Summary FMRP loss-of-function causes Fragile X Syndrome (FXS) and autistic features. FMRP is a polyribosome-associated neuronal RNA-binding protein, suggesting that it plays a key role in regulating neuronal translation, but there has been little consensus regarding either its RNA targets or mechanism of action. Here we use high throughput sequencing of RNAs isolated by crosslinking immunoprecipitation (HITS-CLIP) to identify FMRP interactions with mouse brain polyribosomal mRNAs. FMRP interacts with the coding region of transcripts encoding pre- and postsynaptic proteins, and transcripts implicated in autism spectrum disorders (ASD). We developed a brain polyribosome-programmed translation system, revealing that FMRP reversibly stalls ribosomes specifically on its target mRNAs. Our results indicate that loss of a translational brake on the synthesis of a subset of synaptic proteins may contribute to FXS. In addition, they provide insight into the molecular basis of the cognitive and allied defects in FXS and ASD, and suggest multiple targets for clinical intervention.

Loss of the RNA binding protein FMRP causes Fragile X Syndrome (FXS), the most common cause of inherited intellectual disability, yet it is unknown how FMRP function varies across brain regions and cell types and how this contributes to disease pathophysiology. Here we use conditional tagging of FMRP and CLIP (FMRP cTag CLIP) to examine FMRP mRNA targets in hippocampal CA1 pyramidal neurons, a critical cell type for learning and memory relevant to FXS phenotypes. Integrating these data with analysis of ribosome-bound transcripts in these neurons revealed CA1-enriched binding of autism-relevant mRNAs, and CA1-specific regulation of transcripts encoding circadian proteins. This contrasted with different targets in cerebellar granule neurons, and was consistent with circadian defects in hippocampus-dependent memory in Fmr1 knockout mice. These findings demonstrate differential FMRP-dependent regulation of mRNAs across neuronal cell types that may contribute to phenotypes such as memory defects and sleep disturbance associated with FXS.

Arc is a unique immediate early gene whose expression is induced as synapses are being modified during learning. The uniqueness comes from the fact that newly synthesized Arc mRNA is rapidly transported throughout dendrites where it localizes near synapses that were recently activated. Here, we summarize aspects of Arc mRNA translation in dendrites in vivo, focusing especially on features of its expression that are paradoxical or that donot fit in with current models of how Arc protein operates. Findings from in vivo studies that donot quite fit include: (1) Following induction of LTP in vivo, Arc mRNA and protein localize near active synapses, but are also distributed throughout dendrites. In contrast, Arc mRNA localizes selectively near active synapses when stimulation is continued as Arc mRNA is transported into dendrites; (2) Strong induction of Arc expression as a result of a seizure does not lead to a rundown of synaptic efficacy in vivo as would be predicted by the hypothesis that high levels of Arc cause glutamate receptor endocytosis and LTD. (3) Arc protein is synthesized in the perinuclear cytoplasm rapidly after transcriptional activation, indicating that at least a pool of Arc mRNA is not translationally repressed to allow for dendritic delivery; (4) Increases in Arc mRNA in dendrites are not paralleled by increases in levels of exon junction complex (EJC) proteins. These results of studies of mRNA trafficking in neurons in vivo provide a new perspective on the possible roles of Arc in activity-dependent synaptic modifications.

RNA localization to neuronal dendrites and axons is increasingly recognized as a significant and widespread mechanism of gene expression control in neurons. High-throughput RNA sequencing is rapidly expanding the universe of known localized mRNAs. Although there are inherent difficulties in preparing sequencing libraries from dendrites and axons in the context of intact brain, genetic labeling strategies have paved the way for improved studies of this type. As the list of localized mRNAs grows, there is increasing need for functional validation of localized transcripts-that is, do particular localized transcripts serve demonstrable physiologic functions in axons or dendrites? Finally, specific details about what localized mRNAs do once they reach distal processes have long been elusive. Recent work using single-molecule imaging and other techniques is starting to fill in the picture of how transcripts navigate the localized environment and undergo activity-dependent translational de-repression. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 331-339, 2018.

Highlights 1. We have created a mouse model in which endogenous FMRP can be conditionally tagged.2. Using tag-specific CLIP we describe ranked and specific sets of in vivo FMRP mRNA targets in two types of neurons.3. This ranking was used to reveal that FMRP regulates mRNAs with long coding sequences.4. FMRP mRNA targets in Purkinje cells, including the top-ranked IP3 receptor, are related to cell-autonomous Fragile X phenotypes. 5.We have updated our previous list of whole mouse brain FMRP mRNA targets with more replicates, deeper sequencing and improved analysis 6. The use of tagged FMRP in less abundant cell populations allowed identification of novel mRNA targets missed in a whole brain analysis

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