The Human Orphan Nuclear Receptor Tailless (TLX, NR2E1) Is Druggable

Abstract

Nuclear receptors (NRs) are an important group of ligand-dependent transcriptional factors. Presently, no natural or synthetic ligand has been identified for a large group of orphan NRs. Small molecules to target these orphan NRs will provide unique resources for uncovering regulatory systems that impact human health and to modulate these pathways with drugs. The orphan NR tailless (TLX, NR2E1), a transcriptional repressor, is a major player in neurogenesis and Neural Stem Cell (NSC) derived brain tumors. No chemical probes that modulate TLX activity are available, and it is not clear whether TLX is druggable. To assess TLX ligand binding capacity, we created homology models of the TLX ligand binding domain (LBD). Results suggest that TLX belongs to an emerging class of NRs that lack LBD helices α1 and α2 and that it has potential to form a large open ligand binding pocket (LBP). Using a medium throughput screening strategy, we investigated direct binding of 20,000 compounds to purified human TLX protein and verified interactions with a secondary (orthogonal) assay. We then assessed effects of verified binders on TLX activity using luciferase assays. As a result, we report identification of three compounds (ccrp1, ccrp2 and ccrp3) that bind to recombinant TLX protein with affinities in the high nanomolar to low micromolar range and enhance TLX transcriptional repressive activity. We conclude that TLX is druggable and propose that our lead compounds could serve as scaffolds to derive more potent ligands. While our ligands potentiate TLX repressive activity, the question of whether it is possible to develop ligands to de-repress TLX activity remains open.

Structural basis for corepressor assembly by the orphan nuclear receptor TLX

“…In adult mouse brains, TLX functions as a transcriptional repressor that is critical for the regulation of neurogenesis and the progression of neural stem cell-dependent gliomagenesis (Sun et al 2007;Liu et al 2008Liu et al , 2010Zou et al 2012). Thus, TLX has been suggested as a therapeutic target for the treatment of human neurological disorders and brain tumors (Islam and Zhang 2014), and a recent study showed that TLX activity can be regulated by small molecules (Benod et al 2014). These exciting findings suggest that our crystal structures of TLX may also provide a rational template to develop drugs for the treatment of TLX-related diseases.…”

The Orphan Nuclear Receptor TLX/NR2E1 in Neural Stem Cells and Diseases

“…This study confirmed that the TLX LBD structure resembles other NR LBD conformations as a canonical helical sandwich, and also revealed notable differences, in that the TLX LBD lacks the first helices a1 and a2 and contains a unique 5-amino-acid insertion between helices a8 and a9. In contrast to the previous description of the full-length TLXforming monomer [21], Benod et al suggested that the recombinant TLX LBD can form a homodimer in solution [18]. More interestingly, three compounds were shown to be able to bind the recombinant TLX LBD, indicating that TLX functions can be manipulated using selected compounds [18].…”

“…In contrast to the previous description of the full-length TLXforming monomer [21], Benod et al suggested that the recombinant TLX LBD can form a homodimer in solution [18]. More interestingly, three compounds were shown to be able to bind the recombinant TLX LBD, indicating that TLX functions can be manipulated using selected compounds [18]. Most recently, Xu et al have reported the crystal structure of the corepressor assembly in TLX.…”

“…It is notable that TLX contains a ligand-dependent C-terminal activation function 2 (AF-2) domain but not a ligand-independent N-terminal activation function 1 (AF-1) domain, which is found in many NR family members [18]. In order to explore the ligand-binding potential of TLX, Benod et al recently developed three homology models for the TLX LBD in a transcriptionally-active conformation using a chimera LBD template based on the crystal structures of two human NRs, COUP-TFII (COUP transcription factor II) and RXRa (retinoid X receptor a) [18][19][20]. This study confirmed that the TLX LBD structure resembles other NR LBD conformations as a canonical helical sandwich, and also revealed notable differences, in that the TLX LBD lacks the first helices a1 and a2 and contains a unique 5-amino-acid insertion between helices a8 and a9.…”

The Tailless Root of Glioma: Cancer Stem Cells

“…potentially interacting with TLX may also inform its contribution to tumor growth. While no TLX inhibitors have been identified, the TLX mutant mouse is viable, albeit with developmental abnormalities in the brain, and TLX has been shown to be a druggable target (Benod et al, 2014). …”