Summary N 6 -methyladenosine (m 6 A), the most prevalent internal RNA modification on mammalian messenger RNAs (mRNAs), regulates fates and functions of modified transcripts through m 6 A-specific binding proteins 1 – 5 . m 6 A is abundant in the nervous system and modulates various neural functions 6 – 11 . While m 6 A marks groups of mRNAs for coordinated degradation in various physiological processes 12 – 15 , the relevance of m 6 A in mRNA translation remains largely unknown in vivo . Here we show that, through its binding protein Ythdf1, m 6 A promotes protein synthesis of target transcripts in response to neuronal stimuli in the adult mouse hippocampus, thereby facilitating learning and memory. Mice with genetic deletion of Ythdf1 ( Ythdf1 -KO) exhibit learning and memory defects as well as impaired hippocampal synaptic transmission and long-term potentiation. Ythdf1 re-expression in the hippocampus of adult Ythdf1 -KO mice rescues behavioral and synaptic defects, while hippocampus-specific acute knockdown of Ythdf1 or Mettl3 , the catalytic component of m 6 A methyltransferase complex, recapitulates the hippocampal deficiency. Transcriptome-wide mapping of Ythdf1 binding sites and m 6 A sites on hippocampal mRNAs uncovered key neuronal genes. Nascent protein labeling and tether reporter assays in hippocampal neurons revealed that Ythdf1 enhances protein synthesis in a neuronal-stimulus-dependent manner. Collectively, our results uncover a pathway of mRNA m 6 A methylation in learning and memory, which is mediated through Ythdf1 in response to stimuli.

Summary Contusive spinal cord injury (SCI) leads to a variety of disabilities due to limited neuronal regeneration and functional plasticity. It is well established that an upregulation of glial derived chondroitin sulfate proteoglycans (CSPGs) within the glial scar and perineuronal net (PNN) creates a barrier to axonal regrowth and sprouting1–5. Protein Tyrosine Phosphatase σ (PTPσ), along with its sister phosphatase Leukocyte common Antigen-Related (LAR), and the Nogo Receptors 1 and 3 (NgR) have recently been identified as receptors for the inhibitory glycosylated side chains of CSPGs6–8. We found that PTPσ plays a critical role in converting growth cones into a dystrophic state by tightly stabilizing them within CSPG-rich substrates. We generated a membrane-permeable peptide mimetic of the PTPσ wedge domain that binds to PTPσ and relieves CSPG-mediated inhibition. Systemic delivery of this peptide over weeks restored substantial serotonergic innervation to the spinal cord below the level of injury and facilitated functional recovery of both locomotor and urinary systems. Our results add a new layer of understanding to the critical role of PTPσ in mediating the growth-inhibited state of neurons due to CSPGs within the injured adult spinal cord.

Contrary to the long-held belief that DNA methylation of terminally differentiated cells is permanent and essentially immutable, post-mitotic neurons exhibit extensive DNA demethylation. The cellular function of active DNA demethylation in neurons, however, remains largely unknown. Tet family proteins oxidize 5-methylcytosine to initiate active DNA demethylation through the base-excision repair pathway. Here, we show that synaptic activity bi-directionally regulates neuronal Tet3 expression. Functionally, knockdown of Tet or inhibition of base-excision repair in hippocampal neurons elevates excitatory glutamatergic synaptic transmission, whereas overexpressing Tet3 or Tet1 catalytic domain decreases it. Furthermore, dysregulation of Tet3 signalling prevents homeostatic synaptic plasticity. Mechanistically, Tet3 dictates neuronal surface GluR1 levels. RNA-seq analyses further revealed a pivotal role of Tet3 in regulating gene expression in response to global synaptic activity changes. Thus, Tet3 serves as a synaptic activity sensor to epigenetically regulate fundamental properties and meta-plasticity of neurons via active DNA demethylation.