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.

N-methyladenosine (mA) regulates mRNA metabolism and translation, serving as an important source of post-transcriptional regulation. To date, the functional consequences of mA deficiency within the adult brain have not been determined. To achieve mA deficiency, we deleted Mettl14, an essential component of the mA methyltransferase complex, in two related yet discrete mouse neuronal populations: striatonigral and striatopallidal. Mettl14 deletion reduced striatal mA levels without altering cell numbers or morphology. Transcriptome-wide profiling of mA-modified mRNAs in Mettl14-deleted striatum revealed downregulation of similar striatal mRNAs encoding neuron- and synapse-specific proteins in both neuronal types, but striatonigral and striatopallidal identity genes were uniquely downregulated in each respective manipulation. Upregulated mRNA species encoded non-neuron-specific proteins. These changes increased neuronal excitability, reduced spike frequency adaptation, and profoundly impaired striatal-mediated behaviors. Using viral-mediated, neuron-specific striatal Mettl14 deletion in adult mice, we further confirmed the significance of mA in maintaining normal striatal function in the adult mouse.

The c-myc mRNA coding region determinant-binding protein (CRD-BP) was first identified as a masking protein that stabilizes c-myc mRNA in a cell-free mRNA degradation system. Thus, CRD-BP is thought to promote cell proliferation by maintaining c-Myc at critical levels. CRD-BP also appears to be an oncofetal protein, based upon its expression during mammalian development and in some tumors. By using K562 leukemia cells as a model, we show that CRD-BP gene silencing by RNA interference significantly promoted proliferation, indicating an inhibitory effect of CRD-BP on proliferation. Unexpectedly, CRD-BP knockdown had no discernible effect on c-myc mRNA levels. CRD-BP is also known as insulin-like growth factor II (IGF-II) mRNA-binding protein-1. It has been reported to repress translation of a luciferase reporter mRNA containing an IGF-II 5 -untranslated region known as leader 3 but not one containing IGF-II leader 4. CRD-BP knockdown markedly increased IGF-II mRNA and protein levels but did not alter translation of luciferase reporter mRNAs containing 5 -untranslated regions consisting of either IGF-II leader 3 or leader 4. Addition of antibody against IGF-II to cell cultures inhibited the proliferative effect of CRD-BP knockdown, suggesting that regulation of IGF-II gene expression, rather than c-myc mRNA levels, mediates the proliferative effect of CRD-BP knockdown. Thus, we have identified a dominant function for CRD-BP in cell proliferation of human K562 cells, involving a possible IGF-II-dependent mechanism that appears independent of its ability to serve as a c-myc mRNA masking protein.

The most ventral structure of the developing neural tube, the floor plate (FP), differs in neurogenic capacity along the neuraxis. The FP is largely non-neurogenic at the hindbrain and spinal cord levels, but generates large numbers of dopamine (mDA) neurons at the midbrain levels. Wnt1, and other Wnts are expressed in the ventral midbrain, and Wnt/beta catenin signaling can at least in part account for the difference in neurogenic capacity of the FP between midbrain and hindbrain levels. To further develop the hypothesis that canonical Wnt signaling promotes mDA specification and FP neurogenesis, we have generated a model wherein beta–catenin is conditionally stabilized throughout the FP. Here, we unambiguously show by fate mapping FP cells in this mutant, that the hindbrain and spinal cord FP are rendered highly neurogenic, producing large numbers of neurons. We reveal that a neurogenic hindbrain FP results in the altered settling pattern of neighboring precerebellar neuronal clusters. Moreover, in this mutant, mDA progenitor markers are induced throughout the rostrocaudal axis of the hindbrain FP, although TH+ mDA neurons are produced only in the rostral aspect of rhombomere (r)1. This is, at least in part, due to depressed Lmx1b levels by Wnt/beta catenin signaling; indeed, when Lmx1b levels are restored in this mutant, mDA are observed not only in rostral r1, but also at more caudal axial levels in the hindbrain, but not in the spinal cord. Taken together, these data elucidate both patterning and neurogenic functions of Wnt/beta catenin signaling in the FP, and thereby add to our understanding of the molecular logic of mDA specification and neurogenesis.

The floor plate (FP), a ventral midline structure of the developing neural tube, has differential neurogenic capabilities along the anterior-posterior axis. The midbrain FP, unlike the hindbrain and spinal cord floor plate, is highly neurogenic and produces midbrain dopaminergic (mDA) neurons. Canonical Wnt/beta–catenin signaling, at least in part, is thought to account for the difference in neurogenic capability. Removal of beta–catenin results in mDA progenitor specification defects as well as a profound reduction of neurogenesis. To examine the effects of excessive Wnt/beta–catenin signaling on mDA specification and neurogenesis, we have analyzed a model wherein beta–catenin is conditionally stabilized in the Shh+ domain. Here, we show that the Foxa2+/Lmx1a+ domain is extended rostrally in mutant embryos, suggesting that canonical Wnt/beta–catenin signaling can drive FP expansion along the rostrocaudal axis. Although excess canonical Wnt/beta–catenin signaling generally promotes neurogenesis at midbrain levels, less tyrosine hydroxylase (Th)+, mDA neurons are generated, particularly impacting the Substantia nigra pars compacta. This is likely because of improper progenitor specification. Excess canonical Wnt/beta–catenin signaling causes downregulation of net Lmx1b, Shh and Foxa2 levels in mDA progenitors. Moreover, these progenitors assume a mixed identity to that of Lmx1a+/Lmx1b+/Nkx6-1+/Neurog1+ progenitors. We also show by lineage tracing analysis that normally, Neurog1+ progenitors predominantly give rise to Pou4f1+ neurons, but not Th+ neurons. Accordingly, in the mutant embryos, Neurog1+ progenitors at the midline generate ectopic Pou4f1+ neurons at the expense of Th+ mDA neurons. Our study suggests that an optimal dose of Wnt/beta–catenin signaling is critical for proper establishment of the mDA progenitor character. Our findings will impact embryonic stem cell protocols that utilize Wnt pathway reagents to derive mDA neuron models and therapeutics for Parkinson’s disease.