Regulatory Mechanisms Governing Precision in Vertebral Segmentation

项目摘要/摘要 基因程序沿着一系列调控步骤的及时和准确的进展对于 执行一个发展过程。然而，由于不可避免的原因，基因表达是一个高度随机的过程。 复杂生化反应动力学的波动；这种随机性导致大量的细胞间 可变性(基因表达噪声)。由此产生的表型波动只能在 同基因群体中的单细胞水平。科学界最耐人寻味的问题之一是如何 尽管有这些不可避免的因素，但发展模式的形成是如此精确和可重复地执行的 基因表达的波动。据推测，缓冲随机基因表达的机制肯定存在。 脊椎动物体细胞发生学为研究这一问题提供了一个范式系统。体节段( 脊椎的胚胎前体)是从卵裂期前的中胚层顺序地和周期性地产生的 (PSM)在胚胎的尾端。体节分割的周期受分割的控制 钟。分段时钟表现出Hes/Her家族时钟基因的振荡表达，这是由于 自抑性细胞内负反馈环。振荡的Delta配体激活血管内皮细胞Notch受体 并建立同步振荡相位的细胞间正反馈环路 在相邻的细胞之间。这些同步振荡的中断会导致出生缺陷。时间进程 在PSM中，体节的分裂和上皮化发生在后前方。这个 PSM上多个基因的协同表达受三个相互关联的信号控制 梯度(成纤维细胞生长因子、Wnt和维甲酸)。体细胞发生既是精确的--特定物种的胚胎发育 一定数量的节段具有特定物种的节奏性--和多样性--节段总数和 它们的周期性在不同物种之间差异很大。体细胞发生也很健壮，因为胚胎形成的片段带有 特定的大小分布，根据主体大小来缩放分段的大小，即使在总单元数、单元大小 或者通过实验改变增长率。这些特征表明， 有效地缓冲了振荡分段网络。 我们的首要目标是破译基因调控网络中的表达噪音是如何在 发展格局的形成。我们渴望通过以下方式达成对这种缓冲的机械性理解 将数学/计算/统计建模与不同的遗传和化学扰动相结合 改变多个基因的剂量或调节信号反馈强度。

PROJECT SUMMARY/ABSTRACT The timely and precise progression of a genetic program along a cascade of regulatory steps is critical to execute a developmental process. However, gene expression is a highly stochastic process due to inevitable fluctuations in the kinetics of complex biochemical reactions; this randomness leads to substantial cell-to-cell variability (gene expression noise). The resulting phenotypic fluctuations can only be detected and quantified at the single cell level within isogenic populations. One of the most intriguing questions in science is how developmental pattern formation is executed so precisely and reproducibly despite these unavoidable fluctuations in gene expression. Presumably, mechanisms that buffer stochastic gene expression must exist. Vertebrate somitogenesis provides a paradigm system for studying this question. Somite segments (the embryonic precursors of vertebrae) are produced sequentially and periodically from the presomitic mesoderm (PSM) at the tail end of the embryo. The period of somite segmentation is controlled by the segmentation clock. The segmentation clock exhibits oscillatory expression of Hes/her-family “clock” genes due to an autoinhibitory intracellular negative feedback loop. Oscillating Delta ligands activate Notch receptors in neighboring cells and establish an intercellular positive feedback loop that synchronizes oscillation phases among neighboring cells. Disruption of these synchronized oscillations results in birth defects. The time-course of somite segmentation and epithelization occur along the posteroanterior direction in the PSM. The coordinated expression of multiple genes along the PSM are controlled by three interconnected signaling gradients (Fgf, Wnt and retinoic acid). Somitogenesis is both precise – embryos of a given species develop certain number of segments with species-specific rhythmicity – and versatile –total number of segments and their periodicity vary widely among species. Somitogenesis is also robust as embryos form segments with a certain size distribution, scaling the sizes of segments with body size, even when total cell numbers, cell sizes or growth rates are altered experimentally. These characteristics indicate that the expression noise within the oscillating segmentation network is efficiently buffered. Our overarching goal is to decipher how expression noise in gene regulatory networks is buffered during developmental pattern formation. We aspire to reach a mechanistic understanding of this buffering by combining mathematical/computational/statistical modeling with different genetic and chemical perturbations to modify dosage of multiple genes or modulate signal feedback strength.