Ligand Dependent Switch from RXR Homo- to RXR-NURR1 Heterodimerization

Retinoid X receptors (RXRs) play key roles in many physiological processes in both the periphery and central nervous system. In addition, RXRs form heterodimers with other nuclear receptors to exert their physiological effects. The nuclear receptor related 1 protein (NURR1) is particularly interesting because of its role in promoting differentiation and survival of dopamine neurons. However, only a small number of RXR-heterodimer selective modulators are available, with limited chemical diversity. This work describes the synthesis, biochemical evaluation, and structural elucidation of a novel series of RXR ligands with strongly biased interactions with RXRα–NURR1 heterodimers. Targeted modifications to the small molecule biaryl scaffold caused local RXRα side-chain disturbances and displacement of secondary structural elements upon ligand binding. This resulted in the repositioning of protein helices in the heterodimer interface of RXRα, alterations in homo- versus heterodimer formation, and modulation of activation function 2 (AF2). The data provide a rationale for the design of RXR ligands consisting of a highly conserved hydrophilic region, strongly contributing to the ligand affinity, and a variable hydrophobic region, which efficiently probes the effects of structural changes at the level of the ligand on co-regulator recruitment or the RXRα–NURR1 dimerization interface.

Role of Nuclear-Receptor-Related 1 in the Synergistic Neuroprotective Effect of Umbilical Cord Blood and Erythropoietin Combination Therapy in Hypoxic Ischemic Encephalopathy

Neonatal hypoxic–ischemic encephalopathy (HIE) results in neurological impairments; cell-based therapy has been suggested as a therapeutic avenue. Previous research has demonstrated the synergistically potentiated therapeutic efficacy of human umbilical cord blood (UCB) by combining recombinant human erythropoietin (EPO) treatment for recovery from HIE. However, its molecular mechanism is not entirely understood. In the present study, we analyzed the mechanisms underlying the effect of combination treatment with EPO and UCB by transcriptomic analysis, followed by gene enrichment analysis. Mouse HIE model of the neonate was prepared and randomly divided into five groups: sham, HIE, and UCB, EPO, and UCB+EPO treatments after HIE. A total of 376 genes were differentially expressed when |log2FC| ≥ 1-fold change expression values were considered to be differentially expressed between UCB+EPO and HIE. Further assessment through qRT-PCR and gene enrichment analysis confirmed the expression and correlation of its potential target, Nurr1, as an essential gene involved in the synergistic effect of the UCB+EPO combination. The results indicated the remarkable activation of Wnt/β-catenin signaling by reducing the infarct size by UCB+EPO treatment, accompanied by Nurr1 activity. In conclusion, these findings suggest that the regulation of Nurr1 through the Wnt/β-catenin pathway exerts a synergistic neuroprotective effect in UCB and EPO combination treatment.

A Nurr1 Agonist Derived from the Natural Ligand DHI Induces Neuroprotective Gene Expression

The dopamine metabolite 5,6-dihydroxyindole (DHI) has been discovered as a natural Nurr1 ligand with potential biological relevance and is an attractive lead for Nurr1 modulator development but exhibits chemical reactivity and weak potency. We have systematically explored the SAR of 5-chloroindole-6-carboxamide as a DHI mimetic scaffold and identified the first high-affinity (Kd 0.08-0.12 μM) ligands of the DHI binding site of Nurr1. An optimized Nurr1 agonist of this scaffold endowed with favorable physicochemical properties, high selectivity, and low toxicity emerges as a chemical tool to explore the biological impact of Nurr1 activation via the DHI binding site. Treatment of neuronal cells with this compound mediated enhanced expression of Nurr1-regulated neuroprotective genes like brain-derived neurotrophic factor (BDNF), supporting the great potential of Nurr1 activation in neurodegeneration.