Defining the molecular basis of photoperiodism in mammals

地球绕地轴的永久运动和绕太阳的轨道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，这些生物钟和日历可以预测白天和黑夜，以及季节的变化。最近对这种“时间生物学”的研究表明，有少数基因被称为时钟基因，它们产生内在的日常节奏。从昆虫到人类，这些基因在结构和功能上都表现出保守的特征，反映了它们的古老进化。生物钟基因在体内的大多数细胞中起作用，产生近24小时的节律输出，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。对于较长期计时器的分子基础，以及动物对白昼长度变化做出反应以与季节节律同步的机制，人们了解得少得多。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激素。褪黑素只在晚上产生，其模式直接反映了夜晚的长短，正是这种激素信号的变化传达了身体周围一年中的时间信息。每天长时间服用褪黑素是冬季的信号，短时间服用褪黑素是夏季的信号。我们之前的工作表明，褪黑激素的变化模式是通过特定时钟基因的开关来解码的。值得注意的是，黄昏褪黑素增加激活Cry1基因表达，黎明褪黑素下降激活Per1基因表达。在此基础上，我们提出了光周期时间测量的“内部符合假设”。这表明，CRY1和PER1蛋白之间的相互作用程度是由从黄昏到黎明这段时间（褪黑激素信号）决定的，它控制着季节性反应。因此，在大脑和脑垂体中对褪黑激素有反应的细胞中，日常时钟机制被用于季节性定时。现在我们计划用Cry1基因被选择性中和的转基因羊来验证这一假设。这一策略取决于我们最近的证明，即修饰的DNA序列可以有效地引入绵羊胚胎并在羔羊中表达。绵羊之所以被用作模型，是因为它的季节性生物学特征很好。目的是使用两种不同的“转基因”来干扰Cry1基因的内源性表达。预测这不会影响正常发育，但会阻碍光周期反应。这种“一年四季的羊”的生物学将是时间生物学家的主要兴趣，在羊身上使用转基因技术为研究长寿物种的遗传控制开辟了一个新时代。

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.