**3. m6A and Neural Development**

m6A is abundant in the mammalian brain transcriptome, relative to other organs, and more than 25% of human transcripts are m6A-modified [6,7,33]. During embryonic and postnatal brain development, m6A displays temporal and spatial features, and specific m6A modification sites are present in transcripts across brain regions [6,11,21], which suggests an important role of m6A in neural development. Conditional deletion of *Mettl14* led to smaller sizes of newborn pups, and all died before postnatal day 25 (P25) [11]. Mettl14 cKO pups showed enlarged ventricles, delayed depletion of PAX6<sup>+</sup> radial glial cells, a type of neural stem cells, and prolonged cell-cycle progression [11]. Similar phenotypes were also observed in the brains of embryonic mice with *Mettl3* knockdown [11]. m6A sequencing showed that transcripts with m6A modification were related to the cell cycle and neuronal differentiation [11]. In addition, during the postnatal cerebellum development, the global level of m6A decreases from P7 to P60, and m6A is developmentally/temporally modulated [18]. Specific m6A peaks at P7 were close to stop codon regions, whereas P60-specific m6A peaks were near start codons [18]. *Mettl3* deficiency induces embryonic

lethal effects, and the acute knockdown and specific ablation of *Mettl3* both induced remarkable cortical and cerebellar defects, including a reduced number of Purkinje cells and the increased apoptosis of cerebellar granule cells [17,18].

*Fto*-deficient mice showed a decreased body weight compared to control mice, and the sizes of whole and distinct brain regions were also decreased remarkably [15]. In contrast to control mice, which exhibited locomotor activity induced by cocaine, *Fto*deficient mice significantly lost their response to cocaine [34]. Mechanistically, Fto can also demethylase mRNAs involved in dopamine signaling, including Ped1b, Girk2, and Syn1; consequently, Fto can alter dopamine midbrain circuitry [34]. *Alkbh5*-knockout mice also showed drastically smaller cerebella and reduced mature neurons [18]. Collectively, these findings highlight the critical function of m6A in neural development.
