N(6)-Methyladenosine mRNA Modification: From Modification Site Selectivity to Neurological Functions

[Image: see text] The development of various chemical methods has enabled scientists to decipher the distribution features and biological functions of RNA modifications in the past decade. In addition to modifying noncoding RNAs such as tRNAs and rRNAs, N(6)-methyladenosine (m(6)A) has been proven to be the most abundant internal chemical modification on mRNAs in eukaryotic cells and is also the most widely studied mRNA modification to date. Extensive studies have repeatedly demonstrated the important functions of m(6)A in various biological conditions, ranging from embryonic organ development to adult organ function and pathogenesis. Unlike DNA methylation which is relatively stable, the reversible m(6)A modification on mRNA is highly dynamic and easily influenced by various internal or external factors, such as cell type, developmental stage, nutrient supply, circadian rhythm, and environmental stresses. In this Account, we review our previous findings on the site selectivity mechanisms regulating m(6)A formation, as well as the physiological roles of m(6)A modification in cerebellum development and long-term memory consolidation. In our initial efforts to profile m(6)A in various types of mouse and human cells, we surprisingly found that the sequence motifs surrounding m(6)A sites were often complementary with the seed sequences of miRNAs. By manipulating the abundance of the miRNA biogenesis enzyme Dicer or individual miRNAs or mutating miRNA sequences, we were able to reveal a new role of nucleus localized miRNAs, which is to guide the m(6)A methyltransferase METTL3 to bind to mRNAs and to promote m(6)A formation. As a result, we partially answered the question of why only a small proportion of m(6)A motifs within an mRNA could have m(6)A modification at a certain time point. We further explored the functions of m(6)A modification in regulating brain development and brain functions. We found that cerebellum had the most severe defects when Mettl3 was knocked out in developing mouse embryonic brain and revealed that the underlying mechanisms could be attributed to aberrant mRNA splicing and enhanced cell apoptosis under m(6)A deficit conditions. On the other hand, knocking out Mettl3 in postnatal hippocampus did not cause morphological defects in the mouse brain but impaired the efficacy of long-term memory consolidation. Under learning stimuli, formation of m(6)A modifications could be detected on transcripts encoding proteins related to dendrite growth, synapse formation, and other memory related functions. Loss of m(6)A modifications on these transcripts would result in translation deficiency and reduced protein production, particularly in the translation of early response genes, and therefore would compromise the efficacy of long-term memory consolidation. Interestingly, excessive training sessions or increased training intensity could overcome such m(6)A deficiency related memory defects, which is likely due to the longer turnover cycle and the cumulative abundance of proteins throughout the training process. In addition to revealing the roles of m(6)A modification in regulating long-term memory formation, our work also demonstrated an effective method for studying memory formation efficacy. As the lack of an appropriate model for studying memory formation efficacy has been a long-lasting problem in the field of neural science, our hippocampus-specific postnatal m(6)A knockout model could also be utilized to study other questions related to memory formation efficacy.