Nuclear Receptors in Ecdysone-mediated Programmed Cell Death in Drosophila melanogaster

The steroid hormone ecdysone plays vital roles during Drosophila development. Pulses of 20E during Drosophila life cycle function as temporal cues, signaling the onset of metamorphic processes, including the stage specific programmed cell death of larval tissues. Ecdysone is the critical developmental cue orchestrating the metamorphic reformation of CNS, resulting in the formation of adult-specific neural circuitry. Ecdysone signaling is transduced by a heterodimeric receptor complex formed between two nuclear receptors: EcR and Ultraspiracle (USP). There are 18 nuclear receptors known in Drosophila and EcR is the only receptor whose functions in neuronal PCD have been well recognized. Therefore, the current study is aimed to define the role of nuclear receptors in neuronal cell death mechanisms in Drosophila. Here, I examine the function of nuclear receptors in PCD of two groups of peptidergic neurons: vCrz and CCAP.

EcR and USP receptor complex on activation results in the coordinated transcriptional regulation of a host of transcription factors regulating genes essential for PCD. USP plays a dual role in ecdysone response, as its function is necessary for both activation and repression of ecdysone primary response genes. I have developed a possible dominant-negative mutant USP (usp³), and expressed it in flies using the GAL4-UAS system to illustrate the role of USP in ecdysone mediated PCD of vCrz neurons. Targeted expression of usp³ in corazonin neurons results in a complete blockage of PCD pathway. Another interacting partner of USP, Drosophila Hormone Receptor 38, however shows no involvement in PCD of vCrz neurons. I have also designed an ecdysone sensor to monitor the developmental timing of EcR activation in vCrz neurons.

Further, I investigate the survival factors required for preventing the untimely PCD of these two groups of neurons. The study reveals that DIAP1 is required for the survival of larval vCrz and CCAP neurons. Also, the nuclear receptor E75 is shown to be critical for preventing premature PCD of CCAP neurons.