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Assay Method Information

Assay Name:  Calcium Flux Assay
Description:  This assay was used to test compounds for their ability to inhibit TARP γ8 dependent AMPA receptor activity. The AMPA receptor is a non-selective cation channel activated by glutamate. Ionotropic glutamate receptors normally desensitize too rapidly to allow detectable calcium influx in a FLIPR assay (Strange et al. (2006). Functional characterisation of homomeric ionotropic glutamate receptors GluR1-GluR6 in a fluorescence-based high throughput screening assay. Comb Chem High Throughput Screen 9(2): 147-158). But, this desensitization is incomplete, and a substantial steady-state current remains in the sustained presence of glutamate (Cho et al. (2007). Two families of TARP isoforms that have distinct effects on the kinetic properties of AMPA receptors and synaptic currents. Neuron 55(6): 890-904).An in vitro assay was used to determine the potency of test compounds as inhibitors of the glutamate response of the channel formed by GluA10-γ8. To ensure a 1:1 stoichiometry of GluA1o and γ8 subunits in the expressed channel, a fusion of the cDNAs for GRIA1o and CACNG8 was used. Following Shi et al (2009) The stoichiometry of AMPA receptors and TARPs varies by neuronal cell type. Neuron 62(5): 633-640), the C-terminus of the cDNA for GRIA1o was fused to the N-terminus of the cDNA for γ8. The linker sequence was QQQQQQQQQQEFAT. Channels expressed with this construct appear to have similar properties to channels formed by co-expression of GRIA1o with an excess of CACNG8 (Shi et al. 2009). A clonal cell line in HEK293 cells stably expressing this construct, with a geneticin selection marker, was generated for use in this assay.Cell expressing the GRIA1o-CACNG8 fusion construct were grown in a monolayer in 96- or 384-well microtiter plates. They were washed with assay buffer (135 mM NaCl, 4 mM KCl, 3 mM CaCl2, 1 mM MgCl2, 5 mM glucose, 10 mM HEPES, pH 7.4, 300 mOs) using a Biotek EL405 plate washer. The cells were then loaded with a calcium-sensitive dye (Calcium-5 or Calcium-6, Molecular Devices) and the test compounds at a range of concentrations. Calcium flux following the addition of 15 μM glutamate was monitored using a Molecular Devices FLIPR Tetra.The fluorescence in each well was normalized to the fluorescence of negative and positive control wells. The negative control wells had no added compounds, and the positive control wells had been incubated with 10 μM CP465022 (a non-subtype-selective AMPA receptor antagonist) (Lazzaro et al. (2002). ┐Functional characterization of CP-465,022, a selective, noncompetitive AMPA receptor antagonist.┐ Neuropharmacology 42(2): 143-153). The responses to glutamate as functions of the test compound concentrations were fitted to a four-parameter logistic function. The fitted parameter corresponding to the midpoint was taken to be the potency of inhibition of the compound. The data in Table 4 below illustrates the observed potency for the compounds described herein. pIC50 refers to the negative log of the IC50 in molar.
Affinity data for this assay
 

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Last update November 1, 2007
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