*Review* **Emerging Roles of N6-Methyladenosine Modification in Neurodevelopment and Neurodegeneration**

**Liqi Shu 1,†, Xiaoli Huang 2,3,†, Xuejun Cheng 2,3 and Xuekun Li 2,3,4,5,\***

**Abstract:** N6-methyladenosine (m6A), the most abundant modification in messenger RNAs (mRNAs), is deposited by methyltransferases ("writers") Mettl3 and Mettl14 and erased by demethylases ("erasers") Fto and Alkbh5. m6A can be recognized by m6A-binding proteins ("readers"), such as Yth domain family proteins (Ythdfs) and Yth domain-containing protein 1 (Ythdc1). Previous studies have indicated that m6A plays an essential function in various fundamental biological processes, including neurogenesis and neuronal development. Dysregulated m6A modification contributes to neurological disorders, including neurodegenerative diseases. In this review, we summarize the current knowledge about the roles of m6A machinery, including writers, erasers, and readers, in regulating gene expression and the function of m6A in neurodevelopment and neurodegeneration. We also discuss the perspectives for studying m6A methylation.

**Keywords:** N6-methyladenosine; Mettl3; Mettl14; Fto; Ythdf1; neurodevelopment; neurodegeneration

#### **1. Introduction**

Epigenetics refers to the heritable changes in gene expression and cell state caused by some specific mechanisms, aside from the occurrence of potential genetic sequences. More than 170 types of RNA modifications, including N6-methyladenosine (m6A), 5 methylcytidine (m5C), N1-methyladenosine (m1A), and N7-methylguanosine (m7G), have been identified in mammalian transcripts, and the most abundant internal RNA modification is N6-methyladenosine (m6A) [1,2]. m6A is installed by methyltransferases (writers), removed by demethylases (erasers), and recognized by m6A binding proteins (readers). Methyltransferase-like 3 (Mettl3) and methyltransferase-like 14 (Mettl14) form the core of the methyltransferase complex; AlkB homolog 5 protein (Alkbh5) and Fat mass and obesity-associated protein (Fto) are identified as demethylases; YTH domain family proteins (Ythdf1, Ythdf2, Ythdf3) and YTH domain-containing family protein 1 (Ythdc1) are essential reader proteins.

m6A modification is precisely catalyzed by a multi-subunit methyltransferase enzyme complex containing Mettl3, Mettl14, and other accessory components such as Wilms tumor 1-associated protein (Wtap), a mammalian splicing factor [3]. Mettl3 has catalytic activity, while Mettl14 acts as the RNA-binding platform and facilitates the recognition of Mettl3 [4]. Mettl3 and Mettl14 form heterodimers, which interact with Wtap. Wtap does not possess any methylation activity but interacts with Mettl3 and Mettl14 and promotes the recruitment of the Mettl3–Mettl14 complex to target transcripts [5]. The presence of m6A modification induces the preferential binding of certain proteins, i.e., m6A readers,

**Citation:** Shu, L.; Huang, X.; Cheng, X.; Li, X. Emerging Roles of N6-Methyladenosine Modification in Neurodevelopment and Neurodegeneration. *Cells* **2021**, *10*, 2694. https://doi.org/10.3390/ cells10102694

Academic Editors: FengRu Tang and Cord Brakebusch

Received: 18 August 2021 Accepted: 29 September 2021 Published: 9 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Ythdf family proteins, and Ythdc1. In addition, m6A modification is reversible and can be removed by demethylases, including Fto and Alkbh5. Therefore, m6A machinery consists of multiple components that have diverse functions and make the field colorful (Figure 1). (ADHD), and intellectual disability [19,21–24]. In this review, we summarize the recent findings regarding the function and biological consequences of m6A modification in the neural system, from neural development to brain function and neurological disorders.

any methylation activity but interacts with Mettl3 and Mettl14 and promotes the recruitment of the Mettl3–Mettl14 complex to target transcripts [5]. The presence of m6A modification induces the preferential binding of certain proteins, i.e., m6A readers, Ythdf family proteins, and Ythdc1. In addition, m6A modification is reversible and can be removed by demethylases, including Fto and Alkbh5. Therefore, m6A machinery consists of multiple compo-

m6A-specific methylated RNA immunoprecipitation (MeRIP) with next-generation sequencing data has revealed that m6A is non-randomly distributed in mRNAs but is especially enriched at the 5′ and 3′ UTRs [6,7]. m6A has been shown to impact RNA metabolism, including mRNA stability, translation, splicing, and localization; consequently, m6A regulates gene expression and involves diverse biological processes [2,8]. Present findings show that m6A modulates brain function [9,10] and regulates neurogenesis [11– 18], brain development [7,17–19], axon regeneration [20], and learning and memory [13,15]. The dysregulation of m6A has been found in a set of neurological disorders, such as Alzheimer's disease, Fragile X syndrome, attention-deficit/hyperactivity disorder

*Cells* **2021**, *10*, x 2 of 10

nents that have diverse functions and make the field colorful (Figure 1).

**Figure 1.** Schematic illustration of m6A modification. m6A methylation is catalyzed by the methyltransferase complex containing Mettl3, Mettl14, and an adaptor protein, such as WTAP. Fto and Alkbh5 can function as demethylases, and Yth family proteins can recognize m6A sites. m6A modification in mammals is presented on the consensus sequence DRACH (D = A/G/U, R = A/G, H = A/C/U). Reversible m6A modification plays important roles in regulating RNA metabolism, including RNA splicing, nuclear export, translation, and degradation in the specific context.  **Figure 1.** Schematic illustration of m6A modification. m6A methylation is catalyzed by the methyltransferase complex containing Mettl3, Mettl14, and an adaptor protein, such as WTAP. Fto and Alkbh5 can function as demethylases, and Yth family proteins can recognize m6A sites. m6A modification in mammals is presented on the consensus sequence DRACH (D = A/G/U, R = A/G, H = A/C/U). Reversible m6A modification plays important roles in regulating RNA metabolism,including RNA splicing, nuclear export, translation, and degradation in the specific context.

**2. m6A and Neurogenesis**  *2.1. Writers*  During embryonic neurogenesis, Mettl14 displays the highest expression in radial glia cells, and *Mettl14* knockout (KO) in embryonic mouse brains extends the cell cycle of radial glia cells and induces aberrant cortical neurogenesis. Similar defects were induced by Mettl3 knockdown [11]. Mettl14 also regulates the cell cycle of human cortical neuronal progenitor cells [11]. The deletion of *Mettl14* in embryonic neural stem cells (eNSCs) led to a remarkable decrease in proliferation and immature differentiation in vitro and in vivo [16]. In addition, *Mettl3* knockdown reduced the proliferation and skewed the differentiation of adult neural stem cells (aNSCs) towards neuronal lineage, while the newborn m6A-specific methylated RNA immunoprecipitation (MeRIP) with next-generation sequencing data has revealed that m6A is non-randomly distributed in mRNAs but is especially enriched at the 50 and 30 UTRs [6,7]. m6A has been shown to impact RNA metabolism, including mRNA stability, translation, splicing, and localization; consequently, m6A regulates gene expression and involves diverse biological processes [2,8]. Present findings show that m6A modulates brain function [9,10] and regulates neurogenesis [11–18], brain development [7,17–19], axon regeneration [20], and learning and memory [13,15]. The dysregulation of m6A has been found in a set of neurological disorders, such as Alzheimer's disease, Fragile X syndrome, attention-deficit/hyperactivitydisorder (ADHD), and intellectual disability [19,21–24]. In this review, we summarize the recent findings regarding the function and biological consequences of m6A modification in the neural system, from neural development to brain function and neurological disorders.