Activities of the Bicoid Gradient in Drosophila Embryos

描述(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，早期合胞胚胎允许母体贡献的蛋白质，如双核细胞(BCD)自由扩散并形成浓度梯度。BCD是一个从前到后的梯度转录因子，通过以浓度依赖的方式特异性地激活受精卵基因，在胚胎中负责建立前后极性。由于已知BCD的直接和生物相关的下游靶基因，该系统为了解靶基因如何对转录因子梯度做出反应提供了一个很好的模型。以前有人提出，BCD对增强子的亲和力决定了激活胚胎转录所需蛋白质的浓度。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的作用受BCD及其调节辅助因子的特定功能控制。我们解剖对正常发育至关重要的功能的策略是使用遗传和生化方法分析BCD突变和可能导致特定缺陷的突变背景；这些努力将进一步得到BCD同源结构域的合作结构研究的补充。我们提出了三个具体的目标来解决三个问题：1)BCD在发育过程中的活性是如何调节的？2)不同的基因如何对胚胎中不同的BCD浓度做出反应？3)BCD如何在发育过程中特异性地选择其天然的靶基因进行激活？对这些问题的回答不仅将显著提高我们对Bed如何工作的理解，而且还将极大地提高我们对形态发生梯度一般如何工作的理解。

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.