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。形态发生梯度被提议赋予细胞这样的位置信息。在果蝇中，早期合胞胚胎允许母体提供的蛋白质如Bicoid （Bcd）自由扩散并形成浓度梯度。Bcd是一种前后梯度和转录因子，通过在胚胎中以浓度依赖的方式特异性激活合子基因，负责建立前后极性。由于已知Bcd的直接和生物学相关的下游靶基因，该系统为理解靶基因如何响应转录因子梯度提供了一个极好的模型。先前有人提出，Bcd对增强子的亲和力决定了胚胎中激活转录所需的蛋白质浓度。但这个简单的亲和力阈值模型最近受到了包括我们自己的实验室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。我们对正常发育至关重要的解剖功能的策略是分析Bcd突变体和突变背景，这些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；这些努力将通过对Bcd同域的合作结构研究得到进一步补充。我们提出了三个具体目标来解决三个问题：1)在开发过程中如何调节Bcd的活动？2)胚胎中不同基因对不同Bcd浓度有何反应？3) Bcd在发育过程中如何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，不仅是床是如何工作的，而且是如何形成梯度的一般工作。

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.