The retinoid X receptors and their ligands

The molting process in arthropods is regulated by steroid hormones acting via nuclear receptor proteins. The most common molting hormone is the ecdysteroid, 20-hydroxyecdysone. The receptors of 20-hydroxyecdysone have also been identified in many arthropod species, and the amino acid sequences determined. The functional molting hormone receptors consist of two members of the nuclear receptor superfamily, namely the ecdysone receptor and the ultraspiracle, although the ecdysone receptor may be functional, in some instances, without the ultraspiracle. Generally, the ecdysone receptor/ultraspiracle heterodimer binds to a number of ecdysone response elements, sequence motifs that reside in the promoter of various ecdysteroid-responsive genes. In the ensuing transcriptional induction, the ecdysone receptor/ultraspiracle complex binds to 20-hydroxyecdysone or to a cognate ligand that, in turn, leads to the release of a corepressor and the recruitment of coactivators. 3D structures of the ligand-binding domains of the ecdysone receptor and the ultraspiracle have been solved for a few insect species. Ecdysone agonists bind to ecdysone receptors specifically, and ligand-ecdysone receptor binding is enhanced in the presence of the ultraspiracle in insects. The basic mode of ecdysteroid receptor action is highly conserved, but substantial functional differences exist among the receptors of individual species. Even though the transcriptional effects are apparently similar for ecdysteroids and nonsteroidal compounds such as diacylhydrazines, the binding shapes are different between them. The compounds having the strongest binding affinity to receptors ordinarily have strong molting hormone activity. The ability of the ecdysone receptor/ultraspiracle complex to manifest the effects of small lipophilic agonists has led to their use as gene switches for medical and agricultural applications.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1742-4658.2009.07347.x

Arthropod nuclear receptors and their role in molting.

Zooecdysteroids (arthropod steroid hormones) regulate the development of arthropods and probably many other invertebrates. Phytoecdysteroids are analogues occurring in a wide range of plant species, where they contribute to the deterrence of phytophagous invertebrates. The purpose of this short review is to summarise findings on the occurrence, metabolism and pharmacological effects of ecdysteroids in mammalian systems and to draw attention to their potential applications, particularly in gene-switch technology, where ecdysteroid analogues (steroidal and non-steroidal) can be used as effective and potent elicitors.

/pdf/effects-and-applications-of-arthropod-steroid-hormones-3s5z3wf0t4.pdf

Effects and applications of arthropod steroid hormones (ecdysteroids) in mammals.

In 1974, Ashburner and colleagues postulated a model to explain the control of the puffing sequence on Drosophila polytene chromosomes initiated by the molting hormone 20-hydroxyecdysone. This model inspired a generation of molecular biologists to clone and characterize elements of the model, thereby providing insights into the control of gene networks by steroids, diatomic gases, and other small molecules. It led to the first cloning of the EcR subunit of the heterodimeric EcR-USP ecdysone receptor. X-ray diffraction studies of the ligand-binding domain of the receptor are elucidating the specificity of receptor-ecdysteroid interactions, the selectivity of some environmentally friendly insecticides, the evolution of the EcR-USP heterodimer, and indeed Ashburner's classical biochemical evidence for the central role of the ecdysone receptor in his model.

Ecdysone Receptors: From the Ashburner Model to Structural Biology*

Retinoid X receptor (RXR) and Ultraspiracle (USP) play a central role as ubiquitous heterodimerization partners of many nuclear receptors. While it has long been accepted that a wide range of ligands can activate vertebrate/mollusc RXRs, the existence and necessity of specific endogenous ligands activating RXR-USP in vivo is still matter of intense debate. Here we report the existence of a novel type of RXR-USP with a ligand-independent functional conformation. Our studies involved Tribolium USP (TcUSP) as representative of most arthropod RXR-USPs, with high sequence homology to vertebrate/mollusc RXRs. The crystal structure of the ligand-binding domain of TcUSP was solved in the context of the functional heterodimer with the ecdysone receptor (EcR). While EcR exhibits a canonical ligand-bound conformation, USP adopts an original apo structure. Our functional data demonstrate that TcUSP is a constitutively silent partner of EcR, and that none of the RXR ligands can bind and activate TcUSP. These findings together with a phylogenetic analysis suggest that RXR-USPs have undergone remarkable functional shifts during evolution and give insight into receptor–ligand binding evolution and dynamics.

https://www.embopress.org/doi/pdf/10.1038/sj.emboj.7601810

Structural and functional characterization of a novel type of ligand-independent RXR-USP receptor.

Purification and characterization of recombinant ligand-binding domains from the ecdysone receptors of four pest insects.

The present invention relates to structural studies of the functional insect ecdysone receptor. More particularly, the invention relates to the crystal structure of the whitefly ecdysone receptor ligand-binding domain, specifically that of Bemisia tabaci, and uses of the crystal and related structural information to select and screen for compounds that interact with the receptor.

Ecdysone receptor ligand-binding domain structure

The X-ray Structure of a Hemipteran Ecdysone Receptor Ligand-binding Domain COMPARISON WITH A LEPIDOPTERAN ECDYSONE RECEPTOR LIGAND-BINDING DOMAIN AND IMPLICATIONS FOR INSECTICIDE DESIGN

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Purification and characterization of recombinant ligand-binding domains from the ecdysone receptors of four pest insects.

Ecdysone receptor ligand-binding domain structure

The X-ray Structure of a Hemipteran Ecdysone Receptor Ligand-binding Domain COMPARISON WITH A LEPIDOPTERAN ECDYSONE RECEPTOR LIGAND-BINDING DOMAIN AND IMPLICATIONS FOR INSECTICIDE DESIGN