Ecdysteroid Receptors and their Applications in Agriculture and Medicine

Ecdysteroids (Ec) are signaling molecules widespread in the animal as well as in the plant kingdom (Lafont and Wilson, 1992). However, they do not occur naturally in vertebrates, a feature that makes them suitable as ligands in medical gene switch applications due to the reduced likelihood of pleiotropic effects. Ecdysteroids fulfill diverse tasks because they serve as hormones, pheromones, or insect deterrents (Nijhout, 1994). Their most frequent and prominent role, however, is their function as "molting hormones," thereby controlling not only insect and arthropod development but also reproduction and other physiological processes (Spindler, 1997). The action of Ec on target cells, as is the case with steroids in general, may be divided into fast and ephemeral (Tomaschko, 1999), as well as slow and systemic, effects. As will be discussed in the Section II.F., the two modes of Ec action may eventually converge, giving rise to the integral cellular response. Fast Ec effects are generally traced back to a not yet well-defined target on the cell membrane. On the other hand, the systemic effects involve an intracellular receptor, namely the ecdysteroid receptor (EcR) (Koelle et al., 1991). The EcR is a member of the nuclear receptor superfamily (Mangelsdorf and Evans, 1995; Mangelsdorf et al., 1995) and exhibits the typical modular structure composed of the N-terminal A/B domain, the DNA-binding C domain, the hinge (D) region, the ligand-binding E domain, and the C-terminal F domain. The ligand-binding domain is multifunctional and includes ligand-dependent dimerization and transactivation functions, while ligand-independent transactivation and dimerization functions are found in the terminal domains and in the region spanning the DNA binding domain and the N-terminal region of the hinge, respectively. The EcR heterodimerizes with other members of the nuclear receptor superfamily (Henrich, 2004), noticeably with the ultraspiracle protein (USP), which is an orthologue of the vertebrate retinoic acid X receptor (RXR) (Thomas et al., 1993; Yao et al., 1992, 1993). The EcR/USP heterodimers bind to the Ec response elements (EcRE) present in the promoter regions of Ec response genes and regulate their transcription. Most of the nuclear hormone receptors, including EcR, function as ligand-controlled transcription factors, a characteristic that renders these receptors or their key regions (i.e., the ligand- and DNA-binding domains) especially suitable as constituents of gene switches. Several nuclear receptors, including glucocorticoid receptor (GR), progesterone receptor (PR), estrogen receptor (ER), and EcR, are being used to develop gene switches for applications in medicine and agriculture. Since the EcR and its ligands are not found in vertebrates, they are attractive targets for the development of gene switches to be used in humans. This chapter is divided into Section I, which summarizes the present knowledge of the EcR's structure and function with special emphasis on those aspects that are relevant for gene switch applications, and Section II, which describes gene switch technology with a focus on EcR-based gene switches. For a comprehensive and comparative overview on EcR's role within the gene control network and during development and evolution, the reader is referred to Henrich (2004).