Time-Resolved Fluorescence Based GTP Binding Assay for G-Protein Coupled Receptors

Transcription

1 Time-Resolved Fluorescence Based GTP Binding Assay for G-Protein Coupled Receptors Gregory Warner, Rita Syystö, Heini Frang, Christel Gripenberg-Lerche, Jurgen Vanhauwe, Tarja Ahola, and Satu Kovanen

2 Abstract G-protein coupled receptors (GPCRs) participate in a wide range of cell signaling pathways and are believed to play a role in a variety of disease states. Current screening efforts include not only receptor ligand binding assays, but in addition functional assays including: Ca 2+ flux, camp levels, and GTP binding. Until now, GTP binding assays have revolved around the use of the radioactive, non-hydrolyzable GTP analogue, [ 35 S]GTPγS. Time-resolved fluorometry (TRF) is a well-established technology that exploits the unique fluorescence properties of lanthanide chelates to provide a powerful alternative to radioisotopic assays in many HTS applications. Using labeling chemistry, we have developed a non-radioactive, non-hydrolyzable, europium labeled GTP analogue,, for use in GPCR screening assays. The binding assay yielded similar if not better Z values and comparable EC 5 values to the traditional [ 35 S]GTPγS filtration assay when tested using several GPCRs. Agonistinduced stimulation of binding above basal levels was at least equivalent to and up to more than two times greater than that achieved by the [ 35 S]GTPγS assay. Using the PlateTrak robotic liquid handling system, we have shown the ability to automate this assay without sacrificing sensitivity or performance and estimate a minimum daily throughput of 2 plates. Thus, the end result is a highly sensitive, non-radioactive functional assay to monitor GPCR activity through GTP binding that can be automated and used in an HTS environment. Introduction Time-resolved fluorometry (TRF) is a well-established technology that exploits the unique fluorescence properties of lanthanide chelates to provide a powerful alternative to radioisotopic assays in many HTS applications. TRF assays exhibit low background and high signal-to-noise ratios, two attributes critical for robust HTS assays. Long fluorescence decay after excitation allows time-delayed signal detection (microseconds) to virtually eliminate all natural fluorescent background caused by cells and cell debris, screening compounds, plates, and other reagents (half-life of nanoseconds). A large Stokes shift (e.g., excitation and emission wavelengths for the Eu-chelate are ~34 nm and ~615 nm, respectively) minimizes cross-talk, resulting in a high signal-to-noise ratio. Therefore, because of their excellent temporal and spectral resolution, lanthanide chelate labels can provide high sensitivity assays. New screening assays are continually being developed to measure G-protein coupled receptors (GPCRs) activity. GPCRs participate in a wide range of cell signaling pathways and are believed to play a role in a variety of disease states. This in turn makes them attractive target for new therapeutics. Currently around 3% of clinically prescribed drugs act on GPCR family members. The application of TRF to GPCR assays to this point has mainly been for receptor binding assays. Here we describe a new assay that allows the user to examine GPCR activation by monitoring GTP binding through and take advantage of all the benefits of TRF technology. 2

3 Assay Principle Materials: 1X GTP Wash Solution AcroWell filter plates 5 M NaCl 25 mm HEPES (ph 7.4) (Pall Gelman, Cat. #52) 5 M MgCl 2 2 mm GDP Motilin (Bachem, Cat. #H-4385) 25 µm GTPγS 5 mg/ml saponin Neurotensin (Bachem, Cat. #H-4435) Oxotremorine M (Tocris, Cat. #167) FMLF peptide (Bachem, Cat. #H-33) Human Motilin Receptor membranes (PerkinElmer, Cat # RB-HMOTM) Human Neurotensin Receptor membranes (PerkinElmer, Cat # RB-HNT1M) Human Muscarinic M1 Receptor membranes (PerkinElmer, Cat # RB-HM1M) Human FPR1 Receptor membranes (BioSignal, Cat # 61527) [ 35 S]GTPγS (125 Ci/mmol) (PerkinElmer, Cat. # NEG3H) Method: Assay Buffer: 5 mm HEPES, ph 7.4; 1-1 mm MgCl 2 ; -2 mm NaCl;.1-1 µm GDP; -1 µg/ml saponin (components will be unique for each GPCR) Add 4 µl of assay buffer to all wells to be tested Add 35 µl of 5 mm HEPES, ph 7.4 to all wells to be tested Add 5 µl of agonist dilutions or 5 mm HEPES, ph 7.4 (blank) to all wells being tested Add 2 µl of receptor membranes diluted in 5 mm HEPES to give 1 µg membrane protein per well Incubate 3-6 min depending on receptor Add 1 µl of 1 nm to all wells Incubate 1-3 min depending on receptor Filter and wash 2X with 3 µl of ice-cold 1X GTP Wash buffer Measure bound Eu at 615 nm using a time-resolved fluorometer such as VICTOR 2 V 3

4 1 12 Binding (% over basal) GDP (µm) MgCl2 (mm) Figure 1. Optimization of MgCl 2 and GDP concentrations for motilininduced binding of to human motilin receptor. Human motilin receptor (RBHMOT) (1 µg) was incubated in 5 mm HEPES (ph 7.4) with various concentrations of GDP and MgCl 2 in the presence or absence of 1 µm motilin for 3 min. (3 nm) was added for an additional 3 min prior to filtration, washing, and measurement at 615 nm on VICTOR 2 V. 2 1 Binding (% above Basal) N a C l ( m M ) Figure 2. Optimization of NaCl concentration for motilin-induced binding of to human motilin receptor. Human motilin receptor (RBHMOT) (1 µg) was incubated in 5 mm HEPES (ph 7.4) 1 µm GDP, and 1 µm MgCl 2 with varying concentrations of NaCl in the presence or absence of 1 µm motilin for 3 min. (3 nm) was added for an additional 3 min prior to filtration, washing, and measurement at 615 nm on VICTOR 2 V. 4

5 Binding (% above Basal) 1 5 Binding (% above Basal) Saponin (µg/ml) Saponin (µg/ml) Figure 3. Optimization of saponin concentration for motilin-induced binding of to human motilin receptor. Human motilin receptor (RBHMOT) (1 µg membrane protein) was incubated in 5 mm HEPES (ph 7.4) 1 µm GDP, 1 mm MgCl 2, and 2 mm NaCl with varying concentrations of saponin in the presence or absence of 1 µm motilin for 3 min. (3 nm) was added for an additional 3 min prior to filtration, washing, and measurement at 615 nm on VICTOR 2 V Binding (% above Basal) Membrane protein (µg/well) Figure 4. Optimization of human motilin receptor protein concentration. Various amounts of human motilin receptor (RBHMOT) (-13 µg membrane protein) was incubated in 5 mm HEPES (ph 7.4) 1 µm GDP, 1 mm MgCl 2, 2 mm NaCl, and 1 mg/ml saponin in the presence or absence of 1 µm motilin for 3 min. (3 nm) was added for an additional 3 min prior to filtration, washing, and measurement at 615 nm on VICTOR 2 V. 5

6 5 % above % above basal basal % above basal A. [ 35 S]GTPγS C. C. [ 35 S]GTPγS [ 35 S]GTPγS Neurotensin (M) % above basal B. [ 35 S]GTPγS Motilin (M) Oxotremorine M (M) Oxotremorine M (M) Figure 5. Dose curves of agonist-induced binding of and [ 35 S]GTPγS to GPCRs. Membranes expressing: A) human neurotensin; B) human motilin, and C) human muscarinic M1 receptors were incubated with receptor specific agonists under previously optimized conditions (data not shown). Receptors were incubated with either 1 nm or.2 nm [ 35 S]GTPγS for 3 min prior to filtration, wash, and counting on VICTOR 2 V () or MicroBeta Jet ([ 35 S]GTPγS) (counts) FMLF peptide (M) Z EC 5 (nm).7 ± ± 7.3 Figure 6. Automation of assay using the PlateTrak liquid handling system. Six replicate plates of the assay were performed on the PlateTrak system using human FPR1 membranes (1 µg/well) and various concentrations of FMLF peptide as the model assay. Membrane receptors were incubated under the following buffer conditions: 5 mm Hepes, ph 7.4; 1 mm NaCl; 1 mm MgCl 2 ; 1 µm GDP with 3 nm for 3 min prior to filtration and washing using the PlateTrak system. Subsequent counting at 615 nm was performed on VICTOR 2 V. 6

7 7 RBHMOT RBHNT1 RBHM1 Receptor Motilin Neurotensin Muscarinic M1 Cell line HEK-293 HEK-293 CHO-K1 Agonist Motilin Neurotensin Oxotremorine M 5 mm HEPES 5 mm HEPES 5 mm HEPES 2 mm NaCl 1 mm NaCl 2 mm NaCl Optimized buffer 1 mm MgCl 2 1 mm MgCl 2 1 mm MgCl 2 1 µm GDP 1 µm GDP.1 µm GDP 1 µg/ml saponin 1 µg/ml saponin (1) EC 5 () 3-4 nm nm µm 8 nm 3.8 nm;.7 µm; (1) EC 5 ([ 35 S]GTPγS) 2.3 ±.9 nm (3) 7.9 µm (4) % above basal at EC 5 value (GTP- Eu) % above basal at EC 5 value ([ 35 S]GTPγS) Z (2) () Z (2) ([ 35 S]GTPγS) (1) EC 5 values have been measured at Turku and Boston sites (2) Z = 1-[(6*S.D. avg )/ (Avg. high -Avg. low )] (3) Hermans et al. (1997) Br. J. Pharm. 121: (4) DeLapp et al. (1999) J. Pharm. Exp. Ther. 289: Conclusions Easy to optimize, non-radioactive GTP binding assay Performs as well or better than traditional [ 35 S]GTPγS filtration assay in terms of Z values obtained for dose curves of agonist-induced binding Agonist-induced stimulation of binding above basal levels was at least equivalent to and up to more than two times greater than that achieved by the [ 35 S]GTPγS assay Benefits of TRF assay format including reduced compound fluorescence interference Can be automated as exemplified using PlateTrak liquid handling system with an estimated daily throughput of 2 plates PlateTrak and VICTOR are trademarks of. 7

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