METTL3-Regulated m6A Epitranscriptome Plasticity in Pathological Angiogenesis
Authors: Susmita Sahoo
Journal: Molecular Therapy (2020)

In recent years, there have been major advances in our knowledge of the biochemistry and molecular regulations of the N6-methyladenosine (m6A) RNA epitranscriptome in diverse physiological processes. Nevertheless, the functional significance of m6A in the context of pathological manifestations is heavily understudied, impeding our understanding of the role of m6A in disease processes. Particularly, there are very few investigations of the regulatory mechanisms of m6A in physiological and pathological angiogenesis, the process of formation of new blood vessels, which plays a critical role in development, tissue ischemia, diabetes, and cancer. In this issue of Molecular Therapy, Yao et. al.1Yao M.D. Jiang Q. Ma Y. Liu C. Zhu C.Y. Sun Y.N. et al.Role of METTL3-Dependent N6-Methyladenosine mRNA Modification in the Promotion of Angiogenesis.Mol. Ther. 2020; 28 (this issue): 2191-2202Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar added a new dimension to this remarkable area of research to further our understanding of the epitranscriptomic mechanisms of pathological angiogenesis. The authors investigated the functional role of the m6A-writer METTL3-regulated m6A-methylome in endothelial cell (EC) angiogenesis in vitro and in mouse retinas in vivo using an EC-specific knock-out (KO) mouse model. m6A was found to be elevated in the transcriptome of ECs and mouse retinas following hypoxic stress. The authors attributed this increase to elevated METTL3 levels. Depletion of METTL3 decreased endothelial cell angiogenic ability, including viability, proliferation, migration and tube formation in vitro. Post-synthesis, RNA molecules undergo many different modifications that have the potential to alter their function and stability. m6A is one of the more prevalent, functionally relevant, and better characterized internal modification in mRNA. m6A is known to be actively regulated and has emerged as an essential and widespread regulatory mechanism controlling gene expression post-transcriptionally. The functions of m6A methyltransferases (writers), demethylases (erasers), and specific binding proteins (readers) have been studied extensively in a range of systems, revealing regulatory roles in a context-dependent manner. METTL3 is a methyltransferase that acts as the main catalytic subunit in a complex with METTL14 (m6A writer complex) to deposit m6A, which is responsible for the vast majority of m6A on mRNAs. In this new study, the authors demonstrate that EC-specific deletion of METTL3 decreases the vascular area and pathological neovascular tufts in two models in vivo: an oxygen-induced retinopathy model and an alkali-burn-induced corneal neovascularization model. Interestingly, METTL3 silencing resulted in aberrant Wnt signaling, leading to decreased levels of pathological neovascularization in oxygen-induced proliferative retinopathy. Mechanistically, m6A target genes LRP6 and DVL1 are dysregulated by the m6A reader protein YTH-m6A RNA-binding protein1 (YTHDF1) to affect Wnt signaling. The authors observe that intervention of Wnt signaling via regulating m6A methylation is critical for vascular homeostasis. The authors have concluded that "enhanced m6A RNA methylation contributes to the progression of pathological angiogenesis, whereas decreased m6A methylation alleviates vascular dysfunction, highlighting the importance of m6A methylation in vascular homeostasis." Findings from this article align with the observations that high m6A is detected in cancerous tissues with high levels of hypoxia-induced angiogenesis. In the same line, METTL3 levels were found to be elevated in lung adenocarcinoma cell lines compared to healthy tissues, whereas its depletion led to inhibition of cancer cell growth, decreased viability, and increased apoptosis. High expression of METTL3 in cancer cells is known to promote translation of several oncogenes. Contrasting reports show that the m6A writer complex has important oncogenic and tumor suppressor roles in different cancer types. The controversial role of this complex is poorly understood and requires further investigation. Interestingly, previous studies have demonstrated a critical role of METTL3 in endothelial cells in hematopoietic stem and progenitor cell development during mouse and zebrafish embryogenesis via modulating the Notch signaling pathway. Endothelial expression of METTL3 methylates Notch1 mRNA near the stop codon, leading to repression of Notch activity in a YTHDF2-dependent manner, during endothelial-to-hematopoietic transition.2Zhang C. Chen Y. Sun B. Wang L. Yang Y. Ma D. Lv J. Heng J. Ding Y. Xue Y. et al.m6A modulates haematopoietic stem and progenitor cell specification.Nature. 2017; 549: 273-276Crossref PubMed Scopus (223) Google Scholar While the data presented in the current article have demonstrated a clear role for m6A, the authors did not address certain inconsistencies with the existing literature. For example, the authors observed a significant increase in only METTL3 expression following oxygen stress, but, unlike other studies, the expression of other key components of the writer complex, METTL14 and WTAP, remained unchanged. m6A deposits on primary mRNAs are known to be added by the essential proteins of the writer complex, which comprises METTL3 (catalytic subunit), WTAP (a key METTL3 adaptor), and METTL14 (enhancer). Mechanistically, it is not clear how the deposits of m6A on mRNAs are affected when the expression of one of the proteins in the complex is elevated under oxygen stress (METTL3 in this case) without changes to the expression of the other components. There is no obvious compensative effect between METTL3 and METTL14 that has been reported; therefore, it is not clear how this disbalance subsequently regulates the expression of proteins. Interestingly, contrary to the authors' observations, it has been shown that expression of METTL14 and FTO (an eraser) changes under hypoxic stress in other cell types.3Mathiyalagan P. Adamiak M. Mayourian J. Sassi Y. Liang Y. Agarwal N. Jha D. Zhang S. Kohlbrenner E. Chepurko E. et al.FTO-Dependent N6-Methyladenosine Regulates Cardiac Function During Remodeling and Repair.Circulation. 2019; 139: 518-532Crossref PubMed Scopus (118) Google Scholar Moreover, the expression of other m6A regulators, especially METTL14 and ALKBH5 (m6A erasers), has been shown to be affected in METTL3-depleted cells and tissues. It would be interesting to measure the expression of other m6A regulators (FTO, ALKBH5, WTAP, and METTL14) in METTL3-depleted or -deleted cells and tissues so as to obtain a comprehensive picture of the gene regulation. Mapping epitranscriptomic changes in concert with changes in the proteome and functional outcomes in response to external stress such as hypoxia and ischemia has not been performed comprehensively. Discoveries identifying m6A-binding proteins and the m6A-dependent proteome would be critical for our understanding of the functional regulation of the epitranscriptome in angiogenesis under pathological conditions. Angiogenesis induces a critical recovery mechanism after ischemia, especially in wound healing and cardiovascular disease. However, it can also contribute to plaque instability in atherosclerosis, diabetic retinopathy, and tumor growth in cancer. Maintaining a tight balance in the regulation of angiogenesis is therefore imperative. In the future, we can expect several new therapeutic approaches to target a broad array of conditions, including various types of cancer, neurological, and eye diseases and ischemic conditions, based on the epitranscriptomic regulations of angiogenesis. These studies will also yield promising unique targets in therapeutic angiogenesis and provide us with tools to direct and regulate the process. These newly discovered m6A mechanisms from a so-far-hidden layer of gene regulation will also highlight the remaining questions and would provide hints on novel research avenues and potential therapeutic applications. The author declares no competing interests. I thank Samantha Osinski for her help with edits and discussion. This work was supported in part by grants from the National Institutes of Health ( NIH-NHLBI ) grants R01HL140469 , R01HL124187 , R01HL148786 , R01HL145644-01 , and New York State Stem Cell Science (NYSTEM) C32562GG grant to S.S. Role of METTL3-Dependent N6-Methyladenosine mRNA Modification in the Promotion of AngiogenesisYao et al.Molecular TherapyJuly 25, 2020In BriefThe targeting of hypoxia-induced angiogenesis is a promising strategy for treating ischemic diseases. This study reported that the RNA methyltransferase, METTL3, is dysregulated upon hypoxic stress. METTL3 silencing inhibits endothelial angiogenic effects in vitro and pathological angiogenesis in vivo. METTL3 may serve as a therapeutic target for ischemic diseases. Full-Text PDF Open Archive