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

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.

尽管已知内分泌变化可调控主要发育转变的时序，但此类变化背后的遗传机制仍不甚明晰。在昆虫体内，保幼激素（juvenile hormone, JH）与蜕皮激素（ecdysteroids）这两种发育激素彼此协同，诱导与变态相关的发育变化。然而，调控JH与蜕皮激素合成协同的分子机制仍不明了。本研究以赤拟谷盗（Tribolium castaneum，隶属拟步甲科Tenebrionidae）为实验材料，探究了脊椎动物POU结构域蛋白的同源物——腹脉缺失（Ventral veins lacking, Vvl）/漂移因子（Drifter）——在这两条激素通路调控中的功能。通过RNA干扰介导的vvl基因表达沉默，可同时引发幼虫早熟变态与蜕皮抑制现象。向vvl基因敲低的幼虫异位施加JH类似物，能够延缓变态起始时间并延长幼虫期，提示Vvl作用于JH信号通路的上游。与之相符的是，vvl敲低还会降低JH合成基因JH酸甲基转移酶3（juvenile hormone acid methyltransferase 3, jhamt3）的表达水平。此外，vvl敲低幼虫体内的蜕皮激素滴度以及蜕皮激素响应基因激素受体3（hormone receptor 3, HR3）的表达量均出现下调。进一步检测发现，蜕皮激素合成基因幻影（phantom, phm）与幽灵（spook, spo）的表达分别在幼虫前部与后部区域中被抑制，表明Vvl可能同时影响前胸腺与其他内分泌源的蜕皮激素生物合成。向vvl敲低幼虫注射20-羟基蜕皮酮（20-hydroxyecdysone, 20E）可恢复HR3的表达，但无法挽救蜕皮缺陷。上述研究结果表明，Vvl可协同调控JH与蜕皮激素的生物合成以及蜕皮行为，进而调控蜕皮过程与变态的时序。综上，在脊椎动物与昆虫中，POU家族因子均可在性成熟过程中调控主要神经内分泌调节因子的产生。