Mechanical forces and their molecular control during epithelial folding in Drosophila

形态是如何产生的是发育生物学中一个悬而未决的大问题。胚胎是通过产生机械力使覆盖在胚胎表面和内部器官上的上皮细胞片变形而形成的。在原肠胚形成和眼、肠和脑的发育过程中，上皮细胞的基本形状变化是折叠的。在折叠过程中，准二维的细胞薄片转变为三维结构。一个被充分研究的上皮折叠机制是顶端细胞收缩。位于细胞顶端的肌动球蛋白网络的收缩产生机械力，导致细胞顶端变窄。相邻细胞协调的顶端收缩导致上皮折叠。我们最近的工作发现了另外两种参与上皮折叠的新机制。通过实时成像和定量图像分析，我们发现果蝇翅盘中相邻的两个褶皱的形成，是研究上皮发育的一个重要模型系统，它不涉及顶端细胞的收缩，而是涉及细胞基侧的松弛。在第一个褶皱中，基底松弛与细胞外基质的空间定义减少和基底力的减少相关。在第二层中，基底松弛伴随着细胞高度的降低，这与肌动蛋白沿着细胞之间的侧界面流动和侧力的增加有关。提出的工作有三个主要目的：首先，我们将使用光遗传学来改变细胞中的力产生，以测试是否减少基底力或增加侧向力对于褶皱形成是必要和充分的。其次，我们将揭示基底力的减少如何局限于形成第一个褶皱的细胞。我们的初步数据表明，无翼信号转导指定了这个褶皱的位置。我们将结合力测量与分子遗传学来测试是否以及如何无翼信号转导控制基础力。第三，我们将结合实时成像与遗传分析来破译肌动蛋白流是如何在细胞的侧面产生的，以及它们是如何促成横向力的。我们期望所提出的工作将揭示机械力产生和分子控制组织成形的新原理。

How form is generated is one of the big open questions in Developmental Biology. Embryos take shape by generating mechanical forces that deform epithelial sheets of cells covering the embryonic surface and inner organs. A fundamental shape change of epithelia, which takes place for example during gastrulation and the development of the eye, gut and brain, is folding. During folding, the quasi two-dimensional sheet of cells is transformed into a three-dimensional structure. A well-studied mechanism of epithelial folding is apical cell constriction. The contraction of an actomyosin network located at the apical side of cells generates mechanical forces that result in the narrowing of the apical side of cells. Coordinated apical constriction of neighboring cells results in epithelial folding. Our recent work has uncovered two additional novel mechanisms that are involved in epithelial folding. We showed by live imaging and quantitative image analysis that the formation of two neighboring folds in Drosophila wing discs, an important model system to study the development of epithelia, does not involve apical cell constriction, but rather the relaxation of the basal side of cells. In the first fold, basal relaxation correlates with a spatially defined decrease of extracellular matrix and decreased basal forces. In the second fold, basal relaxation is accompanied with a reduction in cell height that correlates with actin flows along the lateral interface between cells and increased lateral forces.The proposed work has three main aims: First, we will use optogenetics to alter force generation in cells to test whether decreased basal forces or increased lateral forces are necessary and sufficient for fold formation. Second, we will reveal how the reduction of basal force is confined to cells that form the first fold. Our preliminary data indicate that Wingless signal transduction specifies the position of this fold. We will combine force measurements with molecular genetics to test whether and how Wingless signal transduction controls basal forces. Third, we will combine live imaging with genetic analysis to decipher how actin flows are generated at the lateral side of cells and how they contribute to lateral forces. We expect that the proposed work will uncover novel principles by which mechanical forces are generated and molecularly controlled for tissues to take shape.