Regulation Of Childhood Growth

哺乳动物的身体在出生后早期快速增长，但随后增长速度减慢，最终随着生物体接近其成年身体大小而下降到零。这种生长速度的下降，主要是由于增殖速度的下降，在多个组织中同时发生，但似乎不是由激素或其他全身机制指挥的。因此，我们假设这种协调的生长减速是由多个组织共同的基因表达程序指导的。 为了验证这一假设，我们进行了微阵列分析，以确定在小鼠出生后早期，随着身体生长放缓，基因表达的变化。我们特别关注在多个器官中上调或下调的基因，因此更有可能对假定的共同生长减速计划做出贡献。在所研究的每个器官中，肾、肺和心脏中，发现有2000多个基因随着年龄的增长显著上调，2800多个基因下调。在所有三个器官中协调调控的基因数量惊人地高，有1207个基因在三个器官中下调，428个基因在三个器官中上调，重叠程度远远超出偶然的预期，这表明除了预期的组织特异性变化外，在出生后的生长过程中，有一个广泛的多个器官共有的基因表达计划。共同的程序包括参与调节G1/S和G2/M检查点、Hedgehog信号和Wnt/β-catenin信号的基因。在这些途径中，随着年龄的增长，下调的基因比上调的基因更多。 接下来，我们将注意力集中在5个基因上，即Igf2、Mest、Peg3、Sox4和Igf2bp3，这些基因在所有三个器官中都显著下调，以获得更详细的特征。首先，我们用实时定量聚合酶链式反应(Real-time PCR)检测了基因芯片研究的三个器官中的mRNA表达水平。实时荧光定量聚合酶链式反应结果证实，随着年龄的增长，该基因的表达显著下降。我们还测量了第四个器官肝脏的mRNA水平，发现随着年龄的增长，基因表达水平急剧下降，与心脏、肾脏和肺的下降模式相似，进一步证实了这种基因表达程序在多个器官中普遍存在。 接下来，我们使用原位杂交来确定在1周大的小鼠中，每个器官中的哪些细胞类型表达Igf2、Mest和Peg3。总的来说，我们发现在器官特异的实质细胞中有表达。这些发现表明，多个器官中表达的协调下降并不是简单地因为基因表达仅限于多个器官共有的细胞，如内皮细胞或间质成纤维细胞。 最后，我们试图确定观察到的基因表达变化是时间本身的函数还是生长的函数。在大鼠中，丙基硫氧嘧啶诱导的甲状腺功能减退和出生前5周的生长迟缓延缓了我们研究的与生长调节有关的三个基因Igf2、Mest和Peg3的表达下降。这些数据与以下假设一致，即这些基因表达的正常下降不是由年龄本身驱动的，而是由生长过程驱动的，因此，之前的生长抑制时期减缓了这种下降。 基于这些发现，我们提出了以下模型来解释哺乳动物体细胞生长减速和最终停止的原因。在胚胎晚期和出生后早期，促进生长的基因网络在高水平表达。由此产生的生长导致这些促进生长的基因的表达下降，这反过来又减缓了生长速度。最终，这些促进生长的基因的表达水平会下降到足以导致体细胞生长停止的程度。如果一些外部条件，如甲状腺功能减退，暂时限制了生长，那么缓慢的生长将减缓基因表达的下降，从而保留未来的生长能力。在生长抑制期后，促生长基因的表达水平会高于正常，因此生长速度也会高于正常。因此，该模型为追赶生长现象提供了解释，追赶生长的现象被定义为在生长抑制时期之后高于正常年龄的生长速度。 我们的发现表明，在出生后阶段存在一个广泛的遗传程序，涉及数千个基因的上调和下调。其中，有些似乎是器官特有的，但许多是在多个器官中以一致的方式调节的。其中一些常见基因调节细胞增殖，因此可能构成导致体细胞生长减慢并最终停止的机制的一部分。我们发现有证据表明，几个可能参与这一生长限制计划的基因，如Igf2、Mest和Peg3，本身受生长调节，这表明，在胚胎中，一种基因表达模式被建立，允许多个组织的快速躯体生长，但随后，在出生后的生活中，这种生长导致基因表达的负反馈变化，进而减缓并最终停止躯体生长，从而对成年人的身体大小施加基本限制。

Mammalian body growth is rapid in early postnatal life, but then the growth rate slows, eventually declining toward zero as the organism approaches its adult body size. This decline in growth rate, which is due primarily to a decrease in the rate of proliferation, occurs simultaneously in multiple tissues but does not appear to be orchestrated by a hormonal or other systemic mechanism. We therefore hypothesized that this coordinated growth deceleration is directed by a program of gene expression that is common to multiple tissues. To test this hypothesis, we performed microarray analysis to identify changes in gene expression that occurred during early postnatal life in mice, as body growth slows. We particularly focused on genes that were up- or downregulated in multiple organs and thus more likely to contribute to the putative common program of growth deceleration. In each of the organs studied, kidney, lung, and heart, more than 2000 genes were found to be significantly upregulated with age and more than 2800 genes downregulated. The number of genes that were regulated coordinately in all three organs was strikingly high, with 1207 genes downregulated in all three organs and 428 upregulated in all three organs, far more overlap than would be expected by chance, indicating that there is an extensive program of gene expression common to multiple organs during postnatal growth, in addition to the expected tissue-specific changes. The common program included genes involved in regulating G1/S and G2/M checkpoints, Hedgehog signaling, and Wnt/beta-catenin signaling. There were more genes that were downregulated with age than were upregulated in these pathways. We next focused our attention on 5 genes, Igf2, Mest, Peg3, Sox4, and Igf2bp3, that were markedly downregulated in all three organs for more detailed characterization. First, we measured mRNA expression levels by real-time PCR in the same three organs studied by microarray. The real-time PCR results confirmed the marked declines in mRNA expression with age. We also measured mRNA levels in a fourth organ, liver, and found dramatic declines with age, similar in pattern to the declines found in heart, kidney, and lung, further confirming that this gene expression program is occurring globally in multiple organs. We next used in situ hybridization to determine which cell types in each organ expressed Igf2, Mest, and Peg3 in the 1-week-old mouse. In general, we found expression in organ-specific parenchymal cells. These findings suggest that the coordinate decline in expression in multiple organs is not occurring simply because gene expression is restricted to cells that are common to multiple organs, such as endothelial cells or stromal fibroblasts. Finally, we sought to determine whether the observed changes in gene expression are a function of time per se or a function of growth. In rats, propylthiouracil-induced hypothyroidism and growth retardation during the first 5 weeks of life delayed the declines in expression of Igf2, Mest, and Peg3, the three genes which we studied that have been implicated in growth regulation. These data are consistent with the hypothesis that the normal decline in expression of these genes is driven, not by age per se, but rather by the process of growth, and therefore, a prior period of growth inhibition slows this decline. Based on these findings we propose the following model to explain the deceleration and eventual cessation of somatic growth in mammals. In late embryonic and early postnatal life, a network of growth-promoting genes is expressed at high levels. The resulting growth causes the expression of these growth-promoting genes to decline, which in turn slows the rate of growth. Eventually, the expression levels of these growth-promoting genes declines sufficiently to cause somatic growth to cease. If some external condition, such as hypothyroidism transiently restricts growth, then the slow growth will slow the decline in gene expression, thus preserving future growth capacity. Following the period of growth inhibition, the expression levels of the growth-promoting genes will be higher than normal, and consequently the growth rate will be greater than normal. Thus the model provides an explanation for the phenomenon of catch-up growth which is defined as a growth rate that is greater than normal for age following a period of growth inhibition. Our findings indicate that there exists an extensive genetic program occurring during the postnatal period involving upregulation and downregulation of thousands of genes. Of these, some appear to be organ-specific, but many are regulated in a concerted fashion in multiple organs. Some of these common genes regulate cell proliferation and thus may constitute part of the mechanism that causes somatic growth to slow and eventually cease. We found evidence that several of the genes likely to participate in this growth-limiting program, Igf2, Mest, and Peg3, are themselves regulated by growth, suggesting that, in the embryo, a gene expression pattern is established that allows for rapid somatic growth of multiple tissues but then, during postnatal life, this growth leads to negative-feedback changes in gene expression that in turn slow and eventually halt somatic growth, thus imposing a fundamental limit on adult body size.