Localization and translation of mRNA in dendrites and axons

Small Differences and Big Changes: The Many Variables of MicroRNA Expression and Function in the Brain

MicroRNAs are emerging as crucial regulators within the complex, dynamic environment of the synapse, and they offer a promising new avenue for the treatment of neurological disease. These small noncoding RNAs modify gene expression in several ways, including posttranscriptional modulation via binding to complementary and semicomplementary sites on target mRNAs. This rapid, finely tuned regulation of gene expression is essential to meet the dynamic demands of the synapse. Here, we provide a detailed review of the multifaceted world of synaptic microRNA regulation. We discuss the many mechanisms by which microRNAs regulate gene expression at the synapse, particularly in the context of neuronal plasticity. We also describe the various factors, such as age, sex, and neurological disease, that can influence microRNA expression and activity in neurons. In summary, microRNAs play a crucial role in the intricate and quickly changing functional requirements of the synapse, and context is essential in the study of microRNAs and their potential therapeutic applications.

Telocytes in the Luminal GI Tract

Telocytes are unique mesenchymal cells characterized by multiple remarkably long cytoplasmic extensions that extend hundreds of micron away from the cell body. Through these extensions, telocytes establish a 3-dimensional network by connecting with other telocytes and various cell types within the tissue. In the intestine, telocytes have emerged as an essential component of the stem cell niche, providing Wnt proteins that are critical for the proliferation of stem and progenitor cells. However, the analysis of single-cell RNA sequencing has revealed other stromal populations and mechanisms for niche organization, raising questions about the role of telocytes as a component of the stem cell niche. This review explores the current state-of-the-art, existing controversies, and potential future directions related to telocytes in the luminal gastrointestinal tract.

Expansion of clinical and variant spectrum of EEF2-related neurodevelopmental disorder: Report of two additional cases

Eukaryotic translation elongation factor 2 (eEF2), encoded by the gene EEF2, is an essential factor involved in the elongation phase of protein translation. A specific heterozygous missense variant (p.P596H) in EEF2 was originally identified in association with autosomal dominant adult-onset spinocerebellar ataxia-26 (SCA26). More recently, additional heterozygous missense variants in this gene have been described to cause a novel, childhood-onset neurodevelopmental disorder with benign external hydrocephalus. Herein, we report two unrelated individuals with a similar gene-disease correlation to support this latter observation. Patient 1 is a 7-year-old male with a previously reported, de novo missense variant (p.V28M) who has motor and speech delay, autism spectrum disorder, failure to thrive with relative macrocephaly, unilateral microphthalmia with coloboma and eczema. Patient 2 is a 4-year-old female with a novel de novo nonsense variant (p.Q145X) with motor and speech delay, hypotonia, macrocephaly with benign ventricular enlargement, and keratosis pilaris. These additional cases help to further expand the genotypic and phenotypic spectrum of this newly described EEF2-related neurodevelopmental syndrome.

Maintenance of a short-lived protein required for long-term memory involves cycles of transcription and local translation

Activity-dependent expression of immediate early genes (IEGs) is critical for long-term synaptic remodeling and memory. It remains unknown how IEGs are maintained for memory despite rapid transcript and protein turnover. To address this conundrum, we monitored Arc, an IEG essential for memory consolidation. Using a knockin mouse where endogenous Arc alleles were fluorescently tagged, we performed real-time imaging of Arc mRNA dynamics in individual neurons in cultures and brain tissue. Unexpectedly, a single burst stimulation was sufficient to induce cycles of transcriptional reactivation in the same neuron. Subsequent transcription cycles required translation, whereby new Arc proteins engaged in autoregulatory positive feedback to reinduce transcription. The ensuing Arc mRNAs preferentially localized at sites marked by previous Arc protein, assembling a "hotspot" of translation, and consolidating "hubs" of dendritic Arc. These cycles of transcription-translation coupling sustain protein expression and provide a mechanism by which a short-lived event may support long-term memory.

Autoimmune RNA dysregulation and seizures: therapeutic prospects in neuropsychiatric lupus

Lupus autoimmunity frequently presents with neuropsychiatric manifestations, but underlying etiology remains poorly understood. Human brain cytoplasmic 200 RNA (BC200 RNA) is a translational regulator in neuronal synapto-dendritic domains. Here, we show that a BC200 guanosine-adenosine dendritic transport motif is recognized by autoantibodies from a subset of neuropsychiatric lupus patients. These autoantibodies impact BC200 functionality by quasi irreversibly displacing two RNA transport factors from the guanosine-adenosine transport motif. Such anti-BC autoantibodies, which can gain access to brains of neuropsychiatric lupus patients, give rise to clinical manifestations including seizures. To establish causality, naive mice with a permeabilized blood-brain barrier were injected with anti-BC autoantibodies from lupus patients with seizures. Animals so injected developed seizure susceptibility with high mortality. Seizure activity was entirely precluded when animals were injected with lupus anti-BC autoantibodies together with BC200 decoy autoantigen. Seizures are a common clinical manifestation in neuropsychiatric lupus, and our work identifies anti-BC autoantibody activity as a mechanistic cause. The results demonstrate potential utility of BC200 decoys for autoantibody-specific therapeutic interventions in neuropsychiatric lupus.

Role of Ca2+/Calmodulin-Dependent Protein Kinase Type II in Mediating Function and Dysfunction at Glutamatergic Synapses

Glutamatergic synapses harbor abundant amounts of the multifunctional Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII). Both in the postsynaptic density as well as in the cytosolic compartment of postsynaptic terminals, CaMKII plays major roles. In addition to its Ca²⁺-stimulated kinase activity, it can also bind to a variety of membrane proteins at the synapse and thus exert spatially restricted activity. The abundance of CaMKII in glutamatergic synapse is akin to scaffolding proteins although its prominent function still appears to be that of a kinase. The multimeric structure of CaMKII also confers several functional capabilities on the enzyme. The versatility of the enzyme has prompted hypotheses proposing several roles for the enzyme such as Ca²⁺ signal transduction, memory molecule function and scaffolding. The article will review the multiple roles played by CaMKII in glutamatergic synapses and how they are affected in disease conditions.

RTN3 - ASC interaction: The potential mechanism behind diabetes-induced cortical neuritic dystrophy

Recent studies have found that people with diabetes are more vulnerable to cognitive dysfunction, particularly Alzheimer's disease (AD). Previous studies revealed that Reticulon 3 (RTN3) oligomers could induce cortical neuritic dystrophy (CND) in the brains of diabetic rats. However, it is not clear how diabetes induces RTN3 aggregation. In this study, we examined in vivo and in vitro diabetes models to explore the underlying effects of RTN3-mediated neurite dystrophy. The results showed that the binding ability of ASC and RTN3 was significantly increased during diabetes- or high glucose-induced neuritic dystrophy, and ASC siRNA or an anti-inflammatory drug (CP 424174) could inhibit neuritic dystrophy in vitro. These results suggest that the ASC and RTN3 interaction is involved in diabetes-induced CND, and anti-inflammatory therapy might be an effective way to prevent and inhibit diabetes-induced CND.

Neonatal Sevoflurane Exposure Impairs Learning and Memory by the Hypermethylation of Hippocampal Synaptic Genes

Sevoflurane anesthesia is widely used in pediatric patients. Clinical studies report memory impairment in those exposed to general anesthesia early in life. DNA methylation is essential for the modulation of synaptic plasticity through regulating the transcription of synaptic genes. Therefore, we tested whether neonatal sevoflurane exposure affects learning and memory underlying the hippocampal DNA methylation of synaptic genes. Male Sprague-Dawley rats were exposed to 3% sevoflurane or air for 2 h daily from postnatal day 7 (P7) to P9. 5-aza-2-deoxycytidine (5-AZA), an inhibitor of DNA methyltransferases (DNMTs), was intraperitoneally injected 30 min before sevoflurane or air exposure on P7-9. The rats were euthanized 6, 12, 24 h, and 28 days after the last sevoflurane exposure, followed by the determination of global and gene-specific DNA methylation. The expression of synaptic proteins and synaptic density and the transcription of Dnmts and ten eleven translocations (Tets) in the hippocampus were measured. The ability of learning and memory was assessed using Morris water maze, novel object recognition, and intruder tests. Repeated neonatal sevoflurane exposure impaired cognitive, social, and spatial memory. The memory impairment was associated with the increased Dnmt1, Dnmt3a, and 5-methylcytosine level and the decreased Tet1 and 5-hydromethylcytosine level. Sevoflurane subsequently induced hypermethylation of Shank2, Psd95, Syn1, and Syp gene and down-regulated the expression of synaptic proteins, which finally led to the decrease of synaptic density in a time-dependent manner. Notably, 5-AZA pretreatment ameliorated learning and memory in sevoflurane-treated rats. In conclusion, neonatal exposure to sevoflurane can impair learning and memory through DNA methylation of synaptic genes.

An activity-dependent local transport regulation via degradation and synthesis of KIF17 underlying cognitive flexibility

Synaptic weight changes among postsynaptic densities within a single dendrite are regulated by the balance between localized protein degradation and synthesis. However, the molecular mechanism via these opposing regulatory processes is still elusive. Here, we showed that the molecular motor KIF17 was locally degraded and synthesized in an N-methyl-d-aspartate receptor (NMDAR)-mediated activity-dependent manner. Accompanied by the degradation of KIF17, its transport was temporarily dampened in dendrites. We also observed that activity-dependent local KIF17 synthesis driven by its 3' untranslated region (3'UTR) occurred at dendritic shafts, and the newly synthesized KIF17 moved along the dendrites. Furthermore, hippocampus-specific deletion of Kif17 3'UTR disrupted KIF17 synthesis induced by fear memory retrieval, leading to impairment in extinction of fear memory. These results indicate that the regulation of the KIF17 transport is driven by the single dendrite-restricted cycle of degradation and synthesis that underlies cognitive flexibility.

Dysregulation of protein synthesis and dendritic spine morphogenesis in ASD: studies in human pluripotent stem cells

Autism spectrum disorder (ASD) is a brain disorder that involves changes in neuronal connections. Abnormal morphology of dendritic spines on postsynaptic neurons has been observed in ASD patients and transgenic mice that model different monogenetic causes of ASD. A number of ASD-associated genetic variants are known to disrupt dendritic local protein synthesis, which is essential for spine morphogenesis, synaptic transmission, and plasticity. Most of our understanding on the molecular mechanism underlying ASD depends on studies using rodents. However, recent advance in human pluripotent stem cells and their neural differentiation provides a powerful alternative tool to understand the cellular aspects of human neurological disorders. In this review, we summarize recent progress on studying mRNA targeting and local protein synthesis in stem cell-derived neurons, and discuss how perturbation of these processes may impact synapse development and functions that are relevant to cognitive deficits in ASD.

miR-181a negatively modulates synaptic plasticity in hippocampal cultures and its inhibition rescues memory deficits in a mouse model of Alzheimer's disease

MicroRNAs play a pivotal role in rapid, dynamic, and spatiotemporal modulation of synaptic functions. Among them, recent emerging evidence highlights that microRNA-181a (miR-181a) is particularly abundant in hippocampal neurons and controls the expression of key plasticity-related proteins at synapses. We have previously demonstrated that miR-181a was upregulated in the hippocampus of a mouse model of Alzheimer's disease (AD) and correlated with reduced levels of plasticity-related proteins. Here, we further investigated the underlying mechanisms by which miR-181a negatively modulated synaptic plasticity and memory. In primary hippocampal cultures, we found that an activity-dependent upregulation of the microRNA-regulating protein, translin, correlated with reduction of miR-181a upon chemical long-term potentiation (cLTP), which induced upregulation of GluA2, a predicted target for miR-181a, and other plasticity-related proteins. Additionally, Aβ treatment inhibited cLTP-dependent induction of translin and subsequent reduction of miR-181a, and cotreatment with miR-181a antagomir effectively reversed the effects elicited by Aβ but did not rescue translin levels, suggesting that the activity-dependent upregulation of translin was upstream of miR-181a. In mice, a learning episode markedly decreased miR-181a in the hippocampus and raised the protein levels of GluA2. Lastly, we observed that inhibition of miR-181a alleviated memory deficits and increased GluA2 and GluA1 levels, without restoring translin, in the 3xTg-AD model. Taken together, our results indicate that miR-181a is a major negative regulator of the cellular events that underlie synaptic plasticity and memory through AMPA receptors, and importantly, Aβ disrupts this process by suppressing translin and leads to synaptic dysfunction and memory impairments in AD.

Molecular correlates of hypothalamic development in songbird ontogeny in comparison with the telencephalon

Development of the songbird brain provides an excellent experimental model for understanding the regulation of sex differences in ontogeny. Considering the regulatory role of the hypothalamus in endocrine, in particular reproductive, physiology, we measured the structural (volume) and molecular correlates of hypothalamic development during ontogeny of male and female zebra finches. We quantified by relative quantitative polymerase chain reaction (rqPCR) the expression of 14 genes related to thyroid and steroid hormones actions as well as 12 genes related to brain plasticity at four specific time points during ontogeny and compared these expression patterns with the expression of the same genes as detected by transcriptomics in the telencephalon. These two different methodological approaches detected specific changes with age and demonstrated that in a substantial number of cases changes observed in both brain regions are nearly identical. Other genes however had a tissue-specific developmental pattern. Sex differences or interactions of sex by age were detected in the expression of a subset of genes, more in hypothalamus than telencephalon. These results correlate with multiple known aspects of the developmental and reproductive physiology but also raise a number of new functional questions.

Emerging Roles of Long Non-Coding RNAs as Drivers of Brain Evolution

Mammalian genomes encode tens of thousands of long-noncoding RNAs (lncRNAs), which are capable of interactions with DNA, RNA and protein molecules, thereby enabling a variety of transcriptional and post-transcriptional regulatory activities. Strikingly, about 40% of lncRNAs are expressed specifically in the brain with precisely regulated temporal and spatial expression patterns. In stark contrast to the highly conserved repertoire of protein-coding genes, thousands of lncRNAs have newly appeared during primate nervous system evolution with hundreds of human-specific lncRNAs. Their evolvable nature and the myriad of potential functions make lncRNAs ideal candidates for drivers of human brain evolution. The human brain displays the largest relative volume of any animal species and the most remarkable cognitive abilities. In addition to brain size, structural reorganization and adaptive changes represent crucial hallmarks of human brain evolution. lncRNAs are increasingly reported to be involved in neurodevelopmental processes suggested to underlie human brain evolution, including proliferation, neurite outgrowth and synaptogenesis, as well as in neuroplasticity. Hence, evolutionary human brain adaptations are proposed to be essentially driven by lncRNAs, which will be discussed in this review.

Dynamic m6A modification regulates local translation of mRNA in axons

N⁶-methyladenosine (m⁶A) is a reversible modification in mRNA and has been shown to regulate processing, translation and decay of mRNA. However, the roles of m⁶A modification in neuronal development are still not known. Here, we found that the m⁶A eraser FTO is enriched in axons and can be locally translated. Axon-specific inhibition of FTO by rhein, or compartmentalized siRNA knockdown of Fto in axons led to increases of m⁶A levels. GAP-43 mRNA is modified by m⁶A and is a substrate of FTO in axons. Loss-of-function of this non-nuclear pool of FTO resulted in increased m⁶A modification and decreased local translation of axonal GAP-43 mRNA, which eventually repressed axon elongation. Mutation of a predicted m⁶A site in GAP-43 mRNA eliminated its m⁶A modification and exempted regulation of its local translation by axonal FTO. This work showed an example of dynamic internal m⁶A demethylation of non-nuclear localized mRNA by the demethylase FTO. Regulation of m⁶A modification of axonal mRNA by axonal FTO might be a general mechanism to control their local translation in neuronal development.

A new chemical approach for proximity labelling of chromatin-associated RNAs and proteins with visible light irradiation

A new nucleus-localized singlet oxygen generator was designed and synthesized.

CeFra-seq reveals broad asymmetric mRNA and noncoding RNA distribution profiles in Drosophila and human cells

Cells are highly asymmetrical, a feature that relies on the sorting of molecular constituents, including proteins, lipids, and nucleic acids, to distinct subcellular locales. The localization of RNA molecules is an important layer of gene regulation required to modulate localized cellular activities, although its global prevalence remains unclear. We combine biochemical cell fractionation with RNA-sequencing (CeFra-seq) analysis to assess the prevalence and conservation of RNA asymmetric distribution on a transcriptome-wide scale in Drosophila and human cells. This approach reveals that the majority (∼80%) of cellular RNA species are asymmetrically distributed, whether considering coding or noncoding transcript populations, in patterns that are broadly conserved evolutionarily. Notably, a large number of Drosophila and human long noncoding RNAs and circular RNAs display enriched levels within specific cytoplasmic compartments, suggesting that these RNAs fulfill extra-nuclear functions. Moreover, fraction-specific mRNA populations exhibit distinctive sequence characteristics. Comparative analysis of mRNA fractionation profiles with that of their encoded proteins reveals a general lack of correlation in subcellular distribution, marked by strong cases of asymmetry. However, coincident distribution profiles are observed for mRNA/protein pairs related to a variety of functional protein modules, suggesting complex regulatory inputs of RNA localization to cellular organization.

Activity-Dependent Regulation of Alternative Cleavage and Polyadenylation During Hippocampal Long-Term Potentiation

Long-lasting forms of synaptic plasticity that underlie learning and memory require new transcription and translation for their persistence. The remarkable polarity and compartmentalization of neurons raises questions about the spatial and temporal regulation of gene expression within neurons. Alternative cleavage and polyadenylation (APA) generates mRNA isoforms with different 3' untranslated regions (3'UTRs) and/or coding sequences. Changes in the 3'UTR composition of mRNAs can alter gene expression by regulating transcript localization, stability and/or translation, while changes in the coding sequences lead to mRNAs encoding distinct proteins. Using specialized 3' end deep sequencing methods, we undertook a comprehensive analysis of APA following induction of long-term potentiation (LTP) of mouse hippocampal CA3-CA1 synapses. We identified extensive LTP-induced APA changes, including a general trend of 3'UTR shortening and activation of intronic APA isoforms. Comparison with transcriptome profiling indicated that most APA regulatory events were uncoupled from changes in transcript abundance. We further show that specific APA regulatory events can impact expression of two molecules with known functions during LTP, including 3'UTR APA of Notch1 and intronic APA of Creb1. Together, our results reveal that activity-dependent APA provides an important layer of gene regulation during learning and memory.

Mechanisms of Long Non-Coding RNAs in the Assembly and Plasticity of Neural Circuitry

The mechanisms underlying development processes and functional dynamics of neural circuits are far from understood. Long non-coding RNAs (lncRNAs) have emerged as essential players in defining identities of neural cells, and in modulating neural activities. In this review, we summarized latest advances concerning roles and mechanisms of lncRNAs in assembly, maintenance and plasticity of neural circuitry, as well as lncRNAs' implications in neurological disorders. We also discussed technical advances and challenges in studying functions and mechanisms of lncRNAs in neural circuitry. Finally, we proposed that lncRNA studies would advance our understanding on how neural circuits develop and function in physiology and disease conditions.

The RNA-binding protein caper is required for sensory neuron development in Drosophila melanogaster

BACKGROUND

Alternative splicing mediated by RNA-binding proteins (RBPs) is emerging as a fundamental mechanism for the regulation of gene expression. Alternative splicing has been shown to be a widespread phenomenon that facilitates the diversification of gene products in a tissue-specific manner. Although defects in alternative splicing are rooted in many neurological disorders, only a small fraction of splicing factors have been investigated in detail.

RESULTS

We find that the splicing factor Caper is required for the development of multiple different mechanosensory neuron subtypes at multiple life stages in Drosophila melanogaster. Disruption of Caper function causes defects in dendrite morphogenesis of larval dendrite arborization neurons and neuronal positioning of embryonic proprioceptors, as well as the development and maintenance of adult mechanosensory bristles. Additionally, we find that Caper dysfunction results in aberrant locomotor behavior in adult flies. Transcriptome-wide analyses further support a role for Caper in alternative isoform regulation of genes that function in neurogenesis.

CONCLUSIONS

Our results provide the first evidence for a fundamental and broad requirement for the highly conserved splicing factor Caper in the development and maintenance of the nervous system and provide a framework for future studies on the detailed mechanism of Caper-mediated RNA regulation. Developmental Dynamics 246:610-624, 2017. © 2017 Wiley Periodicals, Inc.

Components of RNA granules affect their localization and dynamics in neuronal dendrites

In neurons, mRNAs are transported along dendrites as large RNA granules. Localization of these granules was found to be developmentally regulated, and Staufen1, a major component of RNA transport granules, was critical for activity-dependent synaptic localization.

In neurons, RNA transport is important for local protein synthesis. mRNAs are transported along dendrites as large RNA granules. The localization and dynamics of Puralpha and Staufen1 (Stau1), major components of RNA transport granules, were investigated in cultured hippocampal neurons. Puralpha-positive granules were localized in both the shafts and spines of dendrites. In contrast, Stau1-positive granules tended to be localized mainly in dendritic shafts. More than 90% of Puralpha-positive granules were positive for Stau1 in immature dendrites, while only half were positive in mature dendrites. Stau1-negative Puralpha granules tended to be stationary with fewer anterograde and retrograde movements than Stau1-positive Puralpha granules. After metabotropic glutamate receptor 5 activation, Stau1-positive granules remained in the dendritic shafts, while Puralpha granules translocated from the shaft to the spine. The translocation of Puralpha granules was dependent on myosin Va, an actin-based molecular motor protein. Collectively our findings suggest the possibility that the loss of Stau1 in Puralpha-positive RNA granules might promote their activity-dependent translocation into dendritic spines, which could underlie the regulation of protein synthesis in synapses.

Tracking the Fragile X Mental Retardation Protein in a Highly Ordered Neuronal RiboNucleoParticles Population: A Link between Stalled Polyribosomes and RNA Granules

Local translation at the synapse plays key roles in neuron development and activity-dependent synaptic plasticity. mRNAs are translocated from the neuronal soma to the distant synapses as compacted ribonucleoparticles referred to as RNA granules. These contain many RNA-binding proteins, including the Fragile X Mental Retardation Protein (FMRP), the absence of which results in Fragile X Syndrome, the most common inherited form of intellectual disability and the leading genetic cause of autism. Using FMRP as a tracer, we purified a specific population of RNA granules from mouse brain homogenates. Protein composition analyses revealed a strong relationship between polyribosomes and RNA granules. However, the latter have distinct architectural and structural properties, since they are detected as close compact structures as observed by electron microscopy, and converging evidence point to the possibility that these structures emerge from stalled polyribosomes. Time-lapse video microscopy indicated that single granules merge to form cargoes that are transported from the soma to distal locations. Transcriptomic analyses showed that a subset of mRNAs involved in cytoskeleton remodelling and neural development is selectively enriched in RNA granules. One third of the putative mRNA targets described for FMRP appear to be transported in granules and FMRP is more abundant in granules than in polyribosomes. This observation supports a primary role for FMRP in granules biology. Our findings open new avenues for the study of RNA granule dysfunctions in animal models of nervous system disorders, such as Fragile X syndrome.

Fragile X syndrome is the most common form of inherited mental retardation affecting approximately 1 female out of 7000 and 1 male out of 4000 worldwide. The syndrome is due to the silencing of a single gene, the Fragile Mental Retardation 1 (FMR1), that codes for the Fragile X mental retardation protein (FMRP). This protein is highly expressed in brain and controls local protein synthesis essential for neuronal development and maturation as well as the formation of neural circuits. Several studies suggest a role for FMRP in the regulation of mRNA transport along axons and dendrites to distant synaptic locations in structures called RNA granules. Here we report the isolation of a particular subpopulation of these structures and the analysis of their architecture and composition in terms of RNA and protein. Also, using time-lapse video microscopy, we monitored granule transport and fusion throughout neuronal processes. These findings open new avenues for the study of RNA transport dysfunctions in animal models of nervous system disorders.

Memory retrieval requires ongoing protein synthesis and NMDA receptor activity-mediated AMPA receptor trafficking

Whereas consolidation and reconsolidation are considered dynamic processes requiring protein synthesis, memory retrieval has long been considered a passive readout of previously established plasticity. However, previous findings suggest that memory retrieval may be more dynamic than previously thought. This study therefore aimed at investigating the molecular mechanisms underlying memory retrieval in the rat. Infusion of protein synthesis inhibitors (rapamycin or anisomycin) in the amygdala 10 min before memory retrieval transiently impaired auditory fear memory expression, suggesting ongoing protein synthesis is required to enable memory retrieval. We then investigated the role of protein synthesis in NMDA receptor activity-mediated AMPA receptor trafficking. Coinfusion of an NMDA receptor antagonist (ifenprodil) or infusion of an AMPA receptor endocytosis inhibitor (GluA23Y) before rapamycin prevented this memory impairment. Furthermore, rapamycin transiently decreased GluA1 levels at the postsynaptic density (PSD), but did not affect extrasynaptic sites. This effect at the PSD was prevented by an infusion of GluA23Y before rapamycin. Together, these data show that ongoing protein synthesis is required before memory retrieval is engaged, and suggest that this protein synthesis may be involved in the NMDAR activity-mediated trafficking of AMPA receptors that takes place during memory retrieval.

Neuron-wide RNA transport combines with netrin-mediated local translation to spatially regulate the synaptic proteome

The persistence of experience-dependent changes in brain connectivity requires RNA localization and protein synthesis. Previous studies have demonstrated a role for local translation in altering the structure and function of synapses during synapse formation and experience-dependent synaptic plasticity. In this study, we ask whether in addition to promoting local translation, local stimulation also triggers directed trafficking of RNAs from nucleus to stimulated synapses. Imaging of RNA localization and translation in cultured Aplysia sensory-motor neurons revealed that RNAs were delivered throughout the arbor of the sensory neuron, but that translation was enriched only at sites of synaptic contact and/or synaptic stimulation. Investigation of the mechanisms that trigger local translation revealed a role for calcium-dependent retrograde netrin-1/DCC receptor signaling. Spatially restricting gene expression by regulating local translation rather than by directing the delivery of mRNAs from nucleus to stimulated synapses maximizes the readiness of the entire neuronal arbor to respond to local cues.

NoFold: RNA structure clustering without folding or alignment

Structures that recur across multiple different transcripts, called structure motifs, often perform a similar function-for example, recruiting a specific RNA-binding protein that then regulates translation, splicing, or subcellular localization. Identifying common motifs between coregulated transcripts may therefore yield significant insight into their binding partners and mechanism of regulation. However, as most methods for clustering structures are based on folding individual sequences or doing many pairwise alignments, this results in a tradeoff between speed and accuracy that can be problematic for large-scale data sets. Here we describe a novel method for comparing and characterizing RNA secondary structures that does not require folding or pairwise alignment of the input sequences. Our method uses the idea of constructing a distance function between two objects by their respective distances to a collection of empirical examples or models, which in our case consists of 1973 Rfam family covariance models. Using this as a basis for measuring structural similarity, we developed a clustering pipeline called NoFold to automatically identify and annotate structure motifs within large sequence data sets. We demonstrate that NoFold can simultaneously identify multiple structure motifs with an average sensitivity of 0.80 and precision of 0.98 and generally exceeds the performance of existing methods. We also perform a cross-validation analysis of the entire set of Rfam families, achieving an average sensitivity of 0.57. We apply NoFold to identify motifs enriched in dendritically localized transcripts and report 213 enriched motifs, including both known and novel structures.

Neuronal BC RNAs cooperate with eIF4B to mediate activity-dependent translational control

Regulatory brain cytoplasmic RNAs cooperate with eukaryotic initiation factor 4B to couple translation to receptor activation in support of long-term plastic changes in neurons.

In neurons, translational regulation of gene expression has been implicated in the activity-dependent management of synapto-dendritic protein repertoires. However, the fundamentals of stimulus-modulated translational control in neurons remain poorly understood. Here we describe a mechanism in which regulatory brain cytoplasmic (BC) RNAs cooperate with eukaryotic initiation factor 4B (eIF4B) to control translation in a manner that is responsive to neuronal activity. eIF4B is required for the translation of mRNAs with structured 5′ untranslated regions (UTRs), exemplified here by neuronal protein kinase Mζ (PKMζ) mRNA. Upon neuronal stimulation, synapto-dendritic eIF4B is dephosphorylated at serine 406 in a rapid process that is mediated by protein phosphatase 2A. Such dephosphorylation causes a significant decrease in the binding affinity between eIF4B and BC RNA translational repressors, enabling the factor to engage the 40S small ribosomal subunit for translation initiation. BC RNA translational control, mediated via eIF4B phosphorylation status, couples neuronal activity to translational output, and thus provides a mechanistic basis for long-term plastic changes in nerve cells.

Eusocial insects as emerging models for behavioural epigenetics

Understanding the molecular basis of how behavioural states are established, maintained and altered by environmental cues is an area of considerable and growing interest. Epigenetic processes, including methylation of DNA and post-translational modification of histones, dynamically modulate activity-dependent gene expression in neurons and can therefore have important regulatory roles in shaping behavioural responses to environmental cues. Several eusocial insect species - with their unique displays of behavioural plasticity due to age, morphology and social context - have emerged as models to investigate the genetic and epigenetic underpinnings of animal social behaviour. This Review summarizes recent studies in the epigenetics of social behaviour and offers perspectives on emerging trends and prospects for establishing genetic tools in eusocial insects.

A role for dendritic mGluR5-mediated local translation of Arc/Arg3.1 in MEF2-dependent synapse elimination

Experience refines synaptic connectivity through neural activity-dependent regulation of transcription factors. Although activity-dependent regulation of transcription factors has been well described, it is unknown whether synaptic activity and local, dendritic regulation of the induced transcripts are necessary for mammalian synaptic plasticity in response to transcription factor activation. Neuronal depolarization activates the myocyte enhancer factor 2 (MEF2) family of transcription factors that suppresses excitatory synapse number. We report that activation of metabotropic glutamate receptor 5 (mGluR5) on the dendrites, but not cell soma, of hippocampal CA1 neurons is required for MEF2-induced functional and structural synapse elimination. We present evidence that mGluR5 is necessary for synapse elimination to stimulate dendritic translation of the MEF2 target gene Arc/Arg3.1. Activity-regulated cytoskeletal-associated protein (Arc) is required for MEF2-induced synapse elimination, where it plays an acute, cell-autonomous, and postsynaptic role. This work reveals a role for dendritic activity in local translation of specific transcripts in synapse refinement.

Interactions of noncanonical motifs with hnRNP A2 promote activity-dependent RNA transport in neurons

Ca²⁺-dependent RNA–protein interactions enable activity-inducible RNA transport in dendrites.

A key determinant of neuronal functionality and plasticity is the targeted delivery of select ribonucleic acids (RNAs) to synaptodendritic sites of protein synthesis. In this paper, we ask how dendritic RNA transport can be regulated in a manner that is informed by the cell’s activity status. We describe a molecular mechanism in which inducible interactions of noncanonical RNA motif structures with targeting factor heterogeneous nuclear ribonucleoprotein (hnRNP) A2 form the basis for activity-dependent dendritic RNA targeting. High-affinity interactions between hnRNP A2 and conditional GA-type RNA targeting motifs are critically dependent on elevated Ca²⁺ levels in a narrow concentration range. Dendritic transport of messenger RNAs that carry such GA motifs is inducible by influx of Ca²⁺ through voltage-dependent calcium channels upon β-adrenergic receptor activation. The combined data establish a functional correspondence between Ca²⁺-dependent RNA–protein interactions and activity-inducible RNA transport in dendrites. They also indicate a role of genomic retroposition in the phylogenetic development of RNA targeting competence.

Anisomycin injection in area CA3 of the hippocampus impairs both short-term and long-term memories of contextual fear

Protein synthesis is involved in the consolidation of short-term memory into long-term memory. Previous electrophysiological data concerning LTP in CA3 suggest that protein synthesis in that region might also be necessary for short-term memory. We tested this hypothesis by locally injecting the protein synthesis inhibitor anisomycin in hippocampal area CA1 or CA3 immediately after contextual fear conditioning. As previously shown, injections in CA1 impaired long-term memory but spared short-term memory. Conversely, injections in CA3 impaired both long-term and short-term memories. We conclude that early steps of experience-induced plasticity occurring in CA3 and underlying short-term memory require protein synthesis.

Axonal transcription factors: novel regulators of growth cone-to-nucleus signaling

Developing axons contain transcripts that are locally translated to influence the axonal proteome. Recent studies have shown that axonal transcripts include mRNAs encoding transcription factors. These mRNAs are translated to produce transcription factors that can be retrogradely trafficked back to the nucleus, where they regulate gene expression programs. These findings point to a novel mechanism of growth cone-to-nucleus signaling that occurs when growth cones encounter extracellular signaling molecules that stimulate local translation of these transcription factors, thereby influencing gene transcription. Here we summarize recent findings on local translation of transcription factors in axons and their roles in different neuronal processes such as neuronal specification, survival, and axon regeneration. Comprehensive axonal transcriptome studies have revealed transcripts that encode many more transcription factors and cofactors, suggesting a potentially broad role for this type of signaling. We review the progress on the approaches and tools that have been developed to study local translation and retrograde trafficking of transcription factors. We also highlight the challenges in the field and discuss the potential routes to resolving them.

Differentiation in the protein synthesis-dependency of persistent synaptic plasticity in mossy fiber and associational/commissural CA3 synapses in vivo

Long-term potentiation (LTP) and long-term depression (LTD) are two mechanisms involved in the long-term storage of information in hippocampal synapses. In the hippocampal CA1 region, the late phases of LTP and LTD are protein-synthesis dependent. In the dentate gyrus, late-LTP but not LTD requires protein synthesis. The protein synthesis-dependency of persistent plasticity at CA3 synapses has not yet been characterized. Here, the roles of protein transcription and translation at mossy fiber (mf) and associational/commissural (AC)- synapses were studied in freely behaving rats. In control animals, low-frequency stimulation (LFS) evoked robust LTD (>24 h), whereas high-frequency stimulation (HFS) elicited robust LTP (>24 h) at both mf-CA3 and AC-CA3 synapses. Translation inhibitors prevented early and late phases of LTP and LTD at mf-CA3 synapses. In contrast, at AC–CA3 synapses, translation inhibitors prevented intermediate/late-LTP and late-LTD only. Transcription effects were also synapse-specific: whereas transcription inhibitors inhibited late-LTP and late-LTD (>3 h) at mf-CA3 synapses, at AC–CA3 synapses, protein transcription affected early-LTP and late-LTD. These results show that the AC-CA3 and mf-CA3 synapses display different properties in terms of their protein synthesis dependency, suggesting different roles in the processing of short- and long term synaptic plasticity.

SUMOylation is required for glycine-induced increases in AMPA receptor surface expression (ChemLTP) in hippocampal neurons

Multiple pathways participate in the AMPA receptor trafficking that underlies long-term potentiation (LTP) of synaptic transmission. Here we demonstrate that protein SUMOylation is required for insertion of the GluA1 AMPAR subunit following transient glycine-evoked increase in AMPA receptor surface expression (ChemLTP) in dispersed neuronal cultures. ChemLTP increases co-localisation of SUMO-1 and the SUMO conjugating enzyme Ubc9 and with PSD95 consistent with the recruitment of SUMOylated proteins to dendritic spines. In addition, we show that ChemLTP increases dendritic levels of SUMO-1 and Ubc9 mRNA. Consistent with activity dependent translocation of these mRNAs to sites near synapses, levels of the mRNA binding and dendritic transport protein CPEB are also increased by ChemLTP. Importantly, reducing the extent of substrate protein SUMOylation by overexpressing the deSUMOylating enzyme SENP-1 or inhibiting SUMOylation by expressing dominant negative Ubc9 prevent the ChemLTP-induced increase in both AMPAR surface expression and dendritic SUMO-1 mRNA. Taken together these data demonstrate that SUMOylation of synaptic protein(s) involved in AMPA receptor trafficking is necessary for activity-dependent increases in AMPAR surface expression.

Actin isoforms in neuronal development and function

The actin cytoskeleton contributes directly or indirectly to nearly every aspect of neuronal development and function. This diversity of functions is often attributed to actin regulatory proteins, although how the composition of the actin cytoskeleton itself may influence its function is often overlooked. In neurons, the actin cytoskeleton is composed of two distinct isoforms, β- and γ-actin. Functions for β-actin have been investigated in axon guidance, synaptogenesis, and disease. Insight from loss-of-function in vivo studies has also revealed novel roles for β-actin in select brain structures and behaviors. Conversely, very little is known regarding functions of γ-actin in neurons. The dysregulation or mutation of both β- and γ-actin has been implicated in multiple human neurological disorders, however, demonstrating the critical importance of these still poorly understood proteins. This chapter highlights what is currently known regarding potential distinct functions for β- and γ-actin in neurons as well as the significant areas that remain unexplored.

Translational control at the synapse: role of RNA regulators

Translational control of gene expression is instrumental in the regulation of eukaryotic cellular form and function. Neurons in particular rely on this form of control because their numerous synaptic connections need to be independently modulated in an input-specific manner. Brain cytoplasmic (BC) RNAs implement translational control at neuronal synapses. BC RNAs regulate protein synthesis by interacting with eIF4 translation initiation factors. Recent evidence suggests that such regulation is required to control synaptic strength, and that dysregulation of local protein synthesis precipitates neuronal hyperexcitability and a propensity for epileptogenic responses. A similar phenotype results from lack of fragile X mental retardation protein (FMRP), indicating that BC RNAs and FMRP use overlapping and convergent modes of action in neuronal translational regulation.

Quantitative biology of single neurons

The building blocks of complex biological systems are single cells. Fundamental insights gained from single-cell analysis promise to provide the framework for understanding normal biological systems development as well as the limits on systems/cellular ability to respond to disease. The interplay of cells to create functional systems is not well understood. Until recently, the study of single cells has concentrated primarily on morphological and physiological characterization. With the application of new highly sensitive molecular and genomic technologies, the quantitative biochemistry of single cells is now accessible.

Brain region-specific gene expression changes after chronic intermittent ethanol exposure and early withdrawal in C57BL/6J mice

Neuroadaptations that participate in the ontogeny of alcohol dependence are likely a result of altered gene expression in various brain regions. The present study investigated brain region-specific changes in the pattern and magnitude of gene expression immediately following chronic intermittent ethanol (CIE) exposure and 8 hours following final ethanol exposure [i.e. early withdrawal (EWD)]. High-density oligonucleotide microarrays (Affymetrix 430A 2.0, Affymetrix, Santa Clara, CA, USA) and bioinformatics analysis were used to characterize gene expression and function in the prefrontal cortex (PFC), hippocampus (HPC) and nucleus accumbens (NAc) of C57BL/6J mice (Jackson Laboratories, Bar Harbor, ME, USA). Gene expression levels were determined using gene chip robust multi-array average followed by statistical analysis of microarrays and validated by quantitative real-time reverse transcription polymerase chain reaction and Western blot analysis. Results indicated that immediately following CIE exposure, changes in gene expression were strikingly greater in the PFC (284 genes) compared with the HPC (16 genes) and NAc (32 genes). Bioinformatics analysis revealed that most of the transcriptionally responsive genes in the PFC were involved in Ras/MAPK signaling, notch signaling or ubiquitination. In contrast, during EWD, changes in gene expression were greatest in the HPC (139 genes) compared with the PFC (four genes) and NAc (eight genes). The most transcriptionally responsive genes in the HPC were involved in mRNA processing or actin dynamics. Of the few genes detected in the NAc, the most representatives were involved in circadian rhythms. Overall, these findings indicate that brain region-specific and time-dependent neuroadaptive alterations in gene expression play an integral role in the development of alcohol dependence and withdrawal.

Combinatorial use of translational co-factors for cell type-specific regulation during neuronal morphogenesis in Drosophila

The translational regulators Nanos (Nos) and Pumilio (Pum) work together to regulate the morphogenesis of dendritic arborization (da) neurons of the Drosophila larval peripheral nervous system. In contrast, Nos and Pum function in opposition to one another in the neuromuscular junction to regulate the morphogenesis and the electrophysiological properties of synaptic boutons. Neither the cellular functions of Nos and Pum nor their regulatory targets in neuronal morphogenesis are known. Here we show that Nos and Pum are required to maintain the dendritic complexity of da neurons during larval growth by promoting the outgrowth of new dendritic branches and the stabilization of existing dendritic branches, in part by regulating the expression of cut and head involution defective. Through an RNA interference screen we uncover a role for the translational co-factor Brain Tumor (Brat) in dendrite morphogenesis of da neurons and demonstrate that Nos, Pum, and Brat interact genetically to regulate dendrite morphogenesis. In the neuromuscular junction, Brat function is most likely specific for Pum in the presynaptic regulation of bouton morphogenesis. Our results reveal how the combinatorial use of co-regulators like Nos, Pum and Brat can diversify their roles in post-transcriptional regulation of gene expression for neuronal morphogenesis.

Dopamine-regulated microRNA MiR-181a controls GluA2 surface expression in hippocampal neurons

The dynamic expression of AMPA-type glutamate receptors (AMPA-R) at synapses is a key determinant of synaptic plasticity, including neuroadaptations to drugs of abuse. Recently, microRNAs (miRNAs) have emerged as important posttranscriptional regulators of synaptic plasticity, but whether they target glutamate receptors to mediate this effect is not known. Here we used microarray screening to identify miRNAs that regulate synaptic plasticity within the nucleus accumbens, a brain region critical to forming drug-seeking habits. One of the miRNAs that showed a robust enrichment at medium spiny neuron synapses was miR-181a. Using bioinformatics tools, we detected a highly conserved miR-181a binding site within the mRNA encoding the GluA2 subunit of AMPA-Rs. Overexpression and knockdown of miR-181a in primary neurons demonstrated that this miRNA is a negative posttranscriptional regulator of GluA2 expression. Additionally, miR-181a overexpression reduced GluA2 surface expression, spine formation, and miniature excitatory postsynaptic current (mEPSC) frequency in hippocampal neurons, suggesting that miR-181a could regulate synaptic function. Moreover, miR-181a expression was induced by dopamine signaling in primary neurons, as well as by cocaine and amphetamines, in a mouse model of chronic drug treatment. Taken together, our results identify miR-181a as a key regulator of mammalian AMPA-type glutamate receptors, with potential implications for the regulation of drug-induced synaptic plasticity.

Dual nature of translational control by regulatory BC RNAs

In higher eukaryotes, increasing evidence suggests, gene expression is to a large degree controlled by RNA. Regulatory RNAs have been implicated in the management of neuronal function and plasticity in mammalian brains. However, much of the molecular-mechanistic framework that enables neuronal regulatory RNAs to control gene expression remains poorly understood. Here, we establish molecular mechanisms that underlie the regulatory capacity of neuronal BC RNAs in the translational control of gene expression. We report that regulatory BC RNAs employ a two-pronged approach in translational control. One of two distinct repression mechanisms is mediated by C-loop motifs in BC RNA 3' stem-loop domains. These C-loops bind to eIF4B and prevent the factor's interaction with 18S rRNA of the small ribosomal subunit. In the second mechanism, the central A-rich domains of BC RNAs target eIF4A, specifically inhibiting its RNA helicase activity. Thus, BC RNAs repress translation initiation in a bimodal mechanistic approach. As BC RNA functionality has evolved independently in rodent and primate lineages, our data suggest that BC RNA translational control was necessitated and implemented during mammalian phylogenetic development of complex neural systems.

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.

Subcellular profiling reveals distinct and developmentally regulated repertoire of growth cone mRNAs

Cue-directed axon guidance depends partly on local translation in growth cones. Many mRNA transcripts are known to reside in developing axons, yet little is known about their subcellular distribution or, specifically, which transcripts are in growth cones. Here laser capture microdissection (LCM) was used to isolate the growth cones of retinal ganglion cell (RGC) axons of two vertebrate species, mouse and Xenopus, coupled with unbiased genomewide microarray profiling. An unexpectedly large pool of mRNAs defined predominant pathways in protein synthesis, oxidative phosphorylation, cancer, neurological disease, and signaling. Comparative profiling of "young" (pathfinding) versus "old" (target-arriving) Xenopus growth cones revealed that the number and complexity of transcripts increases dramatically with age. Many presynaptic protein mRNAs are present exclusively in old growth cones, suggesting that functionally related sets of mRNAs are targeted to growth cones in a developmentally regulated way. Remarkably, a subset of mRNAs was significantly enriched in the growth cone compared with the axon compartment, indicating that mechanisms exist to localize mRNAs selectively to the growth cone. Furthermore, some receptor transcripts (e.g., EphB4), present exclusively in old growth cones, were equally abundant in young and old cell bodies, indicating that RNA trafficking from the soma is developmentally regulated. Our findings show that the mRNA repertoire in growth cones is regulated dynamically with age and suggest that mRNA localization is tailored to match the functional demands of the growing axon tip as it transforms into the presynaptic terminal.

Proteomics of dense core secretory vesicles reveal distinct protein categories for secretion of neuroeffectors for cell-cell communication

Regulated secretion of neurotransmitters and neurohumoral factors from dense core secretory vesicles provides essential neuroeffectors for cell-cell communication in the nervous and endocrine systems. This study provides comprehensive proteomic characterization of the categories of proteins in chromaffin dense core secretory vesicles that participate in cell-cell communication from the adrenal medulla. Proteomic studies were conducted by nano-HPLC Chip MS/MS tandem mass spectrometry. Results demonstrate that these secretory vesicles contain proteins of distinct functional categories consisting of neuropeptides and neurohumoral factors, protease systems, neurotransmitter enzymes and transporters, receptors, enzymes for biochemical processes, reduction/oxidation regulation, ATPases, protein folding, lipid biochemistry, signal transduction, exocytosis, calcium regulation, as well as structural and cell adhesion proteins. The secretory vesicle proteomic data identified 371 proteins in the soluble fraction and 384 membrane proteins, for a total of 686 distinct secretory vesicle proteins. Notably, these proteomic analyses illustrate the presence of several neurological disease-related proteins in these secretory vesicles, including huntingtin interacting protein, cystatin C, ataxin 7, and prion protein. Overall, these findings demonstrate that multiple protein categories participate in dense core secretory vesicles for production, storage, and secretion of bioactive neuroeffectors for cell-cell communication in health and disease.

Initiating and growing an axon

The ability of neurons to form a single axon and multiple dendrites underlies the directional flow of information transfer in the central nervous system. Dendrites and axons are molecularly and functionally distinct domains. Dendrites integrate synaptic inputs, triggering the generation of action potentials at the level of the soma. Action potentials then propagate along the axon, which makes presynaptic contacts onto target cells. This article reviews what is known about the cellular and molecular mechanisms underlying the ability of neurons to initiate and extend a single axon during development. Remarkably, neurons can polarize to form a single axon, multiple dendrites, and later establish functional synaptic contacts in reductionist in vitro conditions. This approach became, and remains, the dominant model to study axon initiation and growth and has yielded the identification of many molecules that regulate axon formation in vitro (Dotti et al. 1988). At present, only a few of the genes identified using in vitro approaches have been shown to be required for axon initiation and outgrowth in vivo. In vitro, axon initiation and elongation are largely intrinsic properties of neurons that are established in the absence of relevant extracellular cues. However, the importance of extracellular cues to axon initiation and outgrowth in vivo is emerging as a major theme in neural development (Barnes and Polleux 2009). In this article, we focus our attention on the extracellular cues and signaling pathways required in vivo for axon initiation and axon extension.

A role for huntington disease protein in dendritic RNA granules

Regulated transport and local translation of mRNA in neurons are critical for modulating synaptic strength, maintaining proper neural circuitry, and establishing long term memory. Neuronal RNA granules are ribonucleoprotein particles that serve to transport mRNA along microtubules and control local protein synthesis in response to synaptic activity. Studies suggest that neuronal RNA granules share similar structures and functions with somatic P-bodies. We recently reported that the Huntington disease protein huntingtin (Htt) associates with Argonaute (Ago) and localizes to cytoplasmic P-bodies, which serve as sites of mRNA storage, degradation, and small RNA-mediated gene silencing. Here we report that wild-type Htt associates with Ago2 and components of neuronal granules and co-traffics with mRNA in dendrites. Htt was found to co-localize with RNA containing the 3'-untranslated region sequence of known dendritically targeted mRNAs. Knockdown of Htt in neurons caused altered localization of mRNA. When tethered to a reporter construct, Htt down-regulated reporter gene expression in a manner dependent on Ago2, suggesting that Htt may function to repress translation of mRNAs during transport in neuronal granules.

An RNA transport system in Candida albicans regulates hyphal morphology and invasive growth

Localization of specific mRNAs is an important mechanism through which cells achieve polarity and direct asymmetric growth. Based on a framework established in Saccharomyces cerevisiae, we describe a She3-dependent RNA transport system in Candida albicans, a fungal pathogen of humans that grows as both budding (yeast) and filamentous (hyphal and pseudohyphal) forms. We identify a set of 40 mRNAs that are selectively transported to the buds of yeast-form cells and to the tips of hyphae, and we show that many of the genes encoded by these mRNAs contribute to hyphal development, as does the transport system itself. Although the basic system of mRNA transport is conserved between S. cerevisiae and C. albicans, we find that the cargo mRNAs have diverged considerably, implying that specific mRNAs can easily move in and out of transport control over evolutionary timescales. The differences in mRNA cargos likely reflect the distinct selective pressures acting on the two species.

BC1 regulation of metabotropic glutamate receptor-mediated neuronal excitability

Regulatory RNAs have been suggested to contribute to the control of gene expression in eukaryotes. Brain cytoplasmic (BC) RNAs are regulatory RNAs that control translation initiation. We now report that neuronal BC1 RNA plays an instrumental role in the protein-synthesis-dependent implementation of neuronal excitation-repression equilibria. BC1 repression counter-regulates translational stimulation resulting from synaptic activation of group I metabotropic glutamate receptors (mGluRs). Absence of BC1 RNA precipitates plasticity dysregulation in the form of neuronal hyperexcitability, elicited by group I mGluR-stimulated translation and signaled through the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway. Dysregulation of group I mGluR function in the absence of BC1 RNA gives rise to abnormal brain function. Cortical EEG recordings from freely moving BC1(-/-) animals show that group I mGluR-mediated oscillations in the gamma frequency range are significantly elevated. When subjected to sensory stimulation, these animals display an acute group I mGluR-dependent propensity for convulsive seizures. Inadequate RNA control in neurons is thus causally linked to heightened group I mGluR-stimulated translation, neuronal hyperexcitability, heightened gamma band oscillations, and epileptogenesis. These data highlight the significance of small RNA control in neuronal plasticity.

Synapse- and stimulus-specific local translation during long-term neuronal plasticity

Long-term memory and synaptic plasticity require changes in gene expression and yet can occur in a synapse-specific manner. Messenger RNA localization and regulated translation at synapses are thus critical for establishing synapse specificity. Using live-cell microscopy of photoconvertible fluorescent protein translational reporters, we directly visualized local translation at synapses during long-term facilitation of Aplysia sensory-motor synapses. Translation of the reporter required multiple applications of serotonin, was spatially restricted to stimulated synapses, was transcript- and stimulus-specific, and occurred during long-term facilitation but not during long-term depression of sensory-motor synapses. Translational regulation only occurred in the presence of a chemical synapse and required calcium signaling in the postsynaptic motor neuron. Thus, highly regulated local translation occurs at synapses during long-term plasticity and requires trans-synaptic signals.

Regulation of glutamate receptor subunit availability by microRNAs

The efficacy of synaptic transmission depends, to a large extent, on postsynaptic receptor abundance. The molecular mechanisms controlling receptor abundance are poorly understood. We tested whether abundance of postsynaptic glutamate receptors (GluRs) in Drosophila neuromuscular junctions is controlled by microRNAs, and provide evidence that it is. We show here that postsynaptic knockdown of dicer-1, the endoribonuclease necessary for microRNA synthesis, leads to large increases in postsynaptic GluR subunit messenger RNA and protein. Specifically, we measured increases in GluRIIA and GluRIIB but not GluRIIC. Further, knockout of MiR-284, a microRNA predicted to bind to GluRIIA and GluRIIB but not GluRIIC, increases expression of GluRIIA and GluRIIB but not GluRIIC proportional to the number of predicted binding sites in each transcript. Most of the de-repressed GluR protein, however, does not appear to be incorporated into functional receptors, and only minor changes in synaptic strength are observed, which suggests that microRNAs primarily regulate Drosophila receptor subunit composition rather than overall receptor abundance or synaptic strength.