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

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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.