In Pursuit of Synthetic Modulators for the Orphan Retina-Specific Nuclear Receptor NR2E3

Abstract

NR2E3 is an orphan nuclear receptor expressed exclusively in photoreceptor cells of the retina. NR2E3-specific modulators may prolong photoreceptor survival in patients with dry age-related macular degeneration and other forms of retinal degeneration. To definitively establish NR2E3 as a photoreceptor protection target, identification of small-molecule NR2E3 modulators and their testing in animal models of retinal degeneration are required. Development of the high-throughput screen (HTS)-compatible screen for small-molecule NR2E3 modulators is the first step toward this goal. Purification protocol for isolation of the functionally competent soluble NR2E3 protein after its expression in the insect Sf9 cells was developed. The time-resolved fluorescence energy-transfer (TR-FRET) assay assessing agonist-sensitive interaction between apo-NR2E3 and transcriptional corepressor RetCOR was used for characterization of the previously reported putative NR2E3 agonist, Compound 11a, and to conduct the HTS for novel small-molecule NR2E3 modulators (direct and inverse agonists). A counterscreen TR-FRET assay that measures the affect of test compounds on PPARγ interaction with corepressor NCOR was used for assessing the specificity of compounds identified in the HTS. We developed the cell-free TR-FRET assay for small-molecule NR2E3 modulators, which is based on agonist-induced disruption of the interaction between GST-tagged apo-NR2E3 and MBP-tagged fragment of transcriptional corepressor RetCOR. Compound 11a, a putative NR2E3 agonist, did not affect the NR2E3-RetCOR interaction, as was established by its titration in the developed assay. The assay was miniaturized for an ultralow-volume 1,536-well format and automated into 3 simple pipetting steps. Consistent with excellent assay performance, the test runs established a Z'-score within the 0.6-0.8 range. Analysis of the mid-size National Institutes of Health collection of 315,001 structurally diverse drug-like compounds confirmed excellent assay performance, but did not reveal NR2E3-specific agonists or inverse agonists. A robust and reliable TR-FRET assay for small-molecule NR2E3-specific modulators suitable for the analysis of million compound-strong HTS libraries was developed. A previously described putative NR2E3 agonist, Compound 11a, is unlikely to represent a direct NR2E3 agonist. Application of the developed assay for screening of a more abundant and diverse compound collection be required for identification of synthetic NR2E3 ligands.