1,536-Well Assay Development and Screening Using Whole Cell Displacement Binding and Laser Scanning Imaging

Cell-based assays in GPCR drug discovery

G protein-coupled receptors (GPCRs) transmit extracellular signals into the intracellular space, and play key roles in the physiological regulation of virtually every cell and tissue. Characteristic for the GPCR superfamily of cell surface receptors are their seven transmembrane-spanning alpha-helices, an extracellular N terminus and intracellular C-terminal tail. Besides transmission of extracellular signals, their activity is modulated by cellular signals in an auto- or transregulatory fashion. The molecular complexity of GPCRs and their regulated signaling networks triggered the interest in academic research groups to explore them further, and their drugability and role in pathophysiology triggers pharmaceutical research towards small molecular weight ligands and therapeutic antibodies. About 30% of marketed drugs target GPCRs, which underlines the importance of this target class. This review describes current and emerging cellular assays for the ligand discovery of GPCRs.

A whole-cell assay for the high throughput screening of calmodulin antagonists

Cell-based screening systems for pharmaceuticals are desired over molecular biosensing systems because of the information they provide on toxicity and bioavailability. However, the majority of sensing systems developed are molecular biosensing type screening systems and cannot be easily adapted to cell-based screening. In this study, we demonstrate that protein-based molecular sensing systems that employ a fluorescent protein as a signal transducer are amenable to cell-based sensing by expressing the protein molecular sensing system in the cell and employing these cells for screening of desired molecules. To achieve this, we expressed a molecular sensing system based on the fusion protein of calmodulin (CaM) and enhanced green fluorescent protein (EGFP) in bacterial cells, and utilized these cells for the screening of CaM antagonists. In the presence of Ca(2+), CaM undergoes a conformational change exposing a hydrophobic pocket that interacts with CaM-binding proteins, peptides, and drugs. This conformational change induced in CaM leads to a change in the microenvironment of EGFP, resulting in a change in its fluorescence intensity. The observed change in fluorescence intensity of EGFP can be correlated to the concentration of the analyte present in the sample. Dose-response curves for various tricyclic antidepressants were generated using cells containing CaM-EGFP fusion protein. Additionally, we demonstrate the versatility of our system for studying protein-protein interactions by using cells to study the binding of a peptide to CaM. The study showed that the CaM-EGFP fusion protein within the intact cells responds similarly to that of the isolated fusion protein, hence eliminating the need for any isolation and purification steps. We have demonstrated that this system can be used for the rapid screening of various CaM antagonists that are potential antipsychotic drugs.

High-Throughput Screening Using β-Lactamase Reporter-Gene Technology for Identification of Low-Molecular-Weight Antagonists of the Human Gonadotropin Releasing Hormone Receptor

G-protein coupled receptors (GPCRs) signal via G-proteins to intracellular second messengers. Assays that link transcription of a detectable reporter to promoters that are activated by such signaling cascades are highly sensitive and allow screening for compounds that either activate or inactivate a GPCR of interest. This study describes the development and performance of an antagonistic screen on the human gonadotropin releasing hormone receptor (GnRH-R). Compounds (245,000) were tested in a high-throughput screen using a Chinese hamster ovary cell line stably expressing the human GnRH-R and the Ca2+ sensitive reporter nuclear factor activated in T-cells/ activator protein-1-beta-lactamase. In total, 4,160 active compounds were identified. Colored and toxic compounds, as well as dust and compound aggregates, have been depicted as artifacts. To deselect non-target hits, several follow-up assays, including luminescent and fluorescent Ca2+ mobilization assays and radioligand binding, were developed for the GnRH-R. These assays were validated using peptide and low-molecular-weight GnRH-R reference compounds before hits from screening were also profiled in these assays. For several reference compounds the use of different assay technologies resulted in a poor correlation of potency values. In conclusion, beta-lactamase as a primary high-throughput screening assay is a powerful complementation to other screening technologies. The beta-lactamase technology has several advantages, including lack of cell lysis and ratiometric read-out, which augments assay robustness. Based on technology comparison, it is not adequate to assume that the same hits would be found regardless of which assay technology is used.

Miniaturized, ultra-high throughput screening of tyrosine kinases using homogeneous, competitive fluorescence immunoassays

Assay miniaturization and the implementation of high-density 1,536-microwell screening increase the speed and efficiency of screening and lead discovery. To serve this need, a platform of miniaturizable assay technologies has been assembled for specific biological targets. This platform will enable initiation and completion of uHTS screens in a straightforward and expeditious manner. Although faster primary screening does contribute to a reduction in timelines, the process of assay development can become a bottleneck. Assay technologies that do not require the use of target-specific reagents can reduce the time necessary for assay development. Assays that measure inhibition of tyrosine kinases can be configured in a competitive format where only the enzyme itself is specific to the assay. In this context, several technologies, including time-resolved fluorometry (also known as DELFIA), time-resolved fluorescence resonance energy transfer (also known as LANCE( trade mark )), fluorescence polarization, enzyme fragmentation complementation assay, and confocal laser scanning imaging, were examined. Quality parameters such as assay reproducibility, signal:background ratio, Z factor, and assay sensitivity were compared. Additionally, the relative merits of each of these technologies are assessed in terms of assay miniaturization, ease of development, ultimate screening capability, efficiency, and cost.