Lam G, Hall B L, Bender M, Thummel C S
Howard Hughes Medical Institute, University of Utah, 15 N 2030 E Rm 5100, Salt Lake City, Utah, 84112-5331, USA.
Dev Biol. 1999 Aug 1;212(1):204-16. doi: 10.1006/dbio.1999.9343.
Pulses of the steroid hormone ecdysone activate genetic regulatory hierarchies that coordinate the developmental changes associated with Drosophila metamorphosis. A high-titer ecdysone pulse at the end of larval development triggers puparium formation and induces expression of the DHR3 orphan nuclear receptor. Here we use both a heat-inducible DHR3 rescue construct and clonal analysis to define DHR3 functions during metamorphosis. Clonal analysis reveals requirements for DHR3 in the development of adult bristles, wings, and cuticle, and no apparent function in eye or leg development. DHR3 mutants rescued to the third larval instar also reveal essential functions during the onset of metamorphosis, leading to lethality during prepupal and early pupal stages. The phenotypes associated with these lethal phases are consistent with the effects of DHR3 mutations on ecdysone-regulated gene expression. Although DHR3 has been shown to be sufficient for early gene repression at puparium formation, it is not necessary for this response, indicating that other negative regulators may contribute to this pathway. In contrast, DHR3 is required for maximal expression of the midprepupal regulatory genes, EcR, E74B, and betaFTZ-1. Reductions in EcR and betaFTZ-F1 expression, in turn, lead to submaximal early gene induction in response to the prepupal ecdysone pulse and corresponding defects in adult head eversion and salivary gland cell death. These studies demonstrate that DHR3 is an essential regulator of the betaFTZ-F1 midprepupal competence factor, providing a functional link between the late larval and prepupal responses to ecdysone. Induction of DHR3 in early prepupae ensures that responses to the prepupal ecdysone pulse will be distinct from responses to the late larval pulse and thus that the animal progresses in an appropriate manner through the early stages of metamorphosis.
类固醇激素蜕皮激素的脉冲激活了基因调控层级，这些层级协调了与果蝇变态相关的发育变化。幼虫发育末期的高滴度蜕皮激素脉冲触发蛹壳形成并诱导DHR3孤儿核受体的表达。在这里，我们使用热诱导的DHR3拯救构建体和克隆分析来定义DHR3在变态过程中的功能。克隆分析揭示了DHR3在成虫刚毛、翅膀和表皮发育中的需求，而在眼睛或腿部发育中没有明显功能。拯救到第三龄幼虫的DHR3突变体也揭示了变态开始时的基本功能，导致在预蛹和早期蛹阶段致死。与这些致死阶段相关的表型与DHR3突变对蜕皮激素调节基因表达的影响一致。虽然DHR3已被证明足以在蛹壳形成时进行早期基因抑制，但这一反应并非必需，表明其他负调节因子可能参与了这一途径。相反，DHR3是预蛹中期调控基因EcR、E74B和βFTZ-1最大表达所必需的。EcR和βFTZ-F1表达的降低反过来又导致对预蛹蜕皮激素脉冲的早期基因诱导不足，以及成虫头部外翻和唾液腺细胞死亡的相应缺陷。这些研究表明，DHR3是βFTZ-F1预蛹中期感受态因子的重要调节因子，在幼虫后期和预蛹期对蜕皮激素的反应之间提供了功能联系。在早期预蛹中诱导DHR3可确保对预蛹蜕皮激素脉冲的反应不同于对幼虫后期脉冲的反应，从而使动物以适当的方式通过变态的早期阶段。
