Monitoring Inositol-Specific Phospholipase C Activity Using a Phospholipid FlashPlate(R)

Current status of HTRF(®) technology in kinase assays

BACKGROUND

Homogeneous time-resolved fluorescence (HTRF) is a fluorescence resonance energy transfer-based technology used to measure bimolecular interactions. It has been applied successfully to kinase assays and has become an important tool in kinase drug discovery.

OBJECTIVE

This article reviews the current status of HTRF technology in biochemical and cellular kinase assays.

METHODS

Recent literature and meeting reports on HTRF kinase assays are reviewed, and their principles, advantages and drawbacks, current status and the potential applications in kinase drug discovery are discussed.

RESULTS/CONCLUSION

HTRF kinase assays are homogeneous, robust, sensitive, easy to miniaturize and high-throughput. This assay format is versatile, as both peptide and protein substrates can be used, and high ATP concentrations are tolerated, which enables the assay to be performed under conditions mimicking the physiological environment. HTRF kinase assays have been applied to both high-throughput screening and compound mechanistic studies. Besides protein kinases, the technology has now been expanded into the lipid kinase family. Furthermore, the utility of HTRF technology in cellular assays is emerging. HTRF kinase assays are a great addition to the toolbox for kinase drug discovery.

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

PLCgamma1 is essential for early events in integrin signalling required for cell motility

Cell motility is a critical event in many processes and is underlined by complex signalling interactions. Although many components have been implicated in different forms of cell migration, identification of early key mediators of these events has proved difficult. One potential signalling intermediate, PLCgamma1, has previously been implicated in growth-factor-mediated chemotaxis but its position and roles in more-complex motility events remain poorly understood. This study links PLCgamma1 to early, integrin-regulated changes leading to cell motility. The key role of PLCgamma1 was supported by findings that specific depletion of PLCgamma1 by small interfering (si)RNA, or by pharmacological inhibition, or the absence of this isoform in PLCgamma1(-/-) cells resulted in the failure to form cell protrusions and undergo cell spreading and elongation in response to integrin engagement. This integrin-PLCgamma1 pathway was shown to underlie motility processes involved in morphogenesis of endothelial cells on basement membranes and invasion of cancer cells into such three-dimensional matrices. By combining cellular and biochemical approaches, we have further characterized this signalling pathway. Upstream of PLCgamma1 activity, beta1 integrin and Src kinase are demonstrated to be essential for phosphorylation of PLCgamma1, formation of protein complexes and accumulation of intracellular calcium. Cancer cell invasion and the early morphological changes associated with cell motility were abolished by inhibition of beta1 integrin or Src. Our findings establish PLCgamma1 as a key player in integrin-mediated cell motility processes and identify other critical components of the signalling pathway involved in establishing a motile phenotype. This suggests a more general role for PLCgamma1 in cell motility, functioning as a mediator of both growth factor and integrin-initiated signals.

Biochemical clustering of monomeric GTPases of the Ras superfamily

To date phylogeny has been used to compare entire families of proteins based on their nucleotide or amino acid sequence. Here we developed a novel analytical platform allowing a systematic comparison of protein families based on their biochemical properties. This approach was validated on the Rho subfamily of GTPases. We used two high throughput methods, referred to as AlphaScreen and FlashPlate, to measure nucleotide binding capacity, exchange, and hydrolysis activities of small monomeric GTPases. These two technologies have the characteristics to be very sensitive and to allow homogenous and high throughput assays. To analyze and integrate the data obtained, we developed an algorithm that allows the classification of GTPases according to their enzymatic activities. Integration and hierarchical clustering of these results revealed unexpected features of the small Rho GTPases when compared with primary sequence-based trees. Hence we propose a novel phylobiochemical classification of the Ras superfamily of GTPases.

Development of a high-throughput assay for two inositol-specific phospholipase Cs using a scintillation proximity format

Inositol-specific PLCs comprise a family of enzymes that utilize phosphoinositide substrates, e.g., PIP(2), to generate intracellular second messengers for the regulation of cellular responses. In the past, monitoring this reaction has been difficult due to the need for radiolabeled substrates, separation of the reaction products by organic-phase extraction, and finally radiometric measurements of the segregated products. In this report, we have studied the enzymatic characteristics of two novel PLCs that were derived from functional genomic analyses using a phospholipid-modified solid scintillating support. This method allows for the hydrophobic capture of the [(3)H]phosphoinositide substrate on a well defined scintillation surface and the homogenous measurement of the enzymatic hydrolysis of the substrate by proximity effects. Our results show that the assay format is robust and well suited for this class of lipid-metabolizing enzymes.

A versatile high-throughput screen for inhibitors of lipid kinase activity: development of an immobilized phospholipid plate assay for phosphoinositide 3-kinase gamma

The family of phosphoinositide 3-kinases (PI3K) regulates fundamental cellular responses such as proliferation, apoptosis, motility, and adhesion. In particular, the PI3K gamma isoform plays a critical role in the control of cell migration. Despite the attractiveness of PI3-kinases as drug targets, drug discovery efforts have been hampered by the lack of appropriate lipid kinase assay formats suitable for high-throughput screening. The authors report the development of a simple and robust 384-well plate assay that is based on(33) P-phosphate transfer from radiolabeled [gamma(33) P]ATP to phosphatidylinositol immobilized on Maxisorp plates. The established assay format for PI3K gamma was easily adapted to the automated screening platform and was successfully employed for high-throughput screening. Enzymatic and inhibition characteristics of recombinant human PI3K gamma determined with the plate assay are in very good agreement with previously reported values determined in other assay formats. Maximal catalytic activity of PI3K gamma was observed at pH 7.0. The apparent K(m) value for ATP using a 1:1 mixture of phosphatidylinositol and phosphatidylserine was determined to be 7.3 microM (6.0-8.6 microM, 95% confidence interval [CI]). IC(50) values for known PI3-kinase inhibitors were determined to be 1.45 nM (1.17-1.80 nM, 95% CI) for wortmannin and estimated from partial inhibition data to be 1400, 2830, and 21,400 nM for quercetin, LY294002, and staurosporine, respectively. This novel assay approach allows for screening of inhibitors of lipid kinases in high-throughput mode and thereby may facilitate the identification of novel inhibitory structures for drug development.

Separation of phospholipids in microfluidic chip device: application to high-throughput screening assays for lipid-modifying enzymes

Phospholipid molecules such as ceramide and phosphoinositides play crucial roles in signal transduction pathways. Lipid-modifying enzymes including sphingomyelinase and phosphoinositide kinases regulate the generation and degradation of these lipid-signaling molecules and are important therapeutic targets in drug discovery. We now report a sensitive and convenient method to separate these lipids using microfluidic chip-based technology. The method takes advantage of the high-separation power of the microchips that separate lipids based on micellar electrokinetic capillary chromatography (MEKC) and the high sensitivity of fluorescence detection. We further exploited the method to develop a homogenous assay to monitor activities of lipid-modifying enzymes. The assay format consists of two steps: an on-plate enzymatic reaction using fluorescently labeled substrates followed by an on-chip MEKC separation of the reaction products from the substrates. The utility of the assay format for high-throughput screening (HTS) is demonstrated using phospholipase A(2) on the Caliper 250 HTS system: throughput of 80min per 384-well plate can be achieved with unattended running time of 5.4h. This enabling technology for assaying lipid-modifying enzymes is ideal for HTS because it avoids the use of radioactive substrates and complicated separation/washing steps and detects both substrate and product simultaneously.

Homogeneous cell-based fluorescence polarization assay for the direct detection of cAMP

A fluorescence polarization-based functional assay for cyclic AMP (cAMP) production in cells has been proven effective for the detection of agonist-stimulated cAMP production in a HEK 293 recombinant cell line expressing the corticotropin-releasing factor subtype 2alpha (CRF2alpha) receptor. Assays were completed in a single well of a 384-well microplate with no transfer, separation, or wash steps incurred. The assay performance is excellent for adaptation to the high throughput screening environment in terms of speed of analysis, magnitude of displaced signal, precision, and detection limits for cAMP quantitation. Relative potencies of agonists and antagonists are maintained with respect to radiometric assays. The assay withstands up to 5% DMSO and up to 10 microM concentrations of highly colored compound. These attributes suggest that accurate assessment of drug binding can be measured using this assay.