Role of hepatic Mettl14 pathways in liver metabolism and body metabolic homeostasis

信使RNA介导遗传信息向蛋白质合成和细胞功能的转化。翻译能力强的mRNA水平由RNA转录、转录后修饰和RNA与蛋白质的相互作用决定。对转录的遗传和表观遗传调控进行了广泛的研究；相比之下，我们对mRNA修饰、RNA-蛋白质相互作用及其对细胞反应的影响的理解存在差距。N6-甲基腺苷(M6A)是主要的信使核糖核酸，由催化亚单位(METTL3)/结构亚单位(Mettl14)甲基转移酶复合体催化。M6A基序被Ythdc1或相关的RNA结合蛋白识别并结合，这些蛋白反过来控制靶mRNAs的前mRNA剪接、核输出、细胞内定位、衰变和/或翻译效率。METTL3、Mettl14或Ythdc1的全局缺失导致胚胎死亡，揭示了这些表位转录调节因子在发育和生存中的重要作用。肝脏是人体不可缺少的代谢器官。肝脏疾病，包括非酒精性脂肪性肝病(NAFLD)和代谢功能障碍，是导致死亡和发病率的主要原因。然而，基于Mettl14/Ythdc1的表位转录程序尚未在肝脏中被探索。在初步研究中，我们培育了成年发病的、肝细胞特异性的Mettl14和Ythdc1基因敲除小鼠。消融肝脏Mettl14可减轻高脂饮食(HFD)引起的高血糖、糖耐量减低和肝脏脂肪变性。同样，肝细胞特异性Ythdc1基因的缺失也改善了高脂饲料喂养的小鼠的葡萄糖代谢。在肝脏切片培养中，缺乏Mettl14会钝化胰高血糖素刺激的肝脏葡萄糖产生。RNA-seq分析表明，肝脏Mettl14基因的缺失改变了许多参与糖/脂代谢和信号通路的蛋白的转录水平。基于这些结果，我们假设Mettl14在编码肝脏糖异生、脂肪生成和相关信号通路的介体/调节物的mRNAs中介导m6A甲基化。Ythdc1与m6A基序结合，控制前mRNA剪接、核输出和/或Mettl14底物子集的降解。此外，肥胖相关因素增加了肝脏Mettl14和Ythdc1的表达和稳定性，从而引发了基于Mettl14/Ythdc1的肝脏代谢的表位转录重编程。Mettl14/m6A/Ythdc1-诱导的表位转录重编程为肥胖相关的NAFLD和2型糖尿病提供了一种新的机制。目的1确定肝脏Mettl14是否通过表位转录机制直接促进肝脏葡萄糖生成和肝脏脂肪变性。目的2确定Ythdc1是否介导Mettl14在肝脏中的代谢作用。目的3确定Mettl14/Ythdc1诱导的表位转录重编程是否介导肥胖相关的肝病。该项目的结果有望建立一个新的基于Mett14/Ythdc1的表位转录重新编程范例，管理健康和疾病中的肝脏代谢。

Messenger RNA mediates translation of genetic information into protein synthesis and cell functions. Levels of translationally-competent mRNA are determined by RNA transcription, post-transcriptional modifications, and RNA interactions with proteins. Genetic and epigenetic regulations of transcription have been extensively investigated; by contrast, there is a gap in our understanding of mRNA modifications, RNA-protein interactions, and their impacts on cellular responses. N6-methyladenosine (m6A) is the predominant mRNA modification, and is catalyzed by a Mettl3 (catalytic subunit)/Mettl14 (structural subunit) methyltransferase complex. M6A motif is recognized by and bound to Ythdc1 or related RNA-binding proteins, which in turn control pre-mRNA splicing, nuclear export, intracellular localization, decay, and/or translational efficiency of target mRNAs. Global deletion of Mettl3, Mettl14, or Ythdc1 results in embryonic death, revealing the essential role of these epitranscriptomic mediators in development and survival. Liver is an essential metabolic organ. Liver disease, including nonalcoholic fatty liver disease (NAFLD) and metabolic dysfunctions, is a main cause for mortality and morbidity. However, Mettl14/Ythdc1-based epitranscriptomic programs have not been explored in the liver. In the preliminary study, we generated adult-onset, hepatocyte-specific Mettl14 and Ythdc1 knockout mice. Ablation of hepatic Mettl14 attenuated high fat diet (HFD)-induced hyperglycemia, glucose intolerance, and liver steatosis. Likewise, hepatocyte-specific deletion of Ythdc1 also improved glucose metabolism in HFD-fed mice. In liver slice cultures, Mettl14 deficiency blunted glucagon-stimulated liver glucose production. RNA-seq analysis showed that deletion of hepatic Mettl14 changed levels of many liver mRNA transcripts encoding proteins involved in glucose/lipid metabolism and signaling pathways. Based on these results, we hypothesize that Mettl14 mediates m6A methylation in mRNAs encoding mediators/modulates for hepatic gluconeogenesis, lipogenesis, and related signaling pathways. Ythdc1 binds to m6A motif, and controls pre-mRNA splicing, nuclear export, and/or degradation of a subset of Mettl14 substrates. Moreover, obesity-related factors increase expression and stability of hepatic Mettl14 and Ythdc1, thereby eliciting Mettl14/Ythdc1-based epitranscriptomic reprogramming of liver metabolism. Mettl14/m6A/Ythdc1-elicited epitranscriptomic reprogramming provides a new mechanism underlying obesity-associated NAFLD and type 2 diabetes. Aim 1 determines whether hepatic Mettl14 directly promotes liver glucose production and liver steatosis by an epitranscriptomic mechanism. Aim 2 determines whether Ythdc1 mediates the metabolic action of Mettl14 in the liver. Aim 3 determines whether Mettl14/Ythdc1-elicited epitranscriptomic reprogramming mediates obesity- associated liver disease. The outcomes of this project are expected to establish a new Mett14/Ythdc1-based epitranscriptomic reprogramming paradigm governing liver metabolism in health and disease.