AR-A014418 (GSK 3β inhibitor VIII) is an ATP-competitive, and selective GSK3β inhibitor.

GSK3 Scintillation Proximity Assay: The competition experiments are carried out in duplicate with 10 concentrations of the inhibitor in clear-bottomed microtiter plates. The biotinylated peptide substrate, biotin-AAEELDSRAGS(PO3H2)PQL, is added at a final concentration of 2 μM in an assay buffer containing 6 milliunits of recombinant human GSK3 (equal mix of both α and β), 12 mM MOPS, pH 7.0, 0.3 mM EDTA, 0.01% β-mercaptoethanol, 0.004% Brij 35, 0.5% glycerol, and 0.5 μg of bovine serum albumin/25 μl and preincubated for 10-15 min. The reaction is initiated by the addition of 0.04 μCi of [γ-33P]ATP and unlabeled ATP in 50 mM Mg(Ac)2 to a final concentration of 1 μM ATP and assay volume of 25 μl. Blank controls without peptide substrate are used. After incubation for 20 min at room temperature, each reaction is terminated by the addition of 25 μl of stop solution containing 5 mM EDTA, 50 μM ATP, 0.1% Triton X-100, and 0.25 mg of streptavidin-coated SPA beads corresponding to ～35 pmol of binding capacity. After 6 h the radioactivity is determined in a liquid scintillation counter. Inhibition curves are analyzed by non-linear regression using GraphPad Prism.

Cell viability is assessed by calcein/propidium iodide uptake. Calcein AM is taken up and cleaved by esterases present within living cells, yielding yellowish-green fluorescence, whereas PI is only taken up by dead cells, which become orange-red fluorescent. In brief, N2A cells are cultured for 2 days in vitro and then treated with 50 μM LY-294002 in the presence of AR-A014418 or vehicle (DMSO) for 24 h. Subsequently, N2A cells are incubated for 30 min with 2 μM PI and 1 μM calcein-AM. The cultures are then rinsed three times with Hanks' buffered saline solution containing 2 mM CaCl2, and the cells are visualized by fluorescence microscopy using a Zeiss Axiovert 135 microscope. Three fields (selected at random) are analyzed per well (∼300 cells/field) in at least three different experiments. Cell death is expressed as percentage of PI-positive cells from the total number of cells. In every experiment, specific cell death is obtained after subtracting the number of dead cells present in vehicle-treated cultures. (Only for Reference)