Activities of the Bicoid Gradient in Drosophila Embryos

描述（由申请人提供）：发育生物学中最重要的问题之一是细胞如何“知道”它们在发育中的胚胎或组织中的位置。形态发生梯度被提出来赋予细胞这样的位置信息。在果蝇中，早期的合胞体胚胎允许母体贡献的蛋白质如Bicoid（Bcd）自由扩散并形成浓度梯度。Bcd是一种前-后极性梯度和转录因子，在胚胎中以浓度依赖的方式特异性激活合子基因，从而负责建立前-后极性。由于Bcd的直接和生物学相关的下游靶基因是已知的，该系统代表了一个很好的模型，用于了解靶基因如何响应转录因子梯度。先前已经提出，Bcd对增强子的亲和力决定了激活胚胎中转录所需的蛋白质浓度。但是这个简单的亲和力阈值模型最近受到了包括我们自己在内的几个实验室的研究的挑战。我们的总体假设是，Bcd在发育过程中的高度选择性和浓度依赖性作用是由Bcd及其调节辅因子的特定功能控制的。我们的战略解剖功能的正常发展的关键是分析Bcd突变体和突变背景，可以导致特定的缺陷，使用遗传和生化方法，这些努力将进一步补充的Bcd同源结构域的合作结构研究。我们提出了三个具体的目标来解决三个问题：1）在发展过程中如何调节BCD的活动？2)不同的基因如何对胚胎中不同的Bcd浓度作出反应？3)Bcd如何在发育过程中特异性地选择其天然靶基因进行激活？这些问题的答案将大大提高我们的理解，不仅是如何床工作，特别是也如何形态发生梯度一般工作。

DESCRIPTION (provided by applicant): One of the most important questions in developmental biology concerns how cells "know" their positions in a developing embryo or tissue. Morphogenetic gradients are proposed to confer such positional information to cells. In Drosophila, the early syncytial embryo allows maternally contributed proteins such as Bicoid (Bcd) to diffuse freely and form concentration gradients. Bcd, an anterior-to-posterior gradient and a transcription factor, is responsible for establishing the anterior-posterior polarity by specifically activating zygotic genes in a concentration-dependent manner in embryos. Since the direct and biologically relevant downstream target genes of Bcd are known, this system represents an excellent model for understanding how target genes respond to a transcription factor gradient. It has been proposed previously that the Bcd affinity for an enhancer determines the concentration of the protein required for activating transcription in embryos. But this simple affinity-threshold model has recently been challenged by studies from several laboratories including our own. Our overall hypothesis is that the highly selective and concentration-dependent actions of Bcd during development are controlled by specific functions of Bcd and its regulatory cofactors. Our strategy toward dissecting functions critical to normal development is to analyze Bcd mutants and mutant backgrounds that can cause specific defects, using both genetic and biochemical approaches; these efforts will be further complemented by a collaborative structural study of the Bcd homeodomain. We propose three specific aims to address three questions: 1) How is the activity of Bcd regulated during development? 2) How do different genes respond to distinct Bcd concentrations in embryos? 3) How does Bcd specifically select its natural target genes for activation during development? Answers to these questions will significantly enhance our understanding of not only how Bed works in particular but also how morphogenetic gradients work in general.