Cytotoxicity Dose Response with AM2.2-beta2AR cells for PowderSet01
Assay Provider: Jonathan Jarvik, Carnegie Mellon University Screening Center/ PI: UNMCMD/ Larry Sklar ..more
BioActive Compounds: 2
University of New Mexico Assay Overview:
Assay Support: R03 MH093192-01
Project Title: HTS for Non-Canonical Ligands for Beta 2 Adrenergic Receptor Internalization
Assay Provider: Jonathan Jarvik, Carnegie Mellon University Screening Center/ PI: UNMCMD/ Larry Sklar
Lead Biologist: Yang Wu
Chemistry Center/ PI: Vanderbilt Specialty Chemistry Center/Craig Lindsley Chemistry Center Lead: Shaun Stauffer
Assay Implementation: Yang Wu, Phillip Tapia
Assay Background and Significance:
G protein-coupled receptors represent the largest family of proteins in the human genome with an estimated number of approximately 800. Because of their central involvement in almost every aspect of human physiology, they also represent the largest target for medical intervention [Lin and Civelli, Annu Med 36 (2004), 204-14]. Today, GPCRs represent the target of approximately 30-40% of all drugs on the market. Indeed, of the 50 top-selling drugs in the United States in 2007, 18 target GPCRs, with combined sales of approximately 25 billion dollars.
Of about 800 GPCRs, 500 are chemosensory, representing the chemokine/chemoattractant GPCRs, and the olfactory and gustatory GPCRs. Although the former have been thoroughly characterized for the most part, members of the latter, particularly the olfactory receptors which may include the important category of pheromones, have had only a small number of ligands identified. The remaining GPCRs, approximately 360, constitute the transmitter GPCRs. Of these, approximately 100 receptors still have no known ligand. Such receptors with no known physiological ligand are referred to as orphan receptors and arose from cloning strategies based on limited homology within almost all GPCRs (predominantly during the 1990's) as well as the sequencing of the human genome (in 2000) [Chung et al, Br J Pharmacol 153 Suppl 1 (2008), S339-46].
The search for endogenous ligands for orphan GPCRs has been challenging. This process has given rise to the field of reverse pharmacology, which uses orphan GPCRs to identify novel ligands, which together often lead to the characterization of new physiological paradigms. Over the last 20 years, this approach has led to the deorphanization of about 300 GPCRs. Many of the ligands were already known and their biology characterized, but their receptor was unknown. But in some of these instances, this process also led to the identification of novel transmitters [Civelli et al., Pharmacol Ther 110 (2006), 525-32].
During the 1990's, approximately 10 GPCRs were deorphanized per year. However, very few have been deorphanized since 2004. In addition, no novel transmitters have been identified since that time [Chung et al, Br J Pharmacol 153 Suppl 1 (2008), S339-46]. Given these facts, the question arises as to whether the remaining orphan GPCRs will be readily deorphanized. One major issue is that the pool of known transmitters for which no receptor is known has been essentially depleted. Since all the possible transmitters have now been matched to GPCRs, the current orphan GPCRs can only bind unknown ligands, for which the identification is also slow and resource intense. This is particularly true given that the concentrations of many transmitters in vivo is exceedingly low, making their identification difficult. It is also possible that the expression of some ligands may be developmentally or environmentally regulated.
Almost all current reverse pharmacological/screening approaches rely on monitoring second messenger levels such as calcium mobilization, cAMP production and transcriptional activation. Thus, successful screening requires knowledge of the pathway for a given receptor, in particular the G protein to which the receptor couples. As the number of heterotrimeric G protein combinations is large, this is not a trivial task.
An alternate approach to measuring signal transduction is the monitoring receptor internalization. Virtually all known GPCRs undergo activation-dependent internalization as a mechanism to reduce cell surface receptor numbers. Internalization does not require G protein coupling. Instead, the activity of one of four G protein receptor kinases (GRKs) is required. In most instances, the binding of an accessory protein, arrestin, is also required. One screening approach that has been developed is the recruitment of GFP-tagged arrestin to either the plasma membrane or intracellular endosomes.
The beta-Arrestin clustering assay, developed by Norak Technologies, requires high resolution imaging, well spread, adherent cells, and extensive image analysis to determine the response to a treatment. Other methods, such a receptor desensitization measurements on non-permeabilized cells rely on measurement of subtle changes in intensity. As described below, the CMU TCNP is developing sensors for GPCR responses that are readily compatible with HTS flow cytometry and multiplexing when the GPCRs are expressed in suspension cell lines.
This assay will be used to determine if a compound is causing a decrease in signal by killing the cell instead of actually inhibiting beta2AR internalization.
CellTiter-Glo, a luminescent cell viability assay kit from Promega (Madison, WI), will be used according to the manufacturer's instruction. Briefly, the cell cultures of AM2.2-beta2AR cells are seeded in complete medium at twelve different cell densities in 96-well white polypropylene opaque plates (50 microL/well per 384 well) (Corning, Corning, NY). At the time of passage, 90microL of cell suspension (10^5 cells/milliL) are added into the plates. After stabilization for 2h, test compounds are added to the wells at 10 microL per well to a final concentration range of 380 nM to 100 microM. Vehicle control wells contain 0.01% DMSO alone. Following treatment, cells are incubated at 37 degrees C and 5% CO2 for 18 hours. At the end of the respective time point, 1x CellTiter-Glo is added to each well (10 microL/well per 384 well). Plates are read after 30 minutes. Luminescence intensity (LI) is collected using a Wallac 1420 plate reader (PerkinElmer, Norwalk, CT).
Background luminescence were subtracted from all readings and then luminescence values for the entire concentration range of a test compound were fitted by Prism(R) software (GraphPad Software, Inc., San Diego, CA) using nonlinear least-squares regression in a sigmoidal dose response model with variable slope, also known as the four parameter logistic equation. Curve fit statistics were used to determine the following parameters of the model: EC50, microM - concentration of added test compound competitor that inhibited fluorescent ligand binding by 50 percent; LOGEC50 - the logarithm of EC50; TOP - the response value at the top plateau; BOTTOM - the response value at the bottom plateau; HILLSLOPE - the slope factor, or the Hill coefficient; STD_LOGEC50, STD_TOP, STD_BOTTOM, STD_HILLSLOPE - standard errors of LOGEC50, TOP, BOTTOM, and HILLSLOPE ; EC50_95CI_LOW, EC50_95CI_HIGH - the low and high boundaries of the 95% confidence interval of the EC50 estimate, RSQR - the correlation coefficient (r squared) indicative of goodness-of-fit.
Compounds with percent viability at the highest concentration is less than 50% are labeled active and the PubChem_Score is calculated based on EC50 of cytotoxicity by the following equation:
PubChem Score = 100 * (1 - EC50/30 microM)
Keywords: beta2AR, UNMCMD
Categorized Comment - additional comments and annotations
Assay Type: Toxicity
Assay Cell Type: AM2.2-beta2AR
Assay Format: Cell-based
Assay Type: Functional
* Activity Concentration. ** Test Concentration.
Data Table (Concise)