Specificity of receptor-ligand interactions and their effect on dimerisation as observed by electrospray mass spectrometry: bile acids form stable adducts to the RXRalpha.

Electrospray (ES) mass spectrometry data is presented showing that agonist binding to the nuclear receptor (NR), retinoid X receptor alpha (RXRalpha), is competitive. The competitive nature of agonist binding can be used to discriminate between the specific and non-specific binding of small lipophilic molecules to NRs. Further, data is presented which show that high-affinity ligand binding to the RXRalpha ligand-binding domain (LBD) stabilises the domain homodimer. The results indicate that homodimerisation, a functional property of the receptor associated with the binding of agonist ligands, could be used to discriminate between specific and non-specific binding events. Additionally, we report on the remarkable stability of the gas-phase complex between the RXRalpha LBD protein and endogenous bile acids. Protein-bile acid interactions in the gas phase were found to be surprisingly strong, withstanding 'in-source' fragmentation in the ES interface, and, in the case of taurocholic acid (TCA) and lithocholic acid-3-sulphate (LCA-3-sulphate), collision-induced dissociation within the collision cell of a tandem mass spectrometer. Bile acids were found to be inactive towards RXRalpha in transfection assays, and have not been reported to be ligands for the RXRalpha, although lithocholic acid (LCA) has been found to be a competitor in the photoaffinity labelling of RXRbeta with 9-cis-retinoic acid (9-cis-RA). The observation of strong RXRalpha-bile acid non-covalent complexes in ES mass spectrometry highlight the danger of extrapolating gas-phase binding data to the solution phase and further to a possible biological activity, particularly when surface-active compounds such as bile acids are involved. The introduction of a competitive ligand-binding experiment can alleviate this problem and allow the differentiation between specific and non-specific binding.

Polyunsaturated fatty acids including docosahexaenoic and arachidonic acid bind to the retinoid X receptor alpha ligand-binding domain.

Nuclear receptors (NRs) constitute a large and highly conserved family of ligand-activated transcription factors that regulate diverse biological processes such as development, metabolism, and reproduction. As such, NRs have become important drug targets, and the identification of novel NR ligands is a subject of much interest. The retinoid X receptor (RXR) belongs to a subfamily of NRs that bind vitamin A metabolites (i.e. retinoids), including 9-cis-retinoic acid (9-cis-RA). However, although 9-cis-RA has been described as the natural ligand for RXR, its endogenous occurrence has been difficult to confirm. Recently, evidence was provided for the existence of a different natural RXR ligand in mouse brain, the highly enriched polyunsaturated fatty acid (PUFA) docosahexaenoic acid (DHA) (Mata de Urquiza et al. (2000) Science 290, 2140-2144). However, the results suggested that supra-physiological levels of DHA were required for efficient RXR activation. Using a refined method for ligand addition to transfected cells, the current study shows that DHA is a more potent RXR ligand than previously observed, inducing robust RXR activation already at low micromolar concentrations. Furthermore, it is shown that other naturally occurring PUFAs can activate RXR with similar efficiency as DHA. In additional experiments, the binding of fatty acid ligands to RXRalpha is directly demonstrated by electrospray mass spectrometry of the noncovalent complex between the RXR ligand-binding domain (LBD) and its ligands. Data is presented that shows the noncovalent interaction between the RXR LBD and a number of PUFAs including DHA and arachidonic acid, corroborating the results in transfected cells. Taken together, these results show that RXR binds PUFAs in solution and that these compounds induce receptor activation, suggesting that RXR could function as a fatty acid receptor in vivo.

Nurr1-RXR heterodimers mediate RXR ligand-induced signaling in neuronal cells.

The retinoid X receptor (RXR) is essential as a common heterodimerization partner of several nuclear receptors (NRs). However, its function as a bona fide receptor for endogenous ligands has remained poorly understood. Such a role would depend on the existence of RXR activating ligands in vivo and on the ability of such ligands to influence relevant biological functions. Here we demonstrate the presence of endogenous RXR ligands in the embryonic central nervous system (CNS) and show that they can activate heterodimers formed between RXR and the orphan NR Nurr1 in vivo. Moreover, RXR ligands increase the number of surviving dopaminergic cells and other neurons in a process mediated by Nurr1-RXR heterodimers. These results provide evidence for a role of Nurr1 as a ligand-independent partner of RXR in its function as a bona fide ligand-activated NR. Finally, our findings identify RXR-Nurr1 heterodimers as a potential target in the treatment of neurodegenerative disease.

A triple mutant of the Drosophila ERR confers ligand-induced suppression of activity.

The steroid hormone (NR3) subfamily of nuclear receptors was until recently believed to be restricted to deuterostomes. However, a novel nuclear receptor belonging to the NR3 subfamily was recently identified in the Drosophila melanogaster genome, indicating the existence of an ancestor before the evolutionary split of deuterostomes and protostomes. This receptor, termed the Drosophila estrogen-related receptor (dERR), most closely resembles the human and mouse estrogen-related receptors (ERRs) in both the DNA binding domain (DBD) (approximately 85% identical) and the ligand binding domain (LBD) (approximately 35% identical). Here we describe the functional analysis and rational design of ligand responsive dERR mutants created by protein engineering of the LBD. On the basis of homology modeling, three amino acid residues in the LBD were identified and mutated to enable ligand-dependent suppression of transcriptional activity. Our results show that the Y295A/T333I/Y365L triple mutant is significantly suppressed by the known ERR inverse agonists 4-hydroxytamoxifen (OHT) and diethylstilbestrol (DES), in comparison to the wild-type dERR receptor, which was inefficiently suppressed by these substances. The coactivator mGRIP-1 (mouse glucocorticoid receptor interacting protein 1) was shown to significantly increase the activity of the triple mutant in transfection experiments, and the addition of OHT resulted in an efficient suppression of the activity. Accordingly, the ability to functionally interact with a coactivator is still maintained by the Y295A/T333I/Y365L mutant. These findings demonstrate the potential of using rational design and engineering of the LBD to study the function of a nuclear receptor lacking identified ligands.