Radial patterning of smooth muscle in the vertebrate gut

项目摘要 开始时，肠道是一个简单的内胚层管，周围是间充质，肠道的模式早在20世纪70年代。 沿着其径向轴沿着发育以产生具有不同取向的平滑肌的离散层。 需要将这些肌肉层正确组织成周向和纵向带， 肠腔的折叠和产生驱动肠腔折叠的适当的物理力。然而， 在发育过程中用精确的时空分辨率来确定肌肉的模式是未知的。 基于我们实验室和其他人的工作，我们预测这种精度是通过特定水平的 肠间充质内的刺猬（Hh）和BMP信号传导活性。通过调节Hh通路的水平 活性，我们在初步研究中表明，低水平和高水平的Hh信号抑制分化的细胞， 早期形成的环形平滑肌层。我们预测，这种抑制作用的Hh在高水平是 由于Hh依赖性BMP信号传导活性增加，其在Hh的高阈值水平下被激活， 阻止肌肉分化。重要的是，当Hh信号仅轻微减少时， 邻近内胚层形成平滑肌。在此基础上，我们提出了目标1中的模型，并对模型进行了检验。 平滑肌的环带在Hh信号传导的精确径向位置形成， BMP的抑制作用足够高，但BMP的抑制作用足够低，不会干扰。那么是什么触发了 在圆周层之后形成的外纵向层和内纵向层的形成？我们 初步结果表明，持续的BMP信号传导抑制了细胞的分化， 纵向层，因为外源Noggin导致其早熟分化。我们还表明， Noggin通过分化的肠神经元特异性表达，并提出了一个模型， Noggin在肠神经元中的特异性和表达可能有助于启动纵向神经元的分化， 肌肉在正确的地点和时间通过局部拮抗骨形态发生蛋白。在目标2中，我们通过以下方式测试此模型： 鸡的胚胎学操作和小鼠的遗传功能丧失，以确定是否来自 NCC是纵向肌肉分化所必需的。此外，我们的目标是确定细胞起源的 纵肌本文提出的实验结果将建立一个机理模型， 解释脊椎动物肠道中的平滑肌模式，并有可能提高我们对 肌肉发育不良损害消化的潜在机制。

PROJECT SUMMARY Beginning as a simple tube of endoderm surrounded by mesenchyme, the gut is patterned early in development along its radial axis to generate discrete layers of smooth muscle with distinct orientations. Correct organization of these muscle layers into circumferential and longitudinal bands is required for peristalsis and to generate appropriate physical forces that drive folding of the gut lumen. Yet, how the radial pattern of muscle is determined with precise spatiotemporal resolution during development is not known. Building upon work from our lab and others, we predict that this precision is achieved through specific levels of Hedgehog (Hh) and BMP signaling activity within the gut mesenchyme. By modulating levels of Hh pathway activity, we show in preliminary studies that both low and high levels of Hh signaling inhibit differentiation of the early-forming circumferential smooth muscle layer. We predict that this inhibitory effect of Hh at high levels is due to increased Hh-dependent BMP signaling activity, which is activated at high threshold levels of Hh and acts to block muscle differentiation. Importantly, when Hh signaling is only slightly decreased smooth muscle smooth muscle forms adjacent to the endoderm. Given these data, we put forth and test model in Aim 1 in which the circumferential band of smooth muscle forms at an exact radial location where Hh signaling is sufficiently high, yet the inhibitory effects of BMP are low enough to not interfere. What then triggers the formation of the outer and inner longitudinal layers that form subsequent to the circumferential layer? Our preliminary results indicate that continuous BMP signaling acts to suppress the differentiation of the longitudinal layers, as exogenous Noggin causes their precocious differentiation. We additionally show that Noggin is expressed specifically by differentiating enteric neurons, and put forth a model wherein temporally specific and expression of Noggin in enteric neurons may serve to initiate the differentiation of longitudinal muscle at the correct place and time by locally antagonizing BMPs. In Aim 2, we test this model through embryological manipulations in chick and genetic loss of function in mouse to determine if Noggin from the NCCs is required for longitudinal muscle differentiation. Furthermore, we aim to identify the cellular origin of the longitudinal muscle. The results of the experiments proposed herein will establish a mechanistic model to explain smooth muscle patterning in the vertebrate gut and have the potential to improve our understanding of the mechanism underlying muscle dysplasias that impair digestion.