consuming immunoblotting, and a need of large quantities of purified proteins are decreasing the user-friendliness of these methods. Here, we have developed a homogeneous H-Ras activity assay based on a single-label utilizing the homogeneous quenching resonance energy transfer technique (QRET). In the QRET method, the binding of a terbium-labeled GTP (Tb-GTP) to small GTPase protein H-Ras protects the signal of the label from quenching, whereas the signal of the nonbound fraction of Tb-GTP is quenched by a soluble quencher. This enables a rapid determination of the changes in the activity status of Ras. The assay optimization showed that only 60 nM concentration of purified H-Ras protein was needed. The functionality of the assay was proved by detecting the effect of H-Ras guanine nucleotide exchange factor, Son of Sevenless. The signal-to-background ratio up to 7.7 was achieved with an average assay coefficient of variation of 9.1%. The use of a low concentration of purified protein is desirable and the signal-to-background ratio of 3.4 was achieved in the assay at a concentration of 60 nM for H-Ras and SOS proteins. The need of only one labeled molecule and the ability to decrease the quantities of purified proteins used in the experiments are valuable qualities in HTS showing the potential of the QRET method.

Use of the GTPγS ([35S]GTPγS and Eu-GTPγS) binding assay for analysis of ligand potency and efficacy at G protein-coupled receptors

In this review I consider assays for G protein-coupled receptor (GPCR) activity based on the binding of labelled analogues of GTPγS ([(35) S]GTPγS or Eu-GTPγS) to G proteins in tissues (GTPγS binding assays). Such assays provide convenient measures of GPCR activity close to the receptor in the signalling cascade. In order to set up a GTPγS binding assay, the requirements of the assay must be considered. These are tissue source, GTPγS analogue, G protein, GDP, Mg(2+) /Na(+) ions, saponin, incubation time. The assay, once optimized, can be used to generate concentration/response curves for GPCRs signalling via G(i/o) proteins (or to other G proteins with a modified assay) and actions of agonists, inverse agonists and antagonists may, in principle, be assessed. For agonists and inverse agonists, data for the maximal agonist effect, the concentration of ligand giving a half-maximal response and the Hill coefficient may be derived. For antagonists, data for the equilibrium dissociation constant can be obtained. The mechanistic basis of the assay is considered. Although the assay can be used to profile ligands, under the conditions it is used, it may not be measuring the same event that determines GPCR action in cells.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6

A homogeneous single-label time-resolved fluorescence cAMP assay

G-protein-coupled receptors (GPCRs) are an important class of pharmaceutical drug targets. Functional high-throughput GPCR assays are needed to test an increasing number of synthesized novel drug compounds and their function in signal transduction processes. Measurement of changes in the cyclic adenosine monophosphate (cAMP) concentration is a widely used method to verify GPCR activation in the adenylyl cyclase pathway. Here, a single-label time-resolved fluorescence and high-throughput screening (HTS)-feasible method was developed to measure changes in cAMP levels in HEK293(i) cells overexpressing either β(2)-adrenergic or δ-opioid receptors. In the quenching resonance energy transfer (QRET) technique, soluble quenchers reduce the signal of unbound europium(III)-labeled cAMP in solution, whereas the antibody-bound fraction is fluorescent. The feasibility of this homogeneous competitive assay was proven by agonist-mediated stimulation of receptors coupled to either the stimulatory G(s) or inhibitory G(i) proteins. The reproducibility of the assays was excellent, and Z' values exceeded 0.7. The dynamic range, signal-to-background ratio, and detection limit were compared with a commercial time-resolved fluorescence resonance energy transfer (TR-FRET) assay. In both homogeneous assays, similar assay parameters were obtained when adenylyl cyclase was stimulated directly by forskolin or via agonist-mediated activation of the G(s)-coupled β(2)AR. The advantage of using the single-label approach relates to the cost-effectiveness of the QRET system compared with the two-label TR-FRET assay as there is no need for labeling of two binding partners leading to reduced requirements for assay optimization.

A homogeneous method to measure nucleotide exchange by α-subunits of heterotrimeric G-proteins using fluorescence polarization

The mainstay of assessing guanosine diphosphate release by the α-subunit of a heterotrimeric G-protein is the [³⁵S]guanosine 5'-O-(3-thiotriphosphate) (GTPγS) radionucleotide-binding assay. This assay requires separation of protein-bound GTPγS from free GTPγS at multiple time points followed by quantification via liquid scintillation. The arduous nature of this assay makes it difficult to quickly characterize multiple mutants, determine the effects of individual variables (e.g., temperature and Mg(2+) concentration) on nucleotide exchange, or screen for small molecule modulators of Gα nucleotide binding/cycling properties. Here, we describe a robust, homogeneous, fluorescence polarization assay using a red-shifted fluorescent GTPγS probe that can rapidly determine the rate of GTPγS binding by Gα subunits.