The POU Factor Ventral Veins Lacking/Drifter Directs the Timing of Metamorphosis through Ecdysteroid and Juvenile Hormone Signaling

Abstract

Although endocrine changes are known to modulate the timing of major developmental transitions, the genetic mechanisms underlying these changes remain poorly understood. In insects, two developmental hormones, juvenile hormone (JH) and ecdysteroids, are coordinated with each other to induce developmental changes associated with metamorphosis. However, the regulation underlying the coordination of JH and ecdysteroid synthesis remains elusive. Here, we examined the function of a homolog of the vertebrate POU domain protein, Ventral veins lacking (Vvl)/Drifter, in regulating both of these hormonal pathways in the red flour beetle, Tribolium castaneum (Tenebrionidae). RNA interference-mediated silencing of vvl expression led to both precocious metamorphosis and inhibition of molting in the larva. Ectopic application of a JH analog on vvl knockdown larvae delayed the onset of metamorphosis and led to a prolonged larval stage, indicating that Vvl acts upstream of JH signaling. Accordingly, vvl knockdown also reduced the expression of a JH biosynthesis gene, JH acid methyltransferase 3 (jhamt3). In addition, ecdysone titer and the expression of the ecdysone response gene, hormone receptor 3 (HR3), were reduced in vvl knockdown larvae. The expression of the ecdysone biosynthesis gene phantom (phm) and spook (spo) were reduced in vvl knockdown larvae in the anterior and posterior halves, respectively, indicating that Vvl might influence ecdysone biosynthesis in both the prothoracic gland and additional endocrine sources. Injection of 20-hydroxyecdysone (20E) into vvl knockdown larvae could restore the expression of HR3 although molting was never restored. These findings suggest that Vvl coordinates both JH and ecdysteroid biosynthesis as well as molting behavior to influence molting and the timing of metamorphosis. Thus, in both vertebrates and insects, POU factors modulate the production of major neuroendocrine regulators during sexual maturation.

The Insect Prothoracic Gland as a Model for Steroid Hormone Biosynthesis and Regulation

“…The regulatory group comprises a total of 13 DNA-binding proteins. Of these, molting defective ( mld ), vvl, ouija board ( ouib , aka CG11762 ), snail , and timeless ( tim ) were already known to have specific expression in the RG (Cheng et al, 2014; Danielsen et al, 2014; Komura-Kawa et al, 2015; Morioka et al, 2012; Neubueser et al, 2005;Sánchez-Higueras et al, 2014), while tinman has been linked to the embryonic development of the CC, but no expression in the RG has been demonstrated (Park et al, 2011). Among the identified transcription factors are Escargot, which is related to Snail, and Hand, which ranks among the 50 most highly enriched mRNAs in the RG (Table S2), as well as pdm3 , which encodes—like vvl —a POU-domain transcription factor.…”

The Homeodomain Transcription Factors Antennapedia and POU-M2 Regulate the Transcription of the Steroidogenic Enzyme Gene Phantom in the Silkworm

“…POU-3F1 affects the activity of the gonadotropin-releasing hormone promoter (60,61). In insects, Har-POU regulates the biosynthesis of diapause hormone in the cotton bollworm (Helicoverpa armigera) (62), and Vvl mediates the production of both juvenile hormone and ecdysone in the fruit fly and the red flour beetle (23,63). Drifter plays an essential role during the development and differentiation of the neuroendocrine system in the fruit fly (64).…”

Transcriptional regulation of insect steroid hormone biosynthesis and its role in controlling timing of molting and metamorphosis

“…For example, Vvl described above is involved in cellular differentiation of several types of cells including the PG (Cheng et al . ; Danielsen et al . ; Sánchez‐Higueras et al .…”

“…Insect orthologues of vvl are also involved in ecdysteroid biosynthesis not only in D. melanogaster but also in other insects such as the red flour beetle Tribolium castaneum (Cheng et al . ) and B. mori (referred as POU‐M2 ) (Meng et al . ).…”