Abstract
Ecdysteroids are steroid hormones that induce molting and determine developmental timing in arthropods. In insect larva, the prothoracic gland (PG) is a major organ for ecdysone synthesis and release. Released ecdysone is converted into the active form, 20-hydroxyecdysone (20E) in the peripheral tissues. All processes from ecdysone synthesis and release from the PG to its conversion to 20E are called ecdysteroidogenesis and are under the regulation of numerous factors expressed in the PG and peripheral tissues. Classical genetic approaches and recent transcriptomic screening in the PG identified several genes responsible for ecdysone synthesis and release, whereas the regulatory mechanism remains largely unknown. We analyzed RNA-seq data of the silkworm Bombyx mori PG and employed the fruit fly Drosophila melanogaster GAL4/UAS binary RNAi system to comprehensively screen for genes involved in ecdysone synthesis and/or release. We found that the genes encoding δ-aminolevulinic acid synthase (CG3017/alas) and putative NAD kinase (CG33156) were highly expressed in the PG of both B. mori and D. melanogaster. Neither alas nor CG33156 RNAi-induced larvae could enter into the pupal stage, and they had a lower abundance of the active form ecdysteroids in their prolonged larval stage. These results demonstrated that alas and CG33156 are indispensable for ecdysteroidogenesis.
Highlights
Steroid hormones play crucial roles in the regulation of many biological processes in vertebrates and invertebrates
Regarding alas and CG33156, we showed that antisense probe specific signals were detected in the ring gland (RG) (Fig 2B–2E), whereas no clear signals were detected with probes for the other six candidate genes. alas and CG33156 showed typical expression profiles for genes responsible for ecdysone synthesis or release, and while the other six genes might be expressed in the prothoracic gland (PG), they may be less expressed or temporally restricted during development
Since alas and CG33156 were predominantly expressed in the RG and disrupting their expression in the PG resulted in the developmental arrest at the larval stage, loss of gene function was expected to cause defects in ecdysteroid synthesis and/or release
Summary
Steroid hormones play crucial roles in the regulation of many biological processes in vertebrates and invertebrates. Growth is controlled by molting and metamorphosis, and characteristic developmental events are regulated by steroid hormones called ecdysteroids. Ecdysone is synthesized from cholesterol through serial oxidation/ hydroxylation steps in the prothoracic gland (PG) and secreted into the hemolymph [1,2,3,4]. The released ecdysone is converted to the biologically active form of ecdysone, 20-hydroxyecdysone (20E), in peripheral tissues [5]. The sequence of biological processes from ecdysone synthesis in the PG to 20E production in the peripheral tissues is called ecdysteroidogenesis.
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