Defining the molecular basis of photperiodism in mammals

地球在其轴心和围绕太阳的轨道上的永久运动为地球上的生命创造了一个有节奏的环境。所有类型的生物体都通过进化生物钟和日历来做出反应，这些生物钟和日历允许预测昼夜和季节的变化。最近对这种“时间生物学”的研究表明，有一小部分基因被称为时钟基因，它们可以产生内在的日常节律。这些基因在从昆虫到人类的结构和功能上显示出保守的特征，反映了它们的古老进化。生物钟基因在人体的大多数细胞内发挥作用，产生近24小时的有节奏的输出，细胞之间进行交流，以调节睡眠活动、进食、激素分泌和许多其他特征的日常节奏。关于较长时间计时器的分子基础，以及动物通过改变日长来同步季节性节律的机制，人们了解的要少得多。在哺乳动物中，季节性计时的一个关键方面是松果体产生褪黑素。褪黑激素只在晚上产生，这种模式直接反映了夜晚的长度，正是这种荷尔蒙信号的变化传达了关于身体周围一年中的时间的信息。每天长时间的褪黑素是冬天的信号，而夏天的短时间则是褪黑素的信号。我们之前的工作表明，褪黑素的变化模式是通过打开和关闭特定的时钟基因来解码的。值得注意的是，黄昏时褪黑激素的增加激活了Cry1基因的表达，黎明时褪黑激素的下降激活了PER1基因的表达。在此基础上，我们提出了光周期时间测量的“内符合假设”。这表明，CRY1和PER1蛋白之间的相互作用程度，由从黄昏到黎明的时间段--褪黑素信号--决定了季节性反应。因此，位于大脑和脑下垂体的褪黑素反应细胞的昼夜时钟机制被用于季节性计时。现在，我们计划用转基因绵羊来验证这一假设，在转基因绵羊中，Cry1基因被选择性地中和。这一策略依赖于我们最近的证明，即修改后的DNA序列可以有效地导入绵羊胚胎并在羔羊中表达。绵羊被用作模型，因为它具有很好的季节性生物学特征。目的是用两种不同的“转基因”来干扰Cry1基因的内源表达。据预测，这不会影响正常发育，但会阻碍光周期反应。这种“四季绵羊”的生物学将引起时间生物学家的主要兴趣，利用绵羊的转基因开辟了研究长寿物种遗传控制的新纪元。

The perpetual motion of the Earth on its axis and the orbit around the sun generates a rhythmic environment for life on Earth. Organisms of all types have responded by evolving biological clocks and calendars that allow anticipation of day and night, and the changing seasons. Recent research into this 'chronobiology' has characterised a small number of genes, called clock genes, which generate the intrinsic daily rhythm. These genes show conserved features in their structure and function from insects to man, reflecting their ancient evolution. Clock genes act within most cells of the body to produce a near 24-hour rhythmic output, and cells communicate to regulate daily rhythms in sleep-activity, feeding, hormone secretion and many other characteristics. Much less is understood about the molecular basis of longer-term timers and the mechanism by which animals respond to changing day length to synchronise seasonal rhythms. In mammals, a key aspect of seasonal timekeeping is the production of melatonin by the pineal gland. Melatonin is produced only at night and the pattern directly reflects the length of the night, and it is the changes in this hormonal signal that conveys information about time of year around the body. Long daily bouts of melatonin act as the signal for winter and short bouts for summer. Our previous work has shown that the changing pattern of melatonin is decoded through the switching on and off of specific clock genes. Notably, the melatonin increase at dusk activates Cry1 gene expression, and the melatonin decline at dawn activates Per1 gene expression. Based on this we have proposed an 'internal coincidence hypothesis' for photoperiod time measurement. This states that the extent of interaction between CRY1 and PER1 proteins, which is dictated by the period from dusk to dawn - the melatonin signal, governs the seasonal response. The daily clock mechanism has thus been co-opted for seasonal timing in melatonin-responsive cells that are located in the brain and pituitary gland. Now we plan to test this hypothesis using transgenic sheep in which the Cry1 gene is selectively neutralised. This strategy depends on our recent demonstration that modified DNA sequences can be efficiently introduced into sheep embryos and are expressed in lambs. The sheep is used as a model because of its very well characterised seasonal biology. The aim is to use two different 'transgenes' to interfere with endogenous expression of the Cry1 gene. The prediction is that this will not affect normal development, but will block photoperiodic responsiveness. The biology of this 'sheep for all seasons' will be of major interest to chronobiologists, and the use of transgenesis in sheep opens a new era in the study of genetic control in a long-lived species.