SIGNALING MECHANISMS IN FOLLICLE CELL FATE DETERMINATION

受体酪氨酸激酶（RTK）信号参与生殖细胞-体细胞通信，这是生殖细胞成熟所必需的。它还参与了许多发育过程，包括肢体发育、大脑发育、器官发生等。这种功能多样性的关键是RTK信号可以响应许多不同的细胞外信号，并可以引发各种各样的细胞反应。然而，赋予信号特异性的机制在很大程度上仍不清楚。为了解决这个问题，我们提出了三个问题：1)什么规定了不同的信号，配体和/或受体伙伴？2)是什么传递了不同的信号，不同的激酶和/或不同修饰的激酶？3)除了转录因子活性的调节外，是否存在其他基因调控事件？果蝇卵发生的背腹侧（D/V）模式为解决这些问题提供了一个极好的模型。该图案化过程是由配体从卵母细胞传递到前背卵泡细胞，激活表皮生长因子受体（Egfr）信号通路而启动的。我们已经确定了一个转录因子，称为CF2，在这个位置信号转导过程中起着核心作用。我们发现CF2是菱形（rho）基因的负调控因子，该基因编码背化途径的重要膜结合成分，并且CF2本身的表达受到活化的Egfr的负调控。我们的发现还暗示了这一途径的两个重要特征：(a) D/V模式涉及两步信号传导过程——初始的Egfr信号抑制CF2并诱导rho表达；以及随后的EGFR+Rho信号，后者决定了背侧细胞的命运。我们假设这两种信号，由Rho的缺失或存在而区分，需要不同的配体和/或引发不同的次级信使。因此，解开这个两步信号事件可以帮助回答上面的问题1和2。(b) MAPK级联通过细胞质保留和蛋白酶体介导的蛋白水解等新机制负调控CF2的表达。解开这个机制可以帮助回答上面的问题3。为了探索这些生物学现象，我们提出：目的1，确定Egfr信号介导的CF2蛋白降解机制；目的2，确定Rho是否诱导Egfr信号级联的变化；和Aim 3，筛选CF2功能获得等位基因的第二位点抑制子。

Receptor tyrosine kinase (RTK) signaling is involved in germline-soma communication that is essential for germ cell maturation. It is also involved in a myriad of developmental processes, including limb development, cerebral development, organogenesis, etc. The key to such functional diversity is that RTK signaling can respond to many different extracellular signals and can elicit a wide variety of cellular responses. However, the mechanisms that confer signal specificity have remained largely unclear. To approach this problem, we ask three questions: 1) What specifies the different signals, ligands and/or receptor partners? 2) What transmits the different signals, different kinases and/or differently modified kinases? 3) Are there different gene regulation events other than modulation of transcription factor activities? Dorsoventral (D/V) patterning in Drosophila oogenesis provides an excellent model for resolving these questions. The patterning process is initiated by the transmission of a ligand from the oocyte to the anterior dorsal follicle cells, activating the epidermal growth factor receptor (Egfr) signaling pathway. We have identified a transcription factor, termed CF2, that plays a central role in this positional signal transduction process. We showed that CF2 is a negative regulator of the rhomboid (rho) gene that encodes an essential membrane-bound component of the dorsalizing pathway, and that expression of CF2 itself is negatively regulated by the activated Egfr. Our findings also implicate two important features in this pathway: (a) The D/V patterning involves a two-step signaling process-the initial Egfr signal, which represses CF2 and induces rho expression; and the subsequent EGFR+Rho signal, which determines the dorsal cell fates. We hypothesize that these two signals, differentiated by the absence or presence of Rho, require different ligands and/or elicit different secondary messengers. Therefore unraveling this two-step signaling event can help answer Questions 1 and 2 above. (b) CF2 expression is negatively regulated by the MAPK cascade through a novel mechanism involving cytoplasmic retention and proteasome-mediated proteolysis. Unraveling this mechanism can help answer Question 3 above. To explore these biological phenomena, we propose: Aim 1, to determine the mechanism of CF2 protein degradation mediated by Egfr signaling; Aim 2, to determine whether Rho induces changes in Egfr signaling cascade; and Aim 3, to screen for second-site suppressors of the CF2 gain-of-function allele.