Transcriptional Regulators of Ecdysteroid Biosynthetic Enzymes and Their Roles in Insect Development

Abstract

Steroid hormones are responsible for coordinating many aspects of biological processes in most multicellular organisms, including insects. Ecdysteroid, the principal insect steroid hormone, is biosynthesized from dietary cholesterol or plant sterols. In the last 20 years, a number of ecdysteroidogenic enzymes, including Noppera-bo, Neverland, Shroud, Spook/Spookier, Cyp6t3, Phantom, Disembodied, Shadow, and Shade, have been identified and characterized in molecular genetic studies using the fruit fly Drosophila melanogaster. These enzymes are encoded by genes collectively called the Halloween genes. The transcriptional regulatory network, governed by multiple regulators of transcription, chromatin remodeling, and endoreplication, has been shown to be essential for the spatiotemporal expression control of Halloween genes in D. melanogaster. In this review, we summarize the latest information on transcriptional regulators that are crucial for controlling the expression of ecdysteroid biosynthetic enzymes and their roles in insect development.

Keywords: Drosophila; Halloween genes; ecdysone; ecdysteroids; steroidogenic enzyme; transcription factors (TFs).

FIGURE 1

The ecdysteroid biosynthesis pathway and Halloween enzymes. As the conversion steps between 7-dehydrocholesterol and 5β-ketodiol have not been chemically characterized, they are collectively called the “Black Box.” Notably, spo and spok are paralogous to each other (Ono et al., 2006; Komura-Kawa et al., 2015). Although genetic and physiological analyses have suggested that Nobo plays a role in cholesterol intake or trafficking in the PG, endogenous substrates of Nobo have not been identified. Therefore, the role of Nobo is indicated by the dashed arrow in this illustration.

FIGURE 2

A schematic illustration of regulators of the expression of Halloween genes in the PG. DNA-binding transcription factors are indicated in green circles. Red circles represent histone modification enzymes and cell cycle regulators. Other factors are indicated in red hexagons. H3K4me3 and H3K27me3 indicate trimethylation of the fourth and 27th lysine residues of histone H3, respectively.