Live cell single molecule imaging reveals concentration dependent changes to dynamic behavior of nuclear receptors RAR and RXR.

Abstract

Type 2 Nuclear Receptors (T2NRs) like Retinoic Acid Receptor (RAR) are believed to require heterodimerization with a common factor, the Retinoid X Receptor (RXR), to bind chromatin and regulate its target genes. Here, we deploy stroboscopic photo-activation SPT (spaSPT) to understand the molecular dynamics of the RAR/RXR system and infer different diffusion states acquired by RAR and RXR in the nucleoplasm. These states occupied by RAR and RXR presumably exist in dynamic equilibrium. Next, we ask in a complex heterodimer driven transcriptional regulatory system such as this, how is the dynamic equilibrium shifted when either of the heterodimeric partner is in excess. We find that in live cells, increasing the number of RAR molecules and not RXR molecules leads to altered chromatin binding of RAR. While increasing the number of molecules of RAR and RXR both alter the chromatin binding of RXR. Since pharmacological modulation of the RAR/RXR system is an important avenue for cancer treatments, this study constitutes a step toward understanding how perturbations of this system affect chromatin binding and possibly downstream transcriptional regulatory function.