Unravelling the networks that regulate seasonal rhythmicity in the epigenome

两种相互作用的节律过程主宰着地球上大多数生物的生物学。最好的研究是生物钟，它已经进化到与地球的24小时旋转相匹配。第二个是驱动大约1年节律的年周期时钟，它在许多生命形式中进化，以应对季节性星球的深刻环境挑战。虽然这两种生物钟系统是相互关联的，但生物钟对季节性反应的精确机制尚未在任何动物物种中建立，我们也不知道长期的昼夜节律是如何产生的。我们和其他人的工作已经确定了神经内分泌系统如何使用夜间激素褪黑激素来提供外部光周期的内部表示，从而驱动季节性生殖和代谢反应。一个关键的作用部位是脑垂体，称为结节部（PT），位于紧邻下丘脑的区域，在那里，每天晚上由专门的促甲状腺激素（TSH）表达细胞中的每日褪黑激素激素信号携带局部昼夜节律基因节律。这些PT促甲状腺细胞被称为“日历细胞”。在这里，一个关键的转录辅激活因子（EYA 3）是有节奏地调节，并在长的夏季光周期（LP）强烈增强，导致PT TSH的表达，激活TSH受体和甲状腺激素（TH）代谢在相邻的下丘脑。季节性反应的TH依赖性是脊椎动物物种生物学的保守特征。我们现在已经表明，激素包装蛋白（CHGA）诱导短的冬季光周期（SP）和PT内的专门细胞翻转从CHGA到EYA 3状态（二进制开关）在circannual cycle.我们的目标是发现是否“表观遗传”过程支持这种二进制开关，以及这种开关是否驱动长期的节奏产生。表观遗传学被定义为染色体区域的结构适应，以便在不改变DNA序列的情况下记录、发出信号或保持改变的活性状态。它涉及DNA包裹的组蛋白的修饰，以及DNA核苷酸之一（胞嘧啶）的化学性质（甲基化）。我们的工作部分建立在最近测试的理论模型，由植物科学家研究春化作用，这揭示了组蛋白修饰酶复合物（polycomb阻遏物-2，PRC 2）在提供冬季寒冷暴露的记忆中的关键作用。PRC 2通过作用于组蛋白-3蛋白来抑制基因表达。PRC 2复合物（EZH 2）的一个关键元件在LP上被激活，我们将测试这是否涉及对SP表达基因（包括CHGA）的全面抑制。我们现在有令人兴奋的新证据表明，EZH 2也通过昼夜节律钟激活LP上的EYA 3。这使我们假设染色质修饰酶作为二元开关，激活LP基因，抑制SP基因。这可能是长期预测的生物化学开关机制，驱动脊椎动物的年周期。我们在细胞模型中研究了这种开关机制，并在年周期中使用了来自绵羊的细胞和组织。这将包括研究EZH 2的蛋白质修饰和伴侣是如何参与的。这将使我们能够测试这一假设，即组蛋白-3蛋白甲基化的变化（由EZH 2驱动）通过将这些映射到PT的潜在变化来驱动年周期。最后，我们将联合收割机结合我们的研究结果与现有的基因组数据集的驯化和野生绵羊品种，并建立是否遗传电路驱动的时间已经选择在驯化过程中。因此，我们的目标是解开驱动我们星球上生命季节性节奏的中心途径。

Two interacting rhythmical processes dominate the biology of most organisms on earth. The best studied is the circadian clock, which this has evolved to match the 24h rotation of earth. The second is the circannual clock driving ca 1-year rhythms, which has evolved in many life-forms to meet the profound environmental challenges of a seasonal planet. Although the two clock-work systems are interlocked, the precise mechanisms by which the circadian clock contributes to a seasonal response are yet to be established in any animal species, nor do we know how long-term circannual rhythm generation occurs.The seasonal and circannual timing mechanisms have been studied in sheep. Our work and others has defined how the nocturnal hormone melatonin is used by the neuroendocrine system to provide an internal representation of external photoperiod, driving seasonal reproductive and metabolic responses. A key site of action is the pituitary gland, called pars tuberalis (PT), in a region immediately adjacent to the hypothalamus, where a local circadian clock-gene rhythm is entrained each night by the daily melatonin hormone signal in specialized thyroid-stimulating hormone (TSH) expressing cells. These PT thyrotrophs have been termed "calendar cells". Here, a key transcriptional co-activator (EYA3) is rhythmically regulated, and on long summer photoperiods (LP) is strongly augmented, leading to expression of PT TSH, which activates TSH-receptors and thyroid hormone (TH) metabolism in the adjacent hypothalamus. The TH-dependency for the seasonal response is a conserved feature of the biology of vertebrate species. We have now shown that hormone packaging protein (CHGA) is induced on short winter photoperiods (SP) and that specialised cells within the PT flip from a CHGA to an EYA3 state (binary switching) over the circannual cycle.Our goal is to discover whether an "epigenetic" process underpins this binary switch and whether this switch drives long-term rhythm generation. Epigenetics is defined as the structural adaptation of chromosomal regions so as to register, signal or perpetuate altered activity states without changes to DNA sequence. It involves modification of histone proteins around which DNA is wrapped, and also the chemical nature (methylation) of one of the DNA nucleotides (cytosine). Our work builds in part on recently tested theoretical models developed by plant scientists studying vernalisation, which has revealed a critical role for histone-modifying enzyme complex (polycomb repressor-2, PRC2) in providing a memory of winter cold-exposure. PRC2 represses gene expression by acting on histone-3 proteins. A key element of the PRC2 complex (EZH2) is activated on LP, and we will test whether this is involved in a global suppression of SP-expressed genes, including CHGA. We now have exciting new evidence that EZH2 also activates EYA3 on LP, via the circadian clock. This leads us to the hypothesis that chromatin modifying enzymes act as binary switches, activating LP-genes, and suppressing SP-genes. This could be the long-predicted biochemical switch mechanism driving circannual cycles in vertebrates. We investigate this switch mechanism in cell models, and also using cells and tissues from sheep over the circannual cycle. This will include studies of how protein modifications and partners of EZH2 are involved. This will allow us to test this hypothesis that changes in histone-3 protein methylation (driven by EZH2) drives the circannual cycle by mapping these to underlying changes the PT. Finally, we will combine our findings with existing genomic data sets for domesticated and wild sheep breeds, and establish whether genetic circuits driving timing have been selected in course of domestication. Thus, we aim to unravel the central pathways driving the seasonal rhythm of life on our planet.