The mRNA cap epitranscriptome: Understanding an essential novel layer of gene expression in neuronal differentiation and function

生命的信息被编码在我们染色体中隐藏的基因的DNA中。染色体上的DNA是一条很长的链，由四种不同的核苷酸组成：G、a、C和t。对于大多数基因来说，这些代码随后被转化为信使RNA中间体（mRNA）， mRNA具有帽状结构和poly - a尾部，以保护它不被降解。然后，这种mRNA在细胞质中被翻译成一种叫做蛋白质的氨基酸链，它完成一种功能；例如，从我们吃的营养物质中产生能量的酶促反应，允许大脑神经元之间的电通信。虽然mRNA序列仅由4个核苷酸组成，但许多核苷酸可以通过添加小的化学基团来修饰，以增加调控组合和编码能力。mRNA中最显著的修饰是在帽状结构附近的核苷酸上添加甲基。包括人类在内的动物有两种添加这些修饰的帽甲基转移酶（CMTrs）。此外，许多寄生虫在其基因组中都有一个CMTr基因，这是它们繁殖所必需的。在小鼠中，CMTrs是神经元发育所必需的，然而，CMTrs的生物学功能和mRNA帽修饰在很大程度上仍未得到解释。我们最近发现，缺乏这两种cmtr的突变果蝇是可以存活的，尽管它们患有神经和学习缺陷。有趣的是，我们进一步发现，在这些突变果蝇中，mrna不能正确地转运到突触，而突触是信号传递给邻近神经元的地方。特别是，我们可以证明一些mrna只在突触上形成蛋白质。因此，帽修饰在指导突触局部合成新蛋白质方面起着至关重要的作用，这表明这一过程是学习新关联所必需的，然后作为记忆储存在大脑中。然而，我们目前还不知道哪些基因在突触中以这种方式表达，也不知道将mrna引导到突触进行局部表达的序列代码是什么。我们现在有了理想的动物模型来解决这个非常基本的问题，即这种神秘的修饰是如何将基因的局部表达直接导向突触的。我们的初步数据表明，在不同的动物和条件下，帽的变化是不同的。由于CMTrs也定位于突触，我们的数据表明一个动态代码对局部蛋白质合成很重要。在破解这个密码的第一步中，我们将识别定位于突触的特定mrna，使我们能够建立一个报告系统来测试这个密码。为了补充这一分析，我们将进一步确定生化分析中CMTr的序列偏好，并鉴定对CMTr特异性和帽修饰编码解码重要的蛋白质。这些研究对于理解帽修饰在基因表达调控中的重要功能以及其异常调控如何导致人类神经系统缺陷，或如何被利用来干扰病毒复制（如SARS-CoV-2）至关重要。

The information for life is encoded in the DNA of the genes harboured in our chromosomes. The DNA in a chromosome is a very long chain consisting of four different nucleotides: G, A, C and T. For most genes this code is then converted into a messenger RNA intermediate (mRNA) that has a cap structure and a polyA tail to protect it from degradation. This mRNA is then translated in the cytoplasm into a chain of amino acids called proteins, which fulfil a function; for example an enzymatic reaction to generate energy from the nutrients we eat to allow for the electrical communication among neurons in our brain. Although the sequence of mRNA only consist of four nucleotides, many can be modified by addition of small chemical groups to increase the regulatory portfolio and coding capacity. The most prominent modification in mRNA are methyl groups added to the nucleotides adjacent to the cap structure. Animals including humans have two cap methyltransferase enzymes (CMTrs) that add these modifications. Also many parasites have a CMTr gene in their genome that is required for their propagation. In mice, CMTrs are essential and required for neuronal development, however, the biological functions of CMTrs and the mRNA cap modifications remain largely unexplained.We recently discovered that mutant Drosophila lacking both CMTrs are viable, although they suffer from neurological and learning defects. Intriguingly, we further discovered that in these mutant flies, mRNAs were not properly transported to synapses, which are the sites where signals are transmitted to neighbouring neurons. In particular, we could show that some mRNAs are only made into protein at synapses. Hence, the cap modifications have an essential role in directing the synthesis of new proteins locally at synapses suggesting that this process is required for learning of new associations, that are then stored as memory in the brain. However, we currently do not know which genes are expressed in this way at synapses nor what the sequence code is to direct mRNAs to synapses for localized expression.We now have the ideal animal model to address the very fundamental questions about how this enigmatic modifications direct local expression of genes to synapses. Our preliminary data indicate that the cap modifications vary between different animals and conditions. Since CMTrs also localize to synapses, our data suggest a dynamic code important for local protein synthesis. In a first step to crack this code, we will identify specific mRNAs that localize to synapses allowing us to build a reporter system to test the code. To complement this analysis we will further determine the sequence preferences of CMTrs in biochemical assays and identify proteins important for CMTr specificity and decoding of the cap modification code. These studies are essential to understand the vital function of the cap modifications in the regulation of gene expression and how its aberrant regulation can lead to neurological defects in humans, or can be exploited to interfere with viral replication such as in SARS-CoV-2.