Identification of a cis-acting dendritic targeting element in MAP2 mRNAs

Massively parallel identification of mRNA localization elements in primary cortical neurons

Cells adopt highly polarized shapes and form distinct subcellular compartments in many cases due to the localization of many mRNAs to specific areas, where they are translated into proteins with local functions. This mRNA localization is mediated by specific cis-regulatory elements in mRNAs, commonly called 'zipcodes'. Although there are hundreds of localized mRNAs, only a few zipcodes have been characterized. Here we describe a novel neuronal zipcode identification protocol (N-zip) that can identify zipcodes across hundreds of 3' untranslated regions. This approach combines a method of separating the principal subcellular compartments of neurons-cell bodies and neurites-with a massively parallel reporter assay. N-zip identifies the let-7 binding site and (AU)_n motif as de novo zipcodes in mouse primary cortical neurons. Our analysis also provides, to our knowledge, the first demonstration of an miRNA affecting mRNA localization and suggests a strategy for detecting many more zipcodes.

RNA-Binding Proteins as Epigenetic Regulators of Brain Functions and Their Involvement in Neurodegeneration

A central aspect of nervous system development and function is the post-transcriptional regulation of mRNA fate, which implies time- and site-dependent translation, in response to cues originating from cell-to-cell crosstalk. Such events are fundamental for the establishment of brain cell asymmetry, as well as of long-lasting modifications of synapses (long-term potentiation: LTP), responsible for learning, memory, and higher cognitive functions. Post-transcriptional regulation is in turn dependent on RNA-binding proteins that, by recognizing and binding brief RNA sequences, base modifications, or secondary/tertiary structures, are able to control maturation, localization, stability, and translation of the transcripts. Notably, most RBPs contain intrinsically disordered regions (IDRs) that are thought to be involved in the formation of membrane-less structures, probably due to liquid-liquid phase separation (LLPS). Such structures are evidenced as a variety of granules that contain proteins and different classes of RNAs. The other side of the peculiar properties of IDRs is, however, that, under altered cellular conditions, they are also prone to form aggregates, as observed in neurodegeneration. Interestingly, RBPs, as part of both normal and aggregated complexes, are also able to enter extracellular vesicles (EVs), and in doing so, they can also reach cells other than those that produced them.

Distinct phosphorylation states of mammalian CaMKIIβ control the induction and maintenance of sleep

The reduced sleep duration previously observed in Camk2b knockout mice revealed a role for Ca2+/calmodulin-dependent protein kinase II (CaMKII)β as a sleep-promoting kinase. However, the underlying mechanism by which CaMKIIβ supports sleep regulation is largely unknown. Here, we demonstrate that activation or inhibition of CaMKIIβ can increase or decrease sleep duration in mice by almost 2-fold, supporting the role of CaMKIIβ as a core sleep regulator in mammals. Importantly, we show that this sleep regulation depends on the kinase activity of CaMKIIβ. A CaMKIIβ mutant mimicking the constitutive-active (auto)phosphorylation state promotes the transition from awake state to sleep state, while mutants mimicking subsequent multisite (auto)phosphorylation states suppress the transition from sleep state to awake state. These results suggest that the phosphorylation states of CaMKIIβ differently control sleep induction and maintenance processes, leading us to propose a "phosphorylation hypothesis of sleep" for the molecular control of sleep in mammals.

More than a marker: potential pathogenic functions of MAP2

Microtubule-associated protein 2 (MAP2) is the predominant cytoskeletal regulator within neuronal dendrites, abundant and specific enough to serve as a robust somatodendritic marker. It influences microtubule dynamics and microtubule/actin interactions to control neurite outgrowth and synaptic functions, similarly to the closely related MAP Tau. Though pathology of Tau has been well appreciated in the context of neurodegenerative disorders, the consequences of pathologically dysregulated MAP2 have been little explored, despite alterations in its immunoreactivity, expression, splicing and/or stability being observed in a variety of neurodegenerative and neuropsychiatric disorders including Huntington’s disease, prion disease, schizophrenia, autism, major depression and bipolar disorder. Here we review the understood structure and functions of MAP2, including in neurite outgrowth, synaptic plasticity, and regulation of protein folding/transport. We also describe known and potential mechanisms by which MAP2 can be regulated via post-translational modification. Then, we assess existing evidence of its dysregulation in various brain disorders, including from immunohistochemical and (phospho) proteomic data. We propose pathways by which MAP2 pathology could contribute to endophenotypes which characterize these disorders, giving rise to the concept of a “MAP2opathy”—a series of disorders characterized by alterations in MAP2 function.

m6A and YTHDF proteins contribute to the localization of select neuronal mRNAs

The transport of mRNAs to distal subcellular compartments is an important component of spatial gene expression control in neurons. However, the mechanisms that control mRNA localization in neurons are not completely understood. Here, we identify the abundant base modification, m⁶A, as a novel regulator of this process. Transcriptome-wide analysis following genetic loss of m⁶A reveals hundreds of transcripts that exhibit altered subcellular localization in hippocampal neurons. Additionally, using a reporter system, we show that mutation of specific m⁶A sites in select neuronal transcripts diminishes their localization to neurites. Single molecule fluorescent in situ hybridization experiments further confirm our findings and identify the m⁶A reader proteins YTHDF2 and YTHDF3 as mediators of this effect. Our findings reveal a novel function for m⁶A in controlling mRNA localization in neurons and enable a better understanding of the mechanisms through which m⁶A influences gene expression in the brain.

The Great Escape: mRNA Export through the Nuclear Pore Complex

Nuclear export of messenger RNA (mRNA) through the nuclear pore complex (NPC) is an indispensable step to ensure protein translation in the cytoplasm of eukaryotic cells. mRNA is not translocated on its own, but it forms ribonuclear particles (mRNPs) in association with proteins that are crucial for its metabolism, some of which; like Mex67/MTR2-NXF1/NXT1; are key players for its translocation to the cytoplasm. In this review, I will summarize our current body of knowledge on the basic characteristics of mRNA export through the NPC. To be granted passage, the mRNP cargo needs to bind transport receptors, which facilitate the nuclear export. During NPC transport, mRNPs undergo compositional and conformational changes. The interactions between mRNP and the central channel of NPC are described; together with the multiple quality control steps that mRNPs undergo at the different rings of the NPC to ensure only proper export of mature transcripts to the cytoplasm. I conclude by mentioning new opportunities that arise from bottom up approaches for a mechanistic understanding of nuclear export.

The SARS-CoV-2 RNA interactome

SARS-CoV-2 is an RNA virus whose success as a pathogen relies on its abilities to repurpose host RNA-binding proteins (RBPs) and to evade antiviral RBPs. To uncover the SARS-CoV-2 RNA interactome, we here develop a robust ribonucleoprotein (RNP) capture protocol and identify 109 host factors that directly bind to SARS-CoV-2 RNAs. Applying RNP capture on another coronavirus, HCoV-OC43, revealed evolutionarily conserved interactions between coronaviral RNAs and host proteins. Transcriptome analyses and knockdown experiments delineated 17 antiviral RBPs, including ZC3HAV1, TRIM25, PARP12, and SHFL, and 8 proviral RBPs, such as EIF3D and CSDE1, which are responsible for co-opting multiple steps of the mRNA life cycle. This also led to the identification of LARP1, a downstream target of the mTOR signaling pathway, as an antiviral host factor that interacts with the SARS-CoV-2 RNAs. Overall, this study provides a comprehensive list of RBPs regulating coronaviral replication and opens new avenues for therapeutic interventions.

Translational Control during Mammalian Neocortex Development and Postembryonic Neuronal Function

The control of mRNA translation has key roles in the regulation of gene expression and biological processes such as mammalian cellular differentiation and identity. Methodological advances in the last decade have resulted in considerable progress towards understanding how translational control contributes to the regulation of diverse biological phenomena. In this review, we discuss recent findings in the involvement of translational control in the mammalian neocortex development and neuronal biology. We focus on regulatory mechanisms that modulate translational efficiency during neural stem cells self-renewal and differentiation, as well as in neuronal-related processes such as synapse, plasticity, and memory.

β-Actin mRNA interactome mapping by proximity biotinylation

The molecular function and fate of mRNAs are controlled by RNA-binding proteins (RBPs). Identification of the interacting proteome of a specific mRNA in vivo remains very challenging, however. Based on the widely used technique of RNA tagging with MS2 aptamers for RNA visualization, we developed a RNA proximity biotinylation (RNA-BioID) technique by tethering biotin ligase (BirA*) via MS2 coat protein at the 3' UTR of endogenous MS2-tagged β-actin mRNA in mouse embryonic fibroblasts. We demonstrate the dynamics of the β-actin mRNA interactome by characterizing its changes on serum-induced localization of the mRNA. Apart from the previously known interactors, we identified more than 60 additional β-actin-associated RBPs by RNA-BioID. Among these, the KH domain-containing protein FUBP3/MARTA2 has been shown to be required for β-actin mRNA localization. We found that FUBP3 binds to the 3' UTR of β-actin mRNA and is essential for β-actin mRNA localization, but does not interact with the characterized β-actin zipcode element. RNA-BioID provides a tool for identifying new mRNA interactors and studying the dynamic view of the interacting proteome of endogenous mRNAs in space and time.

Detection of Differentially Expressed Cleavage Site Intervals Within 3' Untranslated Regions Using CSI-UTR Reveals Regulated Interaction Motifs

The length of untranslated regions at the 3' end of transcripts (3'UTRs) is regulated by alternate polyadenylation (APA). 3'UTRs contain regions that harbor binding motifs for regulatory molecules. However, the mechanisms that coordinate the 3'UTR length of specific groups of transcripts are not well-understood. We therefore developed a method, CSI-UTR, that models 3'UTR structure as tandem segments between functional alternative-polyadenylation sites (termed cleavage site intervals-CSIs). This approach facilitated (1) profiling of 3'UTR isoform expression changes and (2) statistical enrichment of putative regulatory motifs. CSI-UTR analysis is UTR-annotation independent and can interrogate legacy data generated from standard RNA-Seq libraries. CSI-UTR identified a set of CSIs in human and rodent transcriptomes. Analysis of RNA-Seq datasets from neural tissue identified differential expression events within 3'UTRs not detected by standard gene-based differential expression analyses. Further, in many instances 3'UTR and CDS from the same gene were regulated differently. This modulation of motifs for RNA-interacting molecules with potential condition-dependent and tissue-specific RNA binding partners near the polyA signal and CSI junction may play a mechanistic role in the specificity of alternative polyadenylation. Source code, CSI BED files and example datasets are available at: https://github.com/UofLBioinformatics/CSI-UTR.

Bioinformatics Approaches to Gain Insights into cis-Regulatory Motifs Involved in mRNA Localization

Messenger RNA (mRNA) is a fundamental intermediate in the expression of proteins. As an integral part of this important process, protein production can be localized by the targeting of mRNA to a specific subcellular compartment. The subcellular destination of mRNA is suggested to be governed by a region of its primary sequence or secondary structure, which consequently dictates the recruitment of trans-acting factors, such as RNA-binding proteins or regulatory RNAs, to form a messenger ribonucleoprotein particle. This molecular ensemble is requisite for precise and spatiotemporal control of gene expression. In the context of RNA localization, the description of the binding preferences of an RNA-binding protein defines a motif, and one, or more, instance of a given motif is defined as a localization element (zip code). In this chapter, we first discuss the cis-regulatory motifs previously identified as mRNA localization elements. We then describe motif representation in terms of entropy and information content and offer an overview of motif databases and search algorithms. Finally, we provide an outline of the motif topology of asymmetrically localized mRNA molecules.

Rapid Cue-Specific Remodeling of the Nascent Axonal Proteome

Axonal protein synthesis and degradation are rapidly regulated by extrinsic signals during neural wiring, but the full landscape of proteomic changes remains unknown due to limitations in axon sampling and sensitivity. By combining pulsed stable isotope labeling of amino acids in cell culture with single-pot solid-phase-enhanced sample preparation, we characterized the nascent proteome of isolated retinal axons on an unparalleled rapid timescale (5 min). Our analysis detects 350 basally translated axonal proteins on average, including several linked to neurological disease. Axons stimulated by different cues (Netrin-1, BDNF, Sema3A) show distinct signatures with more than 100 different nascent protein species up- or downregulated within the first 5 min followed by further dynamic remodeling. Switching repulsion to attraction triggers opposite regulation of a subset of common nascent proteins. Our findings thus reveal the rapid remodeling of the axonal proteomic landscape by extrinsic cues and uncover a logic underlying attraction versus repulsion.

Regulation of Adult CNS Axonal Regeneration by the Post-transcriptional Regulator Cpeb1

Adult mammalian central nervous system (CNS) neurons are unable to regenerate following axonal injury, leading to permanent functional impairments. Yet, the reasons underlying this regeneration failure are not fully understood. Here, we studied the transcriptome and translatome shortly after spinal cord injury. Profiling of the total and ribosome-bound RNA in injured and naïve spinal cords identified a substantial post-transcriptional regulation of gene expression. In particular, transcripts associated with nervous system development were down-regulated in the total RNA fraction while remaining stably loaded onto ribosomes. Interestingly, motif association analysis of post-transcriptionally regulated transcripts identified the cytoplasmic polyadenylation element (CPE) as enriched in a subset of these transcripts that was more resistant to injury-induced reduction at the transcriptome level. Modulation of these transcripts by overexpression of the CPE binding protein, Cpeb1, in mouse and Drosophila CNS neurons promoted axonal regeneration following injury. Our study uncovered a global evolutionarily conserved post-transcriptional mechanism enhancing regeneration of injured CNS axons.

Rat NEURL1 3'UTR is alternatively spliced and targets mRNA to dendrites

Neuralized, an E3 ubiquitin ligase, interacts with and positively modulates the Notch pathway by promoting ubiquitination and subsequent endocytosis of its ligands. NEURL1 mRNA is dendritically localised in the dentate gyrus of an adult rat brain, implying that it may be locally translated, but its transport mechanisms remain unstudied. Here, we report the presence of a previously unknown, shorter splice-variant of rat NEURL1 3'UTR (1477bp in length), and identify it as a potential target of nonsense-mediated decay. We show that endogenous NEURL1 mRNAs with both longer and shorter 3'UTRs are enriched in the neurites of cultured rat primary hippocampal neurons. Both NEURL1 3'UTRs can mediate transport of reporter mRNAs into dendrites in primary hippocampal neurons. By analysing the dendritic trafficking capacity of reporter mRNAs linked to various regions of longer or shorter NEURL1 3'UTR, we localise the dendritic targeting element (DTE) of spliced version of NEURL1 3'UTR to its first half, corresponding to the nucleotides 1-148 and 416-914 of the full-length 3'UTR. In contrast, the dendritic targeting capacity of the full-length NEURL1 3'UTR is abolished by splitting its 3'UTR in two halves (nt 1-914 and nt 915-1744), suggesting that slightly different DTE might mediate dendritic transport of the two transcripts.

Translational control by eIF2α in neurons: Beyond the stress response

The translation of mRNAs is a tightly controlled process that responds to multiple signaling pathways. In neurons, this control is also exerted locally due to the differential necessity of proteins in axons and dendrites. The phosphorylation of the alpha subunit of the translation initiation factor 2 (eIF2α) is one of the mechanisms of translational control. The phosphorylation of eIF2α has classically been viewed as a stress response, halting translation initiation. However, in the nervous system this type of regulation has been related to other mechanisms besides stress response, such as behavior, memory consolidation and nervous system development. Additionally, neurodegenerative diseases have a major stress component, thus eIF2α phosphorylation plays a preeminent role and its modulation is currently viewed as a new opportunity for therapeutic interventions. This review consolidates current information regarding eIF2α phosphorylation in neurons and its impact in neurodegenerative diseases. © 2016 Wiley Periodicals, Inc.

Neuronal activity controls Bdnf expression via Polycomb de-repression and CREB/CBP/JMJD3 activation in mature neurons

It has been recently described that in embryonic stem cells, the expression of some important developmentally regulated genes is repressed, but poised for fast activation under the appropriate stimuli. In this work we show that Bdnf promoters are repressed by Polycomb Complex 2 in mature hippocampal neurons, and basal expression is guaranteed by the coexistence with activating histone marks. Neuronal stimulation triggered by N-methyl-D-aspartate application induces the transcription of these promoters by H3K27Me3 demethylation and H3K27Me3 phosphorylation at Serine 28 leading to displacement of EZH2, the catalytic subunit of Polycomb Repressor Complex 2. Our data show that the fast transient expression of Bdnf promoters II and VI after neuronal stimulation is dependent on acetylation of histone H3K27 by CREB-p/CBP. Thus, regulatory mechanisms established during development seem to remain after differentiation controlling genes induced by different stimuli, as would be the case of early memory genes in mature neurons.

In neurons, brain-derived neurotrophic factor (BDNF) transcription is activated by synaptic activity, in part by epigenetic regulation of its promoter regions. Here the authors characterize histone modifications in response to NMDA treatment that result in different kinetics of Bdnf activation from its different promoter regions.

Proteostasis and RNA Binding Proteins in Synaptic Plasticity and in the Pathogenesis of Neuropsychiatric Disorders

Decades of research have demonstrated that rapid alterations in protein abundance are required for synaptic plasticity, a cellular correlate for learning and memory. Control of protein abundance, known as proteostasis, is achieved across a complex neuronal morphology that includes a tortuous axon as well as an extensive dendritic arbor supporting thousands of individual synaptic compartments. To regulate the spatiotemporal synthesis of proteins, neurons must efficiently coordinate the transport and metabolism of mRNAs. Among multiple levels of regulation, transacting RNA binding proteins (RBPs) control proteostasis by binding to mRNAs and mediating their transport and translation in response to synaptic activity. In addition to synthesis, protein degradation must be carefully balanced for optimal proteostasis, as deviations resulting in excess or insufficient abundance of key synaptic factors produce pathologies. As such, mutations in components of the proteasomal or translational machinery, including RBPs, have been linked to the pathogenesis of neurological disorders such as Fragile X Syndrome (FXS), Fragile X Tremor Ataxia Syndrome (FXTAS), and Autism Spectrum Disorders (ASD). In this review, we summarize recent scientific findings, highlight ongoing questions, and link basic molecular mechanisms to the pathogenesis of common neuropsychiatric disorders.

Activity-associated miRNA are packaged in Map1b-enriched exosomes released from depolarized neurons

Rapid input-restricted change in gene expression is an important aspect of synaptic plasticity requiring complex mechanisms of post-transcriptional mRNA trafficking and regulation. Small non-coding miRNA are uniquely poised to support these functions by providing a nucleic-acid-based specificity component for universal-sequence-dependent RNA binding complexes. We investigated the subcellular distribution of these molecules in resting and potassium chloride depolarized human neuroblasts, and found both selective enrichment and depletion in neurites. Depolarization was associated with a neurite-restricted decrease in miRNA expression; a subset of these molecules was recovered from the depolarization medium in nuclease resistant extracellular exosomes. These vesicles were enriched with primate specific miRNA and the synaptic-plasticity-associated protein MAP1b. These findings further support a role for miRNA as neural plasticity regulators, as they are compartmentalized in neurons and undergo activity-associated redistribution or release into the extracellular matrix.

Regulation of mRNA transport, localization and translation in the nervous system of mammals (Review)

Post-transcriptional control of mRNA trafficking and metabolism plays a critical role in the actualization and fine tuning of the genetic program of cells, both in development and in differentiated tissues. Cis-acting signals, responsible for post-transcriptional regulation, reside in the RNA message itself, usually in untranslated regions, 5′ or 3′ to the coding sequence, and are recognized by trans-acting factors: RNA-binding proteins (RBPs) and/or non-coding RNAs (ncRNAs). ncRNAs bind short mRNA sequences usually present in the 3′-untranslated (3′-UTR) region of their target messages. RBPs recognize specific nucleotide sequences and/or secondary/tertiary structures. Most RBPs assemble on mRNA at the moment of transcription and shepherd it to its destination, somehow determining its final fate. Regulation of mRNA localization and metabolism has a particularly important role in the nervous system where local translation of pre-localized mRNAs has been implicated in developing axon and dendrite pathfinding, and in synapse formation. Moreover, activity-dependent mRNA trafficking and local translation may underlie long-lasting changes in synaptic efficacy, responsible for learning and memory. This review focuses on the role of RBPs in neuronal development and plasticity, as well as possible connections between ncRNAs and RBPs.

Selective distribution and dynamic modulation of miRNAs in the synapse and its possible role in Alzheimer's Disease

MicroRNAs (miRNAs) are small non-coding RNAs that control a wide range of functions in the cell. They act as post-transcriptional gene regulators throughout in development and in adulthood, although recent evidence suggests their potential role in the onset and development of various diseases and neuropathologies. In neurons miRNAs seem to play a key role as regulators of synaptic function. Synapses are vulnerable structures in neurodegenerative diseases. In particular, synaptic loss has been described as an early event in the pathogenesis of Alzheimer's Disease (AD). MicroRNA-mediated gene silencing represents a candidate event for the repression of specific mRNAs and protein synthesis that could account for synaptic dysfunction. In this work, we review the participation of miRNAs in synaptic function and consider their possible role in synaptic alterations in AD. First we review the biogenesis of miRNAs and their role as post-transcriptional regulators. Then we discuss recently published data on the distribution of miRNAs in the brain as well as their role in dynamic regulation at the synapse. In the second part, we briefly introduce the reader to AD, focusing on synaptic alterations in the progression of the pathology. Then we discuss possible implications of miRNAs in the associated synaptic dysfunction.

Degradation of high affinity HuD targets releases Kv1.1 mRNA from miR-129 repression by mTORC1

Little is known about how a neuron undergoes site-specific changes in intrinsic excitability during neuronal activity. We provide evidence for a novel mechanism for mTORC1 kinase-dependent translational regulation of the voltage-gated potassium channel Kv1.1 messenger RNA (mRNA). We identified a microRNA, miR-129, that repressed Kv1.1 mRNA translation when mTORC1 was active. When mTORC1 was inactive, we found that the RNA-binding protein, HuD, bound to Kv1.1 mRNA and promoted its translation. Unexpectedly, inhibition of mTORC1 activity did not alter levels of miR-129 and HuD to favor binding to Kv1.1 mRNA. However, reduced mTORC1 signaling caused the degradation of high affinity HuD target mRNAs, freeing HuD to bind Kv1.1 mRNA. Hence, mTORC1 activity regulation of mRNA stability and high affinity HuD-target mRNA degradation mediates the bidirectional expression of dendritic Kv1.1 ion channels.

Metabolic turnover of synaptic proteins: kinetics, interdependencies and implications for synaptic maintenance

Chemical synapses contain multitudes of proteins, which in common with all proteins, have finite lifetimes and therefore need to be continuously replaced. Given the huge numbers of synaptic connections typical neurons form, the demand to maintain the protein contents of these connections might be expected to place considerable metabolic demands on each neuron. Moreover, synaptic proteostasis might differ according to distance from global protein synthesis sites, the availability of distributed protein synthesis facilities, trafficking rates and synaptic protein dynamics. To date, the turnover kinetics of synaptic proteins have not been studied or analyzed systematically, and thus metabolic demands or the aforementioned relationships remain largely unknown. In the current study we used dynamic Stable Isotope Labeling with Amino acids in Cell culture (SILAC), mass spectrometry (MS), Fluorescent Non-Canonical Amino acid Tagging (FUNCAT), quantitative immunohistochemistry and bioinformatics to systematically measure the metabolic half-lives of hundreds of synaptic proteins, examine how these depend on their pre/postsynaptic affiliation or their association with particular molecular complexes, and assess the metabolic load of synaptic proteostasis. We found that nearly all synaptic proteins identified here exhibited half-lifetimes in the range of 2-5 days. Unexpectedly, metabolic turnover rates were not significantly different for presynaptic and postsynaptic proteins, or for proteins for which mRNAs are consistently found in dendrites. Some functionally or structurally related proteins exhibited very similar turnover rates, indicating that their biogenesis and degradation might be coupled, a possibility further supported by bioinformatics-based analyses. The relatively low turnover rates measured here (∼0.7% of synaptic protein content per hour) are in good agreement with imaging-based studies of synaptic protein trafficking, yet indicate that the metabolic load synaptic protein turnover places on individual neurons is very substantial.

The RNA-binding protein MARTA2 regulates dendritic targeting of MAP2 mRNAs in rat neurons

Dendritic targeting of mRNAs encoding the microtubule-associated protein 2 (MAP2) in neurons involves a cis-acting dendritic targeting element. Two rat brain proteins, MAP2-RNA trans-acting protein (MARTA)1 and MARTA2, bind to the cis-element with both high affinity and specificity. In this study, affinity-purified MARTA2 was identified as orthologue of human far-upstream element binding protein 3. In neurons, it resides in somatodendritic granules and dendritic spines and associates with MAP2 mRNAs. Expression of a dominant-negative variant of MARTA2 disrupts dendritic targeting of endogenous MAP2 mRNAs, while not noticeably altering the level and subcellular distribution of polyadenylated mRNAs as a whole. Finally, MAP2 transcripts associate with the microtubule-based motor KIF5 and inhibition of KIF5, but not cytoplasmic dynein function disrupts extrasomatic trafficking of MAP2 mRNA granules. Thus, in neurons MARTA2 appears to represent a key trans-acting factor involved in KIF5-mediated dendritic targeting of MAP2 mRNAs.

Dendritic protein synthesis in the normal and diseased brain

Synaptic activity is a spatially limited process that requires a precise, yet dynamic, complement of proteins within the synaptic micro-domain. The maintenance and regulation of these synaptic proteins is regulated, in part, by local mRNA translation in dendrites. Protein synthesis within the postsynaptic compartment allows neurons tight spatial and temporal control of synaptic protein expression, which is critical for proper functioning of synapses and neural circuits. In this review, we discuss the identity of proteins synthesized within dendrites, the receptor-mediated mechanisms regulating their synthesis, and the possible roles for these locally synthesized proteins. We also explore how our current understanding of dendritic protein synthesis in the hippocampus can be applied to new brain regions and to understanding the pathological mechanisms underlying varied neurological diseases.

Modulation of dendritic AMPA receptor mRNA trafficking by RNA splicing and editing

RNA trafficking to dendrites and local translation are crucial processes for superior neuronal functions. To date, several α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) mRNAs have been detected in dendrites and are subject to local protein synthesis. Here, we report the presence of all AMPAR GluA1-4 mRNAs in hippocampal and cortical rat synaptic spines by synaptoneurosomes analysis. In particular, we showed that dendritic AMPAR mRNAs are present in the Flip versions in the cortex and hippocampus. To further confirm these data, we demonstrate, using in situ hybridization, the dendritic localization of the GluA2 Flip isoform in vitro and in vivo, whereas the Flop variant is restricted mainly to the soma. In addition, we report that dendritic AMPA mRNAs are edited at low levels at their R/G sites; this result was also supported with transfection experiments using chimeric GluA2 DNA vectors, showing that transcripts carrying an unedited nucleotide at the R/G site, in combination with the Flip exon, are more efficiently targeted to dendrites when compared with the edited-Flip versions. Our data show that post-transcriptional regulations such as RNA splicing, editing and trafficking might be mutually coordinated and that the localization of different AMPAR isoforms in dendrites might play a functional role in the regulation of neuronal transmission.

14-3-3 proteins regulate retinal axon growth by modulating ADF/cofilin activity

Precise navigation of axons to their targets is critical for establishing proper neuronal networks during development. Axon elongation, whereby axons extend far beyond the site of initiation to reach their target cells, is an essential step in this process, but the precise molecular pathways that regulate axon growth remain uncharacterized. Here we show that 14-3-3/14-3-3ς proteins-adaptor proteins that modulate diverse cellular processes including cytoskeletal dynamics-play a critical role in Xenopus retinal ganglion cell (RGC) axon elongation in vivo and in vitro. We have identified the expression of 14-3-3/14-3-3ς transcripts and proteins in retinal growth cones, with higher levels of expression occurring during the phase of rapid pathway extension. Competitive inhibition of 14-3-3/14-3-3ς by expression of a genetically encoded peptide, R18, in RGCs resulted in a marked decrease in the length of the initial retinotectal projection in vivo and a corresponding decrease in axon elongation rate in vitro (30-40%). Furthermore, 14-3-3/14-3-3ς (R1) co-localized with Xenopus actin depolymerizing factor (ADF)/cofilin (XAC) in RGC growth cones. Inhibition of 14-3-3/14-3-3ς function with either R18 or morpholinos reduced the level of inactive pXAC and increased the sensitivity to collapse by the repulsive cue, Slit2. Collectively, these results demonstrate that14-3-3/14-3-3ς participates in the regulation of retinal axon elongation, in part by modulating XAC activity.

Synthesis of two SAPAP3 isoforms from a single mRNA is mediated via alternative translational initiation

In mammalian neurons, targeting and translation of specific mRNAs in dendrites contribute to synaptic plasticity. After nuclear export, mRNAs designated for dendritic transport are generally assumed to be translationally dormant and activity of individual synapses may locally trigger their extrasomatic translation. We show that the long, GC-rich 5′-untranslated region of dendritic SAPAP3 mRNA restricts translation initiation via a mechanism that involves an upstream open reading frame (uORF). In addition, the uORF enables the use of an alternative translation start site, permitting synthesis of two SAPAP3 isoforms from a single mRNA. While both isoforms progressively accumulate at postsynaptic densities during early rat brain development, their levels relative to each other vary in different adult rat brain areas. Thus, alternative translation initiation events appear to regulate relative expression of distinct SAPAP3 isoforms in different brain regions, which may function to influence synaptic plasticity.

Local translation of extranuclear lamin B promotes axon maintenance

Local protein synthesis plays a key role in regulating stimulus-induced responses in dendrites and axons. Recent genome-wide studies have revealed that thousands of different transcripts reside in these distal neuronal compartments, but identifying those with functionally significant roles presents a challenge. We performed an unbiased screen to look for stimulus-induced, protein synthesis-dependent changes in the proteome of Xenopus retinal ganglion cell (RGC) axons. The intermediate filament protein lamin B2 (LB2), normally associated with the nuclear membrane, was identified as an unexpected major target. Axonal ribosome immunoprecipitation confirmed translation of lb2 mRNA in vivo. Inhibition of lb2 mRNA translation in axons in vivo does not affect guidance but causes axonal degeneration. Axonal LB2 associates with mitochondria, and LB2-deficient axons exhibit mitochondrial dysfunction and defects in axonal transport. Our results thus suggest that axonally synthesized lamin B plays a crucial role in axon maintenance by promoting mitochondrial function.

Post-transcriptional trafficking and regulation of neuronal gene expression

Intracellular messenger RNA (mRNA) traffic and translation must be highly regulated, both temporally and spatially, within eukaryotic cells to support the complex functional partitioning. This capacity is essential in neurons because it provides a mechanism for rapid input-restricted activity-dependent protein synthesis in individual dendritic spines. While this feature is thought to be important for synaptic plasticity, the structures and mechanisms that support this capability are largely unknown. Certainly specialized RNA binding proteins and binding elements in the 3′ untranslated region (UTR) of translationally regulated mRNA are important, but the subtlety and complexity of this system suggests that an intermediate “specificity” component is also involved. Small non-coding microRNA (miRNA) are essential for CNS development and may fulfill this role by acting as the guide strand for mediating complex patterns of post-transcriptional regulation. In this review we examine post-synaptic gene regulation, mRNA trafficking and the emerging role of post-transcriptional gene silencing in synaptic plasticity.

Makorin ring zinc finger protein 1 (MKRN1), a novel poly(A)-binding protein-interacting protein, stimulates translation in nerve cells

The poly(A)-binding protein (PABP), a key component of different ribonucleoprotein complexes, plays a crucial role in the control of mRNA translation rates, stability, and subcellular targeting. In this study we identify RING zinc finger protein Makorin 1 (MKRN1), a bona fide RNA-binding protein, as a binding partner of PABP that interacts with PABP in an RNA-independent manner. In rat brain, a so far uncharacterized short MKRN1 isoform, MKRN1-short, predominates and is detected in forebrain nerve cells. In neuronal dendrites, MKRN1-short co-localizes with PABP in granule-like structures, which are morphological correlates of sites of mRNA metabolism. Moreover, in primary rat neurons MKRN1-short associates with dendritically localized mRNAs. When tethered to a reporter mRNA, MKRN1-short significantly enhances reporter protein synthesis. Furthermore, after induction of synaptic plasticity via electrical stimulation of the perforant path in vivo, MKRN1-short specifically accumulates in the activated dendritic lamina, the middle molecular layer of the hippocampal dentate gyrus. Collectively, these data indicate that in mammalian neurons MKRN1-short interacts with PABP to locally control the translation of dendritic mRNAs at synapses.

Huntingtin mediates dendritic transport of β-actin mRNA in rat neurons

Transport of mRNAs to diverse neuronal locations via RNA granules serves an important function in regulating protein synthesis within restricted sub-cellular domains. We recently detected the Huntington's disease protein huntingtin (Htt) in dendritic RNA granules; however, the functional significance of this localization is not known. Here we report that Htt and the huntingtin-associated protein 1 (HAP1) are co-localized with the microtubule motor proteins, the KIF5A kinesin and dynein, during dendritic transport of β-actin mRNA. Live cell imaging demonstrated that β-actin mRNA is associated with Htt, HAP1, and dynein intermediate chain in cultured neurons. Reduction in the levels of Htt, HAP1, KIF5A, and dynein heavy chain by lentiviral-based shRNAs resulted in a reduction in the transport of β-actin mRNA. These findings support a role for Htt in participating in the mRNA transport machinery that also contains HAP1, KIF5A, and dynein.

Lack of Pur-alpha alters postnatal brain development and causes megalencephaly

Pur-alpha (Purα) plays an important role in a variety of cellular processes including transcriptional regulation, cell proliferation and oncogenic transformation. To better understand the role of Purα in the developing and mature brain, we generated Purα-deficient mice, which we were able to raise to the age of six months. Purα(-/-) mice were born with no obvious pathological condition. We obtained convincing evidence that lack of Purα prolongs the postnatal proliferation of neuronal precursor cells both in the hippocampus and in the cerebellum, however, without affecting the overall number of postmitotic neurons. Independent of these findings, we observed alterations in the expression and distribution of the dendritic protein MAP2, the translation of which has been proposed previously to be Purα-dependent. At the age of 2 weeks, Purα(-/-) mice generated a continuous tremor which persisted throughout lifetime. Finally, adult Purα(-/-) mice displayed a megalencephaly and histopathological findings including axonal swellings and hyperphosphorylation of neurofilaments. Our studies underline the importance of Purα in the proliferation of neuronal precursor cells during postnatal brain development and suggest a role for Purα in the regulation of the expression and cellular distribution of dendritic and axonal proteins. Since recent studies implicate a link between Purα and the fragile X tremor/ataxia syndrome, our Purα(-/-) mouse model will provide new opportunities for understanding the mechanisms of neurodegeneration.

Spatial code recognition in neuronal RNA targeting: role of RNA-hnRNP A2 interactions

Recognition of non-canonical purine•purine RNA motifs by hnRNP A2 mediates targeted delivery of neuronal RNAs to dendrites.

In neurons, regulation of gene expression occurs in part through translational control at the synapse. A fundamental requirement for such local control is the targeted delivery of select neuronal mRNAs and regulatory RNAs to distal dendritic sites. The nature of spatial RNA destination codes, and the mechanism by which they are interpreted for dendritic delivery, remain poorly understood. We find here that in a key dendritic RNA transport pathway (exemplified by BC1 RNA, a dendritic regulatory RNA, and protein kinase M ζ [PKMζ] mRNA, a dendritic mRNA), noncanonical purine•purine nucleotide interactions are functional determinants of RNA targeting motifs. These motifs are specifically recognized by heterogeneous nuclear ribonucleoprotein A2 (hnRNP A2), a trans-acting factor required for dendritic delivery. Binding to hnRNP A2 and ensuing dendritic delivery are effectively competed by RNAs with CGG triplet repeat expansions. CGG repeats, when expanded in the 5′ untranslated region of fragile X mental retardation 1 (FMR1) mRNA, cause fragile X–associated tremor/ataxia syndrome. The data suggest that cellular dysregulation observed in the presence of CGG repeat RNA may result from molecular competition in neuronal RNA transport pathways.

Cytoplasmic intron sequence-retaining transcripts can be dendritically targeted via ID element retrotransposons

RNA precursors give rise to mRNA after splicing of intronic sequences traditionally thought to occur in the nucleus. Here, we show that intron sequences are retained in a number of dendritically-targeted mRNAs, by using microarray and Illumina sequencing of isolated dendritic mRNA as well as in situ hybridization. Many of the retained introns contain ID elements, a class of SINE retrotransposon. A portion of these SINEs confers dendritic targeting to exogenous and endogenous transcripts showing the necessity of ID-mediated mechanisms for the targeting of different transcripts to dendrites. ID elements are capable of selectively altering the distribution of endogenous proteins, providing a link between intronic SINEs and protein function. As such, the ID element represents a common dendritic targeting element found across multiple RNAs. Retention of intronic sequence is a more general phenomenon than previously thought and plays a functional role in the biology of the neuron, partly mediated by co-opted repetitive sequences.

Inhibition of the interactions between eukaryotic initiation factors 4E and 4G impairs long-term associative memory consolidation but not reconsolidation

Considerable evidence indicates that the general blockade of protein synthesis prevents both the initial consolidation and the postretrieval reconsolidation of long-term memories. These findings come largely from studies of drugs that block ribosomal function, so as to globally interfere with both cap-dependent and -independent forms of translation. Here we show that intra-amygdala microinfusions of 4EGI-1, a small molecule inhibitor of cap-dependent translation that selectively disrupts the interaction between eukaryotic initiation factors (eIF) 4E and 4G, attenuates fear memory consolidation but not reconsolidation. Using a combination of behavioral and biochemical techniques, we provide both in vitro and in vivo evidence that the eIF4E-eIF4G complex is more stringently required for plasticity induced by initial learning than for that triggered by reactivation of an existing memory.

Cytoplasmic polyadenylation element-like sequences are involved in dendritic targeting of BDNF mRNA in hippocampal neurons

Several mRNAs are known to be targeted to dendrites in hippocampal neurons. In this study, we show that brain-derived neurotrophic factor (BDNF) mRNA has two distinct cis-acting dendritic targeting elements in the short 3' untranslated region (UTR): a constitutive element and an activity-dependent one. Moreover, deletion of serial cytoplasmic polyadenylation element (CPE)-like sequences in the short 3'UTR suppressed both constitutive and activity-dependent dendritic targeting. In addition to the interaction with cytoplasmic polyadenylation element binding protein-1 (CPEB-1), depolarization enhanced CPEB-1 recruitment to the activity-dependent targeting element. These results suggest that CPE-like sequences are involved in the activity-dependent as well as constitutive dendritic targeting of BDNF mRNA.

Binding of alphaII spectrin to 14-3-3beta is involved in NCAM-dependent neurite outgrowth

Members of the 14-3-3 protein family have been implicated in neuronal migration, synaptic plasticity and learning. Using affinity chromatography followed by mass spectrometry analysis, we show here that the cytoskeletal protein alphaII spectrin is a novel ligand of 14-3-3beta. We found that 14-3-3beta interacts with alphaII spectrin via the mode 2 14-3-3 binding motif RLIQS(1302)HP. Binding required phosphorylation of Ser(1302) by casein kinase II and was enhanced in the presence of calmodulin. Co-immunoprecipitation of alphaII spectrin and 14-3-3beta with the neural cell adhesion molecule NCAM suggested that the 14-3-3-spectrin-interaction affects NCAM function. Indeed, disruption of the 14-3-3beta/alphaII spectrin interaction by mutating Ser(1302) to Ala enhanced NCAM-dependent neurite outgrowth. Our results indicate that the phosphorylation-dependent interaction between 14-3-3beta and alphaII spectrin acts as a switch between positive and negative regulation of neurite outgrowth stimulated by NCAM, representing a novel and acute mechanism preventing uncontrolled elongation of neuronal processes.

Conserved 3'-untranslated region sequences direct subcellular localization of chaperone protein mRNAs in neurons

mRNA localization provides polarized cells with a locally renewable source of proteins. In neurons, mRNA translation can occur at millimeters to centimeters from the cell body, giving the dendritic and axonal processes a means to autonomously respond to their environment. Despite that hundreds of mRNAs have been detected in neuronal processes, there are no reliable means to predict mRNA localization elements. Here, we have asked what RNA elements are needed for localization of transcripts encoding endoplasmic reticulum chaperone proteins in neurons. The 3'-untranslated regions (UTRs) of calreticulin and Grp78/BiP mRNAs show no homology to one another, but each shows extensive regions of high sequence identity to their 3'UTRs in mammalian orthologs. These conserved regions are sufficient for subcellular localization of reporter mRNAs in neurons. The 3'UTR of calreticulin has two conserved regions, and either of these is sufficient for axonal and dendritic targeting. However, only nucleotides 1315-1412 show ligand responsiveness to neurotrophin 3 (NT3) and myelin-associated glycoprotein (MAG). This NT3- and MAG-dependent axonal mRNA transport requires activation of JNK, both for calreticulin mRNA and for other mRNAs whose axonal levels are commonly regulated by NT3 and MAG.

Brain-derived neurotrophic factor enhances the expression of the monocarboxylate transporter 2 through translational activation in mouse cultured cortical neurons

MCT2 is the predominant neuronal monocarboxylate transporter allowing lactate use as an alternative energy substrate. It is suggested that MCT2 is upregulated to meet enhanced energy demands after modifications in synaptic transmission. Brain-derived neurotrophic factor (BDNF), a promoter of synaptic plasticity, significantly increased MCT2 protein expression in cultured cortical neurons (as shown by immunocytochemistry and western blot) through a translational regulation at the synaptic level. Brain-derived neurotrophic factor can cause translational activation through different signaling pathways. Western blot analyses showed that p44/p42 mitogen-activated protein kinase (MAPK), Akt, and S6 were strongly phosphorylated on BDNF treatment. To determine by which signal transduction pathway(s) BDNF mediates its upregulation of MCT2 protein expression, the effect of specific inhibitors for p38 MAPK, phosphoinositide 3-kinase (PI3K), mammalian target of rapamycin (mTOR), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK), p44/p42 MAPK (ERK), and Janus kinase 2 (JAK2) was evaluated. It could be observed that the BDNF-induced increase in MCT2 protein expression was almost completely blocked by all inhibitors, except for JAK2. These data indicate that BDNF induces an increase in neuronal MCT2 protein expression by a mechanism involving a concomitant stimulation of PI3K/Akt/mTOR/S6, p38 MAPK, and p44/p42 MAPK. Moreover, our observations suggest that changes in MCT2 expression could participate in the process of synaptic plasticity induced by BDNF.

MPEP reduces seizure severity in Fmr-1 KO mice over expressing human Abeta

Metabotropic glutamate receptor 5 (mGluR(5)) regulates the translation of amyloid precursor protein (APP) mRNA. Under resting conditions, mRNA is bound to and translationally repressed by the fragile X mental retardation protein (FMRP). Upon group 1 mGluR activation, FMRP dissociates from the mRNA and translation ensues. APP levels are elevated in the dendrites of primary neuronal cultures as well as in synaptoneurosomes (SN) prepared from embryonic and juvenile fmr-1 knockout (KO) mice, respectively. In order to study the effects of APP and its proteolytic product Abeta on Fragile X syndrome (FXS) phenotypes, we created a novel mouse model (FRAXAD) that over-expresses human APPSwe/Abeta in an fmr-1 KO background. Herein, we assess (1) human APP(Swe) and Abeta levels as a function of age in FRAXAD mice, and (2) seizure susceptibility to pentylenetetrazol (PTZ) after mGluR(5) blockade. PTZ-induced seizure severity is decreased in FRAXAD mice pre-treated with the mGluR(5) antagonist MPEP. These data suggest that Abeta contributes to seizure incidence and may be an appropriate therapeutic target to lessen seizure pathology in FXS, Alzheimer's disease (AD) and Down syndrome (DS) patients.

To localize or not to localize: mRNA fate is in 3'UTR ends

Translation of localized mRNA is a fast and efficient way of reacting to extracellular stimuli with the added benefit of providing spatial resolution to the cellular response. The efficacy of this adaptive response ultimately relies on the ability to express a particular protein at the right time and in the right place. Although mRNA localization is a mechanism shared by most organisms, it is especially relevant in highly polarized cells, such as differentiated neurons. 3'-Untranslated regions (3'UTRs) of mRNAs are critical both for the targeting of transcripts to specific subcellular compartments and for translational control. Here we review recent studies that indicate how, in response to extracellular cues, nuclear and cytoplasmic remodeling of the 3'UTR contributes to mRNA localization and local protein synthesis.

Fragile X mental retardation protein regulates the levels of scaffold proteins and glutamate receptors in postsynaptic densities

Functional absence of fragile X mental retardation protein (FMRP) causes the fragile X syndrome, a hereditary form of mental retardation characterized by a change in dendritic spine morphology. The RNA-binding protein FMRP has been implicated in regulating postsynaptic protein synthesis. Here we have analyzed whether the abundance of scaffold proteins and neurotransmitter receptor subunits in postsynaptic densities (PSDs) is altered in the neocortex and hippocampus of FMRP-deficient mice. Whereas the levels of several PSD components are unchanged, concentrations of Shank1 and SAPAP scaffold proteins and various glutamate receptor subunits are altered in both adult and juvenile knock-out mice. With the exception of slightly increased hippocampal SAPAP2 mRNA levels in adult animals, altered postsynaptic protein concentrations do not correlate with similar changes in total and synaptic levels of corresponding mRNAs. Thus, loss of FMRP in neurons appears to mainly affect the translation and not the abundance of particular brain transcripts. Semi-quantitative analysis of RNA levels in FMRP immunoprecipitates showed that in the mouse brain mRNAs encoding PSD components, such as Shank1, SAPAP1-3, PSD-95, and the glutamate receptor subunits NR1 and NR2B, are associated with FMRP. Luciferase reporter assays performed in primary cortical neurons from knock-out and wild-type mice indicate that FMRP silences translation of Shank1 mRNAs via their 3'-untranslated region. Activation of metabotropic glutamate receptors relieves translational suppression. As Shank1 controls dendritic spine morphology, our data suggest that dysregulation of Shank1 synthesis may significantly contribute to the abnormal spine development and function observed in brains of fragile X syndrome patients.

Dendritic mRNA targeting of Jacob and N-methyl-d-aspartate-induced nuclear translocation after calpain-mediated proteolysis

Jacob is a recently identified plasticity-related protein that couples N-methyl-d-aspartate receptor activity to nuclear gene expression. An expression analysis by Northern blot and in situ hybridization shows that Jacob is almost exclusively present in brain, in particular in the cortex and the limbic system. Alternative splicing gives rise to multiple mRNA variants, all of which exhibit a prominent dendritic localization in the hippocampus. Functional analysis in primary hippocampal neurons revealed that a predominant cis-acting dendritic targeting element in the 3'-untranslated region of Jacob mRNAs is responsible for dendritic mRNA localization. In the mouse brain, Jacob transcripts are associated with both the fragile X mental retardation protein, a well described trans-acting factor regulating dendritic mRNA targeting and translation, and the kinesin family member 5C motor complex, which is known to mediate dendritic mRNA transport. Jacob is susceptible to rapid protein degradation in a Ca(2+)- and Calpain-dependent manner, and Calpain-mediated clipping of the myristoylated N terminus of Jacob is required for its nuclear translocation after N-methyl-d-aspartate receptor activation. Our data suggest that local synthesis in dendrites may be necessary to replenish dendritic Jacob pools after truncation of the N-terminal membrane anchor and concomitant translocation of Jacob to the nucleus.

3'UTR-dependent localization of a Purkinje cell messenger RNA in dendrites

Pcp2(L7) is a Purkinje cell-specific GoLoco domain protein that modulates activation of Galphai/o proteins by G protein-coupled receptors. A likely downstream effector of this pathway is the P-type Ca(2+) channel, and thereby, the intrinsic electrophysiology of Purkinje cells could be modulated by Pcp2(L7). It has long been known that the Pcp2(L7) mRNA is abundantly localized in dendrites, suggesting the possibility of distal synthesis and local changes in levels of the protein. As a first step to uncover the trafficking and translational mechanisms for this mRNA, we have begun identifying the cis-acting sequences important for its localization in dendrites. Using expression of modified transgenes in vivo, we show that the 3'UTR, only 65 bases long, is necessary in this process.

Dendritic assembly of heteromeric gamma-aminobutyric acid type B receptor subunits in hippocampal neurons

Understanding the mechanisms that control synaptic efficacy through the availability of neurotransmitter receptors depends on uncovering their specific intracellular trafficking routes. gamma-Aminobutyric acid type B (GABA(B)) receptors (GABA(B)Rs) are obligatory heteromers present at dendritic excitatory and inhibitory postsynaptic sites. It is unknown whether synthesis and assembly of GABA(B)Rs occur in the somatic endoplasmic reticulum (ER) followed by vesicular transport to dendrites or whether somatic synthesis is followed by independent transport of the subunits for assembly and ER export throughout the somatodendritic compartment. To discriminate between these possibilities we studied the association of GABA(B)R subunits in dendrites of hippocampal neurons combining live fluorescence microscopy, biochemistry, quantitative colocalization, and bimolecular fluorescent complementation. We demonstrate that GABA(B)R subunits are segregated and differentially mobile in dendritic intracellular compartments and that a high proportion of non-associated intracellular subunits exist in the brain. Assembled heteromers are preferentially located at the plasma membrane, but blockade of ER exit results in their intracellular accumulation in the cell body and dendrites. We propose that GABA(B)R subunits assemble in the ER and are exported from the ER throughout the neuron prior to insertion at the plasma membrane. Our results are consistent with a bulk flow of segregated subunits through the ER and rule out a post-Golgi vesicular transport of preassembled GABA(B)Rs.