Truman J W, Talbot W S, Fahrbach S E, Hogness D S
Department of Zoology, University of Washington, Seattle 98195.
Development. 1994 Jan;120(1):219-34. doi: 10.1242/dev.120.1.219.
In insects, the ecdysteroids act to transform the CNS from its larval to its adult form. A key gene in this response is the ecdysone receptor (EcR), which has been shown in Drosophila to code for 3 protein isoforms. Two of these isoforms, EcR-A and EcR-B1, are prominently expressed in the CNS and we have used isoform-specific antibodies to examine their fluctuations through postembryonic life. EcR expression at the onset of metamorphosis is extremely diverse but specific patterns of EcR expression correlate with distinct patterns of steroid response. Most larval neurons show high levels of EcR-B1 at the start of metamorphosis, a time when they lose larval features in response to ecdysteroids. Earlier, during the larval molts, the same cells have no detectable receptors and show no response to circulating ecdysteroids; later, during the pupal-adult transformation, they switch to EcR-A expression and respond by maturing to their adult form. During the latter period, a subset of the larval neurons hyperexpress EcR-A and these cells are fated to die after the emergence of the adult. The stem cells for the imaginal neurons show prominent EcR-B1 expression during the last larval stage correlated with their main proliferative period. Most imaginal neurons, by contrast, express only EcR-A when they subsequently initiate maturation at the start of metamorphosis. The imaginal neurons of the mushroom bodies are unusual amongst imaginal neurons in expressing the B1 isoform at the start of metamorphosis but they also show regressive changes at this time as they lose their larval axons. Imaginal neurons of the optic lobe show a delayed expression of EcR-B1 through the period when cell-cell interactions are important for establishing connections within this region of the CNS. Overall, the appearance of the two receptor isoforms in cells correlates with different types of steroid responses: EcR-A predominates when cells are undergoing maturational responses whereas EcR-B1 predominates during proliferative activity or regressive responses. The heterogeneity of EcR expression at the start of metamorphosis presumably reflects the diverse origins and requirements of the neurons that nevertheless are all exposed to a common hormonal signal.
在昆虫中，蜕皮甾体作用于将中枢神经系统从幼虫形态转变为成虫形态。这一反应中的一个关键基因是蜕皮激素受体（EcR），在果蝇中已证明它编码3种蛋白质异构体。其中两种异构体，EcR-A和EcR-B1，在中枢神经系统中显著表达，我们已使用异构体特异性抗体来检测它们在胚后发育过程中的波动情况。变态开始时EcR的表达极其多样，但EcR表达的特定模式与不同的类固醇反应模式相关。大多数幼虫神经元在变态开始时显示出高水平的EcR-B1，此时它们会因蜕皮甾体而失去幼虫特征。更早的时候，在幼虫蜕皮期间，相同的细胞没有可检测到的受体，并且对循环中的蜕皮甾体没有反应；后来，在蛹-成虫转变期间，它们转换为EcR-A表达，并通过成熟为成虫形态做出反应。在后期，一部分幼虫神经元过度表达EcR-A，这些细胞在成虫出现后注定会死亡。成虫神经元的干细胞在最后一个幼虫阶段显示出显著的EcR-B1表达，这与其主要增殖期相关。相比之下，大多数成虫神经元在随后变态开始时开始成熟时只表达EcR-A。蕈形体的成虫神经元在变态开始时表达B1异构体，这在成虫神经元中是不寻常的，但此时它们也会出现退行性变化，因为它们会失去幼虫轴突。视叶的成虫神经元在细胞间相互作用对在中枢神经系统的这一区域建立连接很重要的时期，EcR-B1的表达会延迟。总体而言，细胞中两种受体异构体的出现与不同类型的类固醇反应相关：当细胞进行成熟反应时EcR-A占主导，而EcR-B1在增殖活动或退行性反应期间占主导。变态开始时EcR表达的异质性大概反映了尽管都暴露于共同激素信号下，但神经元的不同起源和需求。
