Chapter 248 - Nuclear Receptors in Drosophila Melanogaster

Abstract

This chapter provides significant information on the involvement of nuclear receptors in Drosophila melanogaster development. Two nuclear receptors, EcR and its heterodimeric partner, ultraspiracle (USP), which is the Drosophila ortholog of the vertebrate retinoid X receptor (RXR), form a heterodimer to set off the transcriptional changes that lead to morphogenetic changes. The salivary gland (SG) of Drosophila melanogaster is exposed to at least three ecdysteroid peaks during the approach and onset of metamorphosis, a period of approximately 36 hours, and each wave of transcriptional response is distinct and each one drives developmental changes in the SG. The first and smallest peak of ecdysteroids occurs during the mid-third (final) instar and is necessary for the transcriptional induction of genes encoding glue proteins. It is followed by a second peak at the end of the larval stage that leads to the synthesis of proteins associated with glue protein secretion. A third peak, which occurs after the initiation of the prepupal stage, triggers the programming of SG cell death that occurs during the pupal-adult transition that marks metamorphosis. The action of ecdysteroids, presumably mediated by ecdysone receptor (EcR) induces the transcription of a variety of nuclear receptors, such as E75, DHR3, DHR4, DHR78, DHR96, and βFTZ-F1. EcR's partner, ultraspiracle (USP), interacts with a subset of nuclear receptors including sevenup (SVP, whose vertebrate ortholog is COUP-TF) and DHR38. Some of these nuclear receptors such as DHR78 and DHR96 can interact with promoter elements recognized by the EcR/USP heterodimer. Ecdysteroids coordinate an entire network of physiological and developmental activities that are initially triggered by EcR and USP, and, in turn, act through a variety of nuclear receptor partners and gene targets.