Dynamic visualization of local protein synthesis in hippocampal neurons

Fragile X Mental Retardation Protein Is Required to Maintain Visual Conditioning-Induced Behavioral Plasticity by Limiting Local Protein Synthesis

Fragile X mental retardation protein (FMRP) is thought to regulate neuronal plasticity by limiting dendritic protein synthesis, but direct demonstration of a requirement for FMRP control of local protein synthesis during behavioral plasticity is lacking. Here we tested whether FMRP knockdown in Xenopus optic tectum affects local protein synthesis in vivo and whether FMRP knockdown affects protein synthesis-dependent visual avoidance behavioral plasticity. We tagged newly synthesized proteins by incorporation of the noncanonical amino acid azidohomoalanine and visualized them with fluorescent noncanonical amino acid tagging (FUNCAT). Visual conditioning and FMRP knockdown produce similar increases in FUNCAT in tectal neuropil. Induction of visual conditioning-dependent behavioral plasticity occurs normally in FMRP knockdown animals, but plasticity degrades over 24 h. These results indicate that FMRP affects visual conditioning-induced local protein synthesis and is required to maintain the visual conditioning-induced behavioral plasticity.

SIGNIFICANCE STATEMENT

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. Exaggerated dendritic protein synthesis resulting from loss of fragile X mental retardation protein (FMRP) is thought to underlie cognitive deficits in FXS, but no direct evidence has demonstrated that FMRP-regulated dendritic protein synthesis affects behavioral plasticity in intact animals. Xenopus tadpoles exhibit a visual avoidance behavior that improves with visual conditioning in a protein synthesis-dependent manner. We showed that FMRP knockdown and visual conditioning dramatically increase protein synthesis in neuronal processes. Furthermore, induction of visual conditioning-dependent behavioral plasticity occurs normally after FMRP knockdown, but performance rapidly deteriorated in the absence of FMRP. These studies show that FMRP negatively regulates local protein synthesis and is required to maintain visual conditioning-induced behavioral plasticity in vivo.

Nascent Proteome Remodeling following Homeostatic Scaling at Hippocampal Synapses

Homeostatic scaling adjusts the strength of synaptic connections up or down in response to large changes in input. To identify the landscape of proteomic changes that contribute to opposing forms of homeostatic plasticity, we examined the plasticity-induced changes in the newly synthesized proteome. Cultured rat hippocampal neurons underwent homeostatic up-scaling or down-scaling. We used BONCAT (bio-orthogonal non-canonical amino acid tagging) to metabolically label, capture, and identify newly synthesized proteins, detecting and analyzing 5,940 newly synthesized proteins using mass spectrometry and label-free quantitation. Neither up- nor down-scaling produced changes in the number of different proteins translated. Rather, up- and down-scaling elicited opposing translational regulation of several molecular pathways, producing targeted adjustments in the proteome. We discovered ∼300 differentially regulated proteins involved in neurite outgrowth, axon guidance, filopodia assembly, excitatory synapses, and glutamate receptor complexes. We also identified differentially regulated proteins that are associated with multiple diseases, including schizophrenia, epilepsy, and Parkinson's disease.

Accumulating Evidence for Axonal Translation in Neuronal Homeostasis

The specialized structure of the neuron requires that homeostasis is sustained over the meter or more that may separate a cell body from its axonal terminus. Given this impressive distance and an axonal volume that is many times that of the cell body, how is such a compartment grown during development, re-grown after injury, and maintained throughout adulthood? While early answers to these questions focused on the local environment or the cell soma as supplying the needs of the axon, it is now well-established that the axon has some unique needs that can only be met from within. Decades of research have revealed local translation as an indispensable mechanism of axonal homeostasis during development and regeneration in both invertebrates and vertebrates. In contrast, the extent to which the adult, mammalian axonal proteome is maintained through local translation remains unclear and controversial. This mini-review aims to highlight important experiments that have helped to shape the field of axonal translation, to discuss conceptual arguments and recent evidence that supports local translation as important to the maintenance of adult axons, and to suggest experimental approaches that have the potential to further illuminate the role of axonal translation in neuronal homeostasis.

Long-term memory consolidation: The role of RNA-binding proteins with prion-like domains

Long-term and short-term memories differ primarily in the duration of their retention. At a molecular level, long-term memory (LTM) is distinguished from short-term memory (STM) by its requirement for new gene expression. In addition to transcription (nuclear gene expression) the translation of stored mRNAs is necessary for LTM formation. The mechanisms and functions for temporal and spatial regulation of mRNAs required for LTM is a major contemporary problem, of interest from molecular, cell biological, neurobiological and clinical perspectives. This review discusses primary evidence in support for translational regulatory events involved in LTM and a model in which different phases of translation underlie distinct phases of consolidation of memories. However, it focuses largely on mechanisms of memory persistence and the role of prion-like domains in this defining aspect of long-term memory. We consider primary evidence for the concept that Cytoplasmic Polyadenylation Element Binding (CPEB) protein enables the persistence of formed memories by transforming in prion-like manner from a soluble monomeric state to a self-perpetuating and persistent polymeric translationally active state required for maintaining persistent synaptic plasticity. We further discuss prion-like domains prevalent on several other RNA-binding proteins involved in neuronal translational control underlying LTM. Growing evidence indicates that such RNA regulatory proteins are components of mRNP (RiboNucleoProtein) granules. In these proteins, prion-like domains, being intrinsically disordered, could mediate weak transient interactions that allow the assembly of RNP granules, a source of silenced mRNAs whose translation is necessary for LTM. We consider the structural bases for RNA granules formation as well as functions of disordered domains and discuss how these complicate the interpretation of existing experimental data relevant to general mechanisms by which prion-domain containing RBPs function in synapse specific plasticity underlying LTM.

Local translation in neuronal compartments: how local is local?

Efficient neuronal function depends on the continued modulation of the local neuronal proteome. Local protein synthesis plays a central role in tuning the neuronal proteome at specific neuronal regions. Various aspects of translation such as the localization of translational machinery, spatial spread of the newly translated proteins, and their site of action are carried out in specialized neuronal subcompartments to result in a localized functional outcome. In this review, we focus on the various aspects of these local translation compartments such as size, biochemical and organelle composition, structural boundaries, and temporal dynamics. We also discuss the apparent absence of definitive components of translation in these local compartments and the emerging state-of-the-art tools that could help dissecting these conundrums in greater detail in the future.

Clathrin-Dependent Uptake of Paraquat into SH-SY5Y Cells and Its Internalization into Different Subcellular Compartments

The herbicide paraquat (PQ) is an exogenous toxin that allows the selective activation of dopaminergic neurons in the mesencephalon to induce injury and also causes its apoptosis in vitro. However, uptake mechanisms between PQ and neurons remain elusive. To address this issue, we undertook a study of PQ endocytosis in a dopaminergic SH-SY5Y cell line as well as explored the subsequent subcellular location and potential functional analysis of PQ. The PQ was found to bind the SH-SY5Y cell membrane and then became internalized via a clathrin-dependent pathway. PQ was internalized by many subcellular organelles in a time- and dose-dependent manner. Interestingly, the taken up PQ and secretogranin III (SCG3), which became dysregulated with PQ treatment that induced SH-SY5Y apoptosis in our previous study, colocalized in cytoplasmic vesicles. Taken together, our findings indicate that PQ is endocytosed by SH-SY5Y cells and that its multiple, subcellular localizations indicate PQ may potentially be involved in subcellular-level functions. More importantly, PQ distributing preferentially into SCG3-positive vesicles demonstrates its selective targeting which may affect SCG3 and cargoes carried by SCG3-positive vesicles. Therefore, it is reasonable to infer that PQ toxic insults may potentially interfere with neurotransmitter storage and transport associated with secretory granules.

Screening the Molecular Framework Underlying Local Dendritic mRNA Translation

In the last decade, bioinformatic analyses of high-throughput proteomics and transcriptomics data have enabled researchers to gain insight into the molecular networks that may underlie lasting changes in synaptic efficacy. Development and utilization of these techniques have advanced the field of learning and memory significantly. It is now possible to move from the study of activity-dependent changes of a single protein to modeling entire network changes that require local protein synthesis. This data revolution has necessitated the development of alternative computational and statistical techniques to analyze and understand the patterns contained within. Thus, the focus of this review is to provide a synopsis of the journey and evolution toward big data techniques to address still unanswered questions regarding how synapses are modified to strengthen neuronal circuits. We first review the seminal studies that demonstrated the pivotal role played by local mRNA translation as the mechanism underlying the enhancement of enduring synaptic activity. In the interest of those who are new to the field, we provide a brief overview of molecular biology and biochemical techniques utilized for sample preparation to identify locally translated proteins using RNA sequencing and proteomics, as well as the computational approaches used to analyze these data. While many mRNAs have been identified, few have been shown to be locally synthesized. To this end, we review techniques currently being utilized to visualize new protein synthesis, a task that has proven to be the most difficult aspect of the field. Finally, we provide examples of future applications to test the physiological relevance of locally synthesized proteins identified by big data approaches.

Optimized solubilization of TRIzol-precipitated protein permits Western blotting analysis to maximize data available from brain tissue

BACKGROUND

Techniques simultaneously assessing multiple levels of molecular processing are appealing because molecular signaling underlying complex neural phenomena occurs at complementary levels. The TRIzol method isolates RNA and DNA, but protein retrieval is difficult due to inefficient solubilization of precipitated protein pellets.

NEW METHOD

We optimized a buffer for the efficient solubilization of protein from TRIzol-precipitated brain tissue for Western blotting analysis, which was also more effective at directly homogenizing brain tissue than RIPA buffer.

RESULTS

Protein yield during solubilization, in addition to protein yield via direct homogenization, is increased by optimizing concentrations of chemicals in a standard lysis buffer. Effective incubation parameters for both total protein yield and the analysis of post-translational modifications is remarkably flexible. Importantly, different neural cell types and protein classes are represented in solubilized protein samples. Moreover, we used dissociated mouse brain tissue to isolate microglia from other cell types and successfully resolved cell type-specific proteins from these small and difficult to attain samples.

COMPARISON WITH EXISTING METHOD(S)

Solubilization buffers to date have been comprised primarily of SDS or urea; the data herein demonstrate that components common to lysis buffers can also enhance protein solubilization both after direct homogenization and after precipitation.

CONCLUSIONS

This method is suitable for assessing gene and protein expression from a single brain sample, allowing for a more comprehensive evaluation of neural phenomena while minimizing the number of subjects.

Imaging Single-mRNA Localization and Translation in Live Neurons

Local protein synthesis mediates precise spatio-temporal regulation of gene expression for neuronal functions such as long-term plasticity, axon guidance and regeneration. To reveal the underlying mechanisms of local translation, it is crucial to understand mRNA transport, localization and translation in live neurons. Among various techniques for mRNA analysis, fluorescence microscopy has been widely used as the most direct method to study localization of mRNA. Live-cell imaging of single RNA molecules is particularly advantageous to dissect the highly heterogeneous and dynamic nature of messenger ribonucleoprotein (mRNP) complexes in neurons. Here, we review recent advances in the study of mRNA localization and translation in live neurons using novel techniques for single-RNA imaging.

BDNF and Hippocampal Synaptic Plasticity

Brain-derived neurotrophic factor (BDNF) belongs to a family of small secreted proteins that also include nerve growth factor, neurotrophin 3, and neurotrophin 4. BDNF stands out among all neurotrophins by its high expression levels in the brain and its potent effects at synapses. Several aspects of BDNF biology such as transcription, processing, and secretion are regulated by synaptic activity. Such observations prompted the suggestion that BDNF may regulate activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP), a sustained enhancement of excitatory synaptic efficacy thought to underlie learning and memory. Here, we will review the evidence pointing to a fundamental role of this neurotrophin in LTP, especially within the hippocampus. Prominent questions in the field, including the release and action sites of BDNF during LTP, as well as the signaling and molecular mechanisms involved, will also be addressed. The diverse effects of BDNF at excitatory synapses are determined by the activation of TrkB receptors and downstream signaling pathways, and the functions, typically opposing in nature, of its immature form (proBDNF). The activation of p75^NTR receptors by proBDNF and the implications for long-term depression will also be addressed. Finally, we discuss the synergy between TrkB and glucocorticoid receptor signaling to determine cellular responses to stress.

Visualization of newly synthesized neuronal RNA in vitro and in vivo using click-chemistry

The neuronal transcriptome changes dynamically to adapt to stimuli from the extracellular and intracellular environment. In this study, we adapted for the first time a click chemistry technique to label the newly synthesized RNA in cultured hippocampal neurons and intact larval zebrafish brain. Ethynyl uridine (EU) was incorporated into neuronal RNA in a time- and concentration-dependent manner. Newly synthesized RNA granules observed throughout the dendrites were colocalized with mRNA and rRNA markers. In zebrafish larvae, the application of EU to the swim water resulted in uptake and labeling throughout the brain. Using a GABA receptor antagonist, PTZ (pentylenetetrazol), to elevate neuronal activity, we demonstrate that newly transcribed RNA signal increased in specific regions involved in neurogenesis.

Co-regulation of mRNA translation by TDP-43 and Fragile X Syndrome protein FMRP

For proper mammalian brain development and functioning, the translation of many neuronal mRNAs needs to be repressed without neuronal activity stimulations. We have discovered that the expression of a subclass of neuronal proteins essential for neurodevelopment and neuron plasticity is co-regulated at the translational level by TDP-43 and the Fragile X Syndrome protein FMRP. Using molecular, cellular and imaging approaches, we show that these two RNA-binding proteins (RBP) co-repress the translation initiation of Rac1, Map1b and GluR1 mRNAs, and consequently the hippocampal spinogenesis. The co-repression occurs through binding of TDP-43 to mRNA(s) at specific UG/GU sequences and recruitment of the inhibitory CYFIP1-FMRP complex by its glycine-rich domain. This novel regulatory scenario could be utilized to silence a significant portion of around 160 common target mRNAs of the two RBPs. The study establishes a functional/physical partnership between FMRP and TDP-43 that mechanistically links several neurodevelopmental disorders and neurodegenerative diseases.

Glutamate-induced RNA localization and translation in neurons

Localization of mRNA is required for protein synthesis to occur within discrete intracellular compartments. Neurons represent an ideal system for studying the precision of mRNA trafficking because of their polarized structure and the need for synapse-specific targeting. To investigate this targeting, we derived a quantitative and analytical approach. Dendritic spines were stimulated by glutamate uncaging at a diffraction-limited spot, and the localization of single β-actin mRNAs was measured in space and time. Localization required NMDA receptor activity, a dynamic actin cytoskeleton, and the transacting RNA-binding protein, Zipcode-binding protein 1 (ZBP1). The ability of the mRNA to direct newly synthesized proteins to the site of localization was evaluated using a Halo-actin reporter so that RNA and protein were detected simultaneously. Newly synthesized Halo-actin was enriched at the site of stimulation, required NMDA receptor activity, and localized preferentially at the periphery of spines. This work demonstrates that synaptic activity can induce mRNA localization and local translation of β-actin where the new actin participates in stabilizing the expanding synapse in dendritic spines.

Transactivation of TrkB by Sigma-1 receptor mediates cocaine-induced changes in dendritic spine density and morphology in hippocampal and cortical neurons

Cocaine is a highly addictive narcotic associated with dendritic spine plasticity in the striatum. However, it remains elusive whether cocaine modifies spines in a cell type-specific or region-specific manner or whether it alters different types of synapses in the brain. In addition, there is a paucity of data on the regulatory mechanism(s) involved in cocaine-induced modification of spine density. In the current study, we report that cocaine exposure differentially alters spine density, spine morphology, and the types of synapses in hippocampal and cortical neurons. Cocaine exposure in the hippocampus resulted in increased spine density, but had no significant effect on cortical neurons. Although cocaine exposure altered spine morphology in both cell types, the patterns of spine morphology were distinct for each cell type. Furthermore, we observed that cocaine selectively affects the density of excitatory synapses. Intriguingly, in hippocampal neurons cocaine-mediated effects on spine density and morphology involved sigma-1 receptor (Sig-1 R) and its downstream TrkB signaling, which were not the case in cortical neurons. Furthermore, pharmacological inhibition of Sig-1 R prevented cocaine-induced TrkB activation in hippocampal neurons. Our findings reveal a novel mechanism by which cocaine induces selective changes in spine morphology, spine density, and synapse formation, and could provide insights into the cellular basis for the cognitive impairment observed in cocaine addicts.

Presynaptic CamKII regulates activity-dependent axon terminal growth

Spaced synaptic depolarization induces rapid axon terminal growth and the formation of new synaptic boutons at the Drosophila larval neuromuscular junction (NMJ). Here, we identify a novel presynaptic function for the Calcium/Calmodulin-dependent Kinase II (CamKII) protein in the control of activity-dependent synaptic growth. Consistent with this function, we find that both total and phosphorylated CamKII (p-CamKII) are enriched in axon terminals. Interestingly, p-CamKII appears to be enriched at the presynaptic axon terminal membrane. Moreover, levels of total CamKII protein within presynaptic boutons globally increase within one hour following stimulation. These effects correlate with the activity-dependent formation of new presynaptic boutons. The increase in presynaptic CamKII levels is inhibited by treatment with cyclohexamide suggesting a protein-synthesis dependent mechanism. We have previously found that acute spaced stimulation rapidly downregulates levels of neuronal microRNAs (miRNAs) that are required for the control of activity-dependent axon terminal growth at this synapse. The rapid activity-dependent accumulation of CamKII protein within axon terminals is inhibited by overexpression of activity-regulated miR-289 in motor neurons. Experiments in vitro using a CamKII translational reporter show that miR-289 can directly repress the translation of CamKII via a sequence motif found within the CamKII 3' untranslated region (UTR). Collectively, our studies support the idea that presynaptic CamKII acts downstream of synaptic stimulation and the miRNA pathway to control rapid activity-dependent changes in synapse structure.

CSPGs inhibit axon branching by impairing mitochondria-dependent regulation of actin dynamics and axonal translation

Chondroitin sulfate proteoglycans (CSPGs) inhibit the formation of axon collateral branches. The regulation of the axonal cytoskeleton and mitochondria are important components of the mechanism of branching. Actin-dependent axonal plasticity, reflected in the dynamics of axonal actin patches and filopodia, is greatest along segments of the axon populated by mitochondria. It is reported that CSPGs partially depolarize the membrane potential of axonal mitochondria, which impairs the dynamics of the axonal actin cytoskeleton and decreases the formation and duration of axonal filopodia, the first steps in the mechanism of branching. The effects of CSPGs on actin cytoskeletal dynamics are specific to axon segments populated by mitochondria. In contrast, CSPGs do not affect the microtubule content of axons, or the localization of microtubules into axonal filopodia, a required step in the mechanism of branch formation. It is also reported that CSPGs decrease the mitochondria-dependent axonal translation of cortactin, an actin associated protein involved in branching. Finally, the inhibitory effects of CSPGs on axon branching, actin cytoskeletal dynamics and the axonal translation of cortactin are reversed by culturing neurons with acetyl-l-carnitine, which promotes mitochondrial respiration. Collectively these data indicate that CSPGs impair mitochondrial function in axons, an effect which contributes to the inhibition of axon branching. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.

Fragile X Mental Retardation Protein and Dendritic Local Translation of the Alpha Subunit of the Calcium/Calmodulin-Dependent Kinase II Messenger RNA Are Required for the Structural Plasticity Underlying Olfactory Learning

BACKGROUND

In the adult brain, structural plasticity allowing gain or loss of synapses remodels circuits to support learning. In fragile X syndrome, the absence of fragile X mental retardation protein (FMRP) leads to defects in plasticity and learning deficits. FMRP is a master regulator of local translation but its implication in learning-induced structural plasticity is unknown.

METHODS

Using an olfactory learning task requiring adult-born olfactory bulb neurons and cell-specific ablation of FMRP, we investigated whether learning shapes adult-born neuron morphology during their synaptic integration and its dependence on FMRP. We used alpha subunit of the calcium/calmodulin-dependent kinase II (αCaMKII) mutant mice with altered dendritic localization of αCaMKII messenger RNA, as well as a reporter of αCaMKII local translation to investigate the role of this FMRP messenger RNA target in learning-dependent structural plasticity.

RESULTS

Learning induces profound changes in dendritic architecture and spine morphology of adult-born neurons that are prevented by ablation of FMRP in adult-born neurons and rescued by an metabotropic glutamate receptor 5 antagonist. Moreover, dendritically translated αCaMKII is necessary for learning and associated structural modifications and learning triggers an FMRP-dependent increase of αCaMKII dendritic translation in adult-born neurons.

CONCLUSIONS

Our results strongly suggest that FMRP mediates structural plasticity of olfactory bulb adult-born neurons to support olfactory learning through αCaMKII local translation. This reveals a new role for FMRP-regulated dendritic local translation in learning-induced structural plasticity. This might be of clinical relevance for the understanding of critical periods disruption in autism spectrum disorder patients, among which fragile X syndrome is the primary monogenic cause.

Real-Time Imaging Reveals Properties of Glutamate-Induced Arc/Arg 3.1 Translation in Neuronal Dendrites

The immediate early gene Arc (also Arg3.1) produces rapid changes in synaptic properties that are linked to de novo translation. Here we develop a novel translation reporter that exploits the rapid maturation and "flash" kinetics of Gaussia luciferase (Gluc) to visualize Arc translation. Following glutamate stimulation, discrete Arc-Gluc bioluminescent flashes representing sites of de novo translation are detected within 15 s at distributed sites in dendrites, but not spines. Flashes are episodic, lasting ∼20 s, and may be unitary or repeated at ∼minute intervals at the same sites. Analysis of flash amplitudes suggests they represent the quantal product of one or more polyribosomes, while inter-flash intervals appear random, suggesting they arise from a stochastic process. Surprisingly, glutamate-induced translation is dependent on Arc open reading frame. Combined observations support a model in which stalled ribosomes are reactivated to rapidly generate Arc protein.

Effects of GnRH on Neurite Outgrowth, Neurofilament and Spinophilin Proteins Expression in Cultured Spinal Cord Neurons of Rat Embryos

It has been previously described the presence of GnRH receptor in spinal cord neurons of rat embryos and adult rats. However, the functional role of these receptors has not been studied. In this work, the effect of GnRH on neurite outgrowth and cytoskeletal protein expression in cultured spinal cord neurons of rat embryos was analyzed. Specifically, neurofilaments of 68 and 200 kDa by immunoblot assays and spinophilin mRNA expression by RT-PCR. Results show that GnRH stimulates neurite outgrowth in addition to an increase in neurofilaments and spinophilin expression. These findings suggest that GnRH may play a role as neuromodulator in neuronal plasticity and that could be considered as a potential factor for neuronal regeneration in spinal cord injuries.

Dynamic Axonal Translation in Developing and Mature Visual Circuits

Local mRNA translation mediates the adaptive responses of axons to extrinsic signals, but direct evidence that it occurs in mammalian CNS axons in vivo is scant. We developed an axon-TRAP-RiboTag approach in mouse that allows deep-sequencing analysis of ribosome-bound mRNAs in the retinal ganglion cell axons of the developing and adult retinotectal projection in vivo. The embryonic-to-postnatal axonal translatome comprises an evolving subset of enriched genes with axon-specific roles, suggesting distinct steps in axon wiring, such as elongation, pruning, and synaptogenesis. Adult axons, remarkably, have a complex translatome with strong links to axon survival, neurotransmission, and neurodegenerative disease. Translationally co-regulated mRNA subsets share common upstream regulators, and sequence elements generated by alternative splicing promote axonal mRNA translation. Our results indicate that intricate regulation of compartment-specific mRNA translation in mammalian CNS axons supports the formation and maintenance of neural circuits in vivo.

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Dynamic translatome of retinal axons in vivo matches changing subcellular function

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Adult CNS axons translate mRNAs for synaptic transmission and axon survival in vivo

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Target mRNAs of key translation regulators show developmental co-regulation

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Axon-specific sequence motifs link alternative splicing to axonal translation

Inhibition of Lactate Transport Erases Drug Memory and Prevents Drug Relapse

BACKGROUND

Drug memories that associate drug-paired stimuli with the effects of abused drugs contribute to relapse. Exposure to drug-associated contexts causes consolidated drug memories to be in a labile state, during which manipulations can be given to impair drug memories. Although substantial evidence demonstrates the crucial role of neuronal signaling in addiction, little is known about the contribution of astrocyte-neuron communication.

METHODS

Rats were trained for cocaine-induced conditioned place preference (CPP) or self-administration and microinjected with the glycogen phosphorylation inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol into the basolateral amygdala (BLA) immediately after retrieval. The concentration of lactate was measured immediately after retrieval via microdialysis, and the CPP score and number of nosepokes were recorded 24 hours later. Furthermore, we used antisense oligodeoxynucleotides to disrupt the expression of astrocytic lactate transporters (monocarboxylate transporters 1 and 2) in the BLA after retrieval, tested the expression of CPP 1 day later, and injected L-lactate into the BLA 15 minutes before retrieval to rescue the effects of the oligodeoxynucleotides.

RESULTS

Injection of 1,4-dideoxy-1,4-imino-D-arabinitol into the BLA immediately after retrieval prevented the subsequent expression of cocaine-induced CPP, decreased the concentration of lactate in the BLA, and reduced the number of nosepokes for cocaine self-administration. Disrupting the expression of monocarboxylate transporters 1 and 2 in the BLA also caused subsequent deficits in the expression of cocaine-induced CPP, which was rescued by pretreatment with L-lactate.

CONCLUSIONS

Our results suggest that astrocyte-neuron lactate transport in the BLA is critical for the reconsolidation of cocaine memory.

What is memory? The present state of the engram

The mechanism of memory remains one of the great unsolved problems of biology. Grappling with the question more than a hundred years ago, the German zoologist Richard Semon formulated the concept of the engram, lasting connections in the brain that result from simultaneous “excitations”, whose precise physical nature and consequences were out of reach of the biology of his day. Neuroscientists now have the knowledge and tools to tackle this question, however, and this Forum brings together leading contemporary views on the mechanisms of memory and what the engram means today.

Real-Time Imaging of Translation on Single mRNA Transcripts in Live Cells

Translation is under tight spatial and temporal controls to ensure protein production in the right time and place in cells. Methods that allow real-time, high-resolution visualization of translation in live cells are essential for understanding the spatiotemporal dynamics of translation regulation. Based on multivalent fluorescence amplification of the nascent polypeptide signal, we develop a method to image translation on individual mRNA molecules in real time in live cells, allowing direct visualization of translation events at the translation sites. Using this approach, we monitor transient changes of translation dynamics in responses to environmental stresses, capture distinct mobilities of individual polysomes in different subcellular compartments, and detect 3' UTR-dependent local translation and active transport of polysomes in dendrites of primary neurons.

Translation dynamics of single mRNAs in live cells and neurons

Translation is the fundamental biological process converting mRNA information into proteins. Single-molecule imaging in live cells has illuminated the dynamics of RNA transcription; however, it is not yet applicable to translation. Here, we report single-molecule imaging of nascent peptides (SINAPS) to assess translation in live cells. The approach provides direct readout of initiation, elongation, and location of translation. We show that mRNAs coding for endoplasmic reticulum (ER) proteins are translated when they encounter the ER membrane. Single-molecule fluorescence recovery after photobleaching provides direct measurement of elongation speed (5 amino acids per second). In primary neurons, mRNAs are translated in proximal dendrites but repressed in distal dendrites and display "bursting" translation. This technology provides a tool with which to address the spatiotemporal translation mechanism of single mRNAs in living cells.

Dynamics of Translation of Single mRNA Molecules In Vivo

Regulation of mRNA translation, the process by which ribosomes decode mRNAs into polypeptides, is used to tune cellular protein levels. Currently, methods for observing the complete process of translation from single mRNAs in vivo are unavailable. Here, we report the long-term (>1 hr) imaging of single mRNAs undergoing hundreds of rounds of translation in live cells, enabling quantitative measurements of ribosome initiation, elongation, and stalling. This approach reveals a surprising heterogeneity in the translation of individual mRNAs within the same cell, including rapid and reversible transitions between a translating and non-translating state. Applying this method to the cell-cycle gene Emi1, we find strong overall repression of translation initiation by specific 5' UTR sequences, but individual mRNA molecules in the same cell can exhibit dramatically different translational efficiencies. The ability to observe translation of single mRNA molecules in live cells provides a powerful tool to study translation regulation.

Imaging Single mRNA Dynamics in Live Neurons and Brains

RNA is a key player in the process of gene expression. Whereas fluorescence in situ hybridization allows single mRNA imaging in fixed cells, the MS2-GFP labeling technique enables the observation of mRNA dynamics in living cells. Recently, two genetically engineered mouse models have been developed for the application of the MS2-GFP system in live animals. First, the Actb-MBS mouse was generated by knocking in 24 repeats of the MS2 stem-loop sequence in the 3' untranslated region of the β-actin gene. Second, the MCP mouse was made to express the NLS-HA-MCP-GFP transgene in all cell types. By crossing Actb-MBS and MCP mice, a double homozygous mouse line, MCP×MBS, was established to visualize endogenous β-actin mRNA labeled with multiple green fluorescent proteins. By imaging hippocampal neurons or brain slices from MCP×MBS mice, the dynamics of mRNA, such as transcription, transport, and localization, can be studied at single mRNA resolution. In this chapter, we explain the basics of MCP×MBS mice and describe methods for utilizing these animals.

RNA polymerase I transcription is modulated by spatial learning in different brain regions

Long-term memory is accompanied by changes in neuronal morphology and connectivity. These alterations are thought to depend upon new gene expression and protein synthesis over a distributed network of brain structures. Although much evidence supports the idea that the creation of stable, persistent memory traces requires synthesis of new proteins, the role of rRNA transcription and nucleolar activity in learning and memory has hardly been explored. rRNAs needed for protein synthesis result from the activity of two different RNA polymerases, RNA polymerase I and RNA polymerase III, transcribing for 47S RNA and 5S RNA, respectively. In this study, we first investigated the effects of spatial training in the Morris water maze on 47S RNA transcription in the central nervous system, demonstrating bidirectional modulation of its expression over a distributed neural network. We found learning-induced increases in the nucleolar organizer regions in the hippocampus. Finally, we demonstrated that intrahippocampal administrations of CX-5461 (0.6 μg/side), the specific RNA Polymerase I inhibitor, impair the ability of mice to locate the platform in the same task. These results suggest that de novo rRNA transcription is a necessary step for spatial memory consolidation, and that after learning, it occurs in several brain regions with a complex spatiotemporal dynamic. In this study, we demonstrate for the very first time that spatial learning modulates ribosomal RNA transcription in a wide brain circuit, with anatomical specificities in the dynamic of modulation. Together with pharmacological evidences, data presented here support the hypothesis of a necessary role of RNA Pol-I transcription during spatial memory formation. Read the Editorial Highlight for this article on page 673.

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.

The Ins and Outs of miRNA-Mediated Gene Silencing during Neuronal Synaptic Plasticity

Neuronal connections through specialized junctions, known as synapses, create circuits that underlie brain function. Synaptic plasticity, i.e., structural and functional changes to synapses, occurs in response to neuronal activity and is a critical regulator of various nervous system functions, including long-term memory formation. The discovery of mRNAs, miRNAs, ncRNAs, ribosomes, translational repressors, and other RNA binding proteins in dendritic spines allows individual synapses to alter their synaptic strength rapidly through regulation of local protein synthesis in response to different physiological stimuli. In this review, we discuss our understanding of a number of miRNAs, ncRNAs, and RNA binding proteins that are emerging as important regulators of synaptic plasticity, which play a critical role in memory, learning, and diseases that arise when neuronal circuits are impaired.

Dendritic trafficking faces physiologically critical speed-precision tradeoffs

Nervous system function requires intracellular transport of channels, receptors, mRNAs, and other cargo throughout complex neuronal morphologies. Local signals such as synaptic input can regulate cargo trafficking, motivating the leading conceptual model of neuron-wide transport, sometimes called the 'sushi-belt model' (Doyle and Kiebler, 2011). Current theories and experiments are based on this model, yet its predictions are not rigorously understood. We formalized the sushi belt model mathematically, and show that it can achieve arbitrarily complex spatial distributions of cargo in reconstructed morphologies. However, the model also predicts an unavoidable, morphology dependent tradeoff between speed, precision and metabolic efficiency of cargo transport. With experimental estimates of trafficking kinetics, the model predicts delays of many hours or days for modestly accurate and efficient cargo delivery throughout a dendritic tree. These findings challenge current understanding of the efficacy of nucleus-to-synapse trafficking and may explain the prevalence of local biosynthesis in neurons.

Local protein synthesis in neuronal axons: why and how we study

Adaptive brain function and synaptic plasticity rely on dynamic regulation of local proteome. One way for the neuron to introduce new proteins to the axon terminal is to transport those from the cell body, which had long been thought as the only source of axonal proteins. Another way, which is the topic of this review, is synthesizing proteins on site by local mRNA translation. Recent evidence indicates that the axon stores a reservoir of translationally silent mRNAs and regulates their expression solely by translational control. Different stimuli to axons, such as guidance cues, growth factors, and nerve injury, promote translation of selective mRNAs, a process required for the axon’s ability to respond to these cues. One of the critical questions in the field of axonal protein synthesis is how mRNA-specific local translation is regulated by extracellular cues. Here, we review current experimental techniques that can be used to answer this question. Furthermore, we discuss how new technologies can help us understand what biological processes are regulated by axonal protein synthesis in vivo. [BMB Reports 2015; 48(3): 139-146]

Control of spine maturation and pruning through proBDNF synthesized and released in dendrites

Excess synapses formed during early postnatal development are pruned over an extended period, while the remaining synapses mature. Synapse pruning is critical for activity-dependent refinement of neuronal connections and its dysregulation has been found in neurodevelopmental disorders such as autism spectrum disorders; however, the mechanism underlying synapse pruning remains largely unknown. As dendritic spines are the postsynaptic sites for the vast majority of excitatory synapses, spine maturation and pruning are indicators for maturation and elimination of these synapses. Our previous studies have found that dendritically localized mRNA for brain-derived neurotrophic factor (BDNF) regulates spine maturation and pruning. Here we investigated the mechanism by which dendritic Bdnf mRNA, but not somatically restricted Bdnf mRNA, promotes spine maturation and pruning. We found that neuronal activity stimulates both translation of dendritic Bdnf mRNA and secretion of its translation product mainly as proBDNF. The secreted proBDNF promotes spine maturation and pruning, and its effect on spine pruning is in part mediated by the p75(NTR) receptor via RhoA activation. Furthermore, some proBDNF is extracellularly converted to mature BDNF and then promotes maturation of stimulated spines by activating Rac1 through the TrkB receptor. In contrast, translation of somatic Bdnf mRNA and the release of its translation product mainly as mature BDNF are independent of action potentials. These results not only reveal a biochemical pathway regulating synapse pruning, but also suggest that BDNF synthesized in the soma and dendrites is released through distinct secretory pathways.

The epitranscriptome in modulating spatiotemporal RNA translation in neuronal post-synaptic function

The application of next-generation-sequencing based methods has recently allowed the sequence-specific occurrence of RNA modifications to be investigated in transcriptome-wide settings. This has led to the emergence of a new field of molecular genetics research termed “epitranscriptomics.” Investigations have shown that these modifications can exert control over protein synthesis via various mechanisms, and particularly when occurring on messenger RNAs, can be dynamically regulated. Here, we propose that RNA modifications may be a critical regulator over the spatiotemporal control of protein-synthesis in neurons, which is supported by our finding that the RNA methylase NSun2 colocalizes with the translational-repressor FMRP at neuronal dendrites. We also observe that NSun2 commonly methylates mRNAs which encode components of the postsynaptic proteome, and further find that NSun2 and FMRP likely share a common subset of mRNA targets which include those that are known to be translated at dendrites in an activity-dependent manner. We consider potential roles for RNA modifications in space- time- and activity-dependent regulation of protein synthesis in neuronal physiology, with a particular focus on synaptic plasticity modulation.

A novel form of synaptic plasticity in field CA3 of hippocampus requires GPER1 activation and BDNF release

Estrogen gates metabotropic glutamate receptor–dependent long-term depression at mossy fiber–CA3 synapses through a mechanism involving GPER1-mediated BDNF release, mTOR-dependent protein synthesis, and proteasome activity.

Estrogen is an important modulator of hippocampal synaptic plasticity and memory consolidation through its rapid action on membrane-associated receptors. Here, we found that both estradiol and the G-protein–coupled estrogen receptor 1 (GPER1) specific agonist G1 rapidly induce brain-derived neurotrophic factor (BDNF) release, leading to transient stimulation of activity-regulated cytoskeleton-associated (Arc) protein translation and GluA1-containing AMPA receptor internalization in field CA3 of hippocampus. We also show that type-I metabotropic glutamate receptor (mGluR) activation does not induce Arc translation nor long-term depression (LTD) at the mossy fiber pathway, as opposed to its effects in CA1, and it only triggers LTD after GPER1 stimulation. Furthermore, this form of mGluR-dependent LTD is associated with ubiquitination and proteasome-mediated degradation of GluA1, and is prevented by proteasome inhibition. Overall, our study identifies a novel mechanism by which estrogen and BDNF regulate hippocampal synaptic plasticity in the adult brain.

Biochemical Computation for Spine Structural Plasticity

The structural plasticity of dendritic spines is considered to be essential for various forms of synaptic plasticity, learning, and memory. The process is mediated by a complex signaling network consisting of numerous species of molecules. Furthermore, the spatiotemporal dynamics of the biochemical signaling are regulated in a complicated manner because of geometrical restrictions from the unique morphology of the dendritic branches and spines. Recent advances in optical techniques have enabled the exploration of the spatiotemporal aspects of the signal regulations in spines and dendrites and have provided many insights into the principle of the biochemical computation that underlies spine structural plasticity.

Genomic analysis of RNA localization

The localization of mRNAs to specific subcellular sites is widespread, allowing cells to spatially restrict and regulate protein production, and playing important roles in development and cellular physiology. This process has been studied in mechanistic detail for several RNAs. However, the generality or specificity of RNA localization systems and mechanisms that impact the many thousands of localized mRNAs has been difficult to assess. In this review, we discuss the current state of the field in determining which RNAs localize, which RNA sequences mediate localization, the protein factors involved, and the biological implications of localization. For each question, we examine prominent systems and techniques that are used to study individual messages, highlight recent genome-wide studies of RNA localization, and discuss the potential for adapting other high-throughput approaches to the study of localization.

Single-Molecule Imaging of PSD-95 mRNA Translation in Dendrites and Its Dysregulation in a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is caused by the loss of the fragile X mental retardation protein (FMRP), an RNA binding protein that regulates translation of numerous target mRNAs, some of which are dendritically localized. Our previous biochemical studies using synaptoneurosomes demonstrate a role for FMRP and miR-125a in regulating the translation of PSD-95 mRNA. However, the local translation of PSD-95 mRNA within dendrites and spines, as well as the roles of FMRP or miR-125a, have not been directly studied. Herein, local synthesis of a Venus-PSD-95 fusion protein was directly visualized in dendrites and spines using single-molecule imaging of a diffusion-restricted Venus-PSD-95 reporter under control of the PSD-95 3'UTR. The basal translation rates of Venus-PSD-95 mRNA was increased in cultured hippocampal neurons from Fmr1 KO mice compared with WT neurons, which correlated with a transient elevation of endogenous PSD-95 within dendrites. Following mGluR stimulation with (S)-3,5-dihydroxyphenylglycine, the rate of Venus-PSD-95 mRNA translation increased rapidly in dendrites of WT hippocampal neurons, but not in those of Fmr1 KO neurons or when the binding site of miR125a, previously shown to bind PSD-95 3'UTR, was mutated. This study provides direct support for the hypothesis that local translation within dendrites and spines is dysregulated in FXS. Impairments in the regulated local synthesis of PSD-95, a critical regulator of synaptic structure and function, may affect the spatiotemporal control of PSD-95 levels and affect dendritic spine development and synaptic plasticity in FXS.

BONLAC: A combinatorial proteomic technique to measure stimulus-induced translational profiles in brain slices

Stimulus-triggered protein synthesis is critical for brain health and function. However, due to technical hurdles, de novo neuronal translation is predominantly studied in cultured cells, whereas electrophysiological and circuit analyses often are performed in brain slices. The different properties of these two experimental systems create an information gap about stimulus-induced alterations in the expression of new proteins in mature circuits. To address this, we adapted two existing techniques, BONCAT and SILAC, to a combined proteomic technique, BONLAC, for use in acute adult hippocampal slices. Using BDNF-induced protein synthesis as a proof of concept, we found alterations in expression of proteins involved in neurotransmission, trafficking, and cation binding that differed from those found in a similar screen in cultured neurons. Our results indicate important differences between cultured neurons and slices, and suggest that BONLAC could be used to dissect proteomic changes underlying synaptic events in adult circuits. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Axonal amphoterin mRNA is regulated by translational control and enhances axon outgrowth

High mobility group (HMG) proteins concentrate in the nucleus, interacting with chromatin. Amphoterin is an HMG protein (HMGB1) that has been shown to have extranuclear functions and can be secreted from some cell types. Exogenous amphoterin can increase neurite growth, suggesting that the secreted protein may have growth promoting activities in neurons. Consistent with this, we show that depletion of amphoterin mRNA from cultured adult rat DRG neurons attenuates neurite outgrowth, pointing to autocrine or paracrine mechanisms for its growth-promoting effects. The mRNA encoding amphoterin localizes to axonal processes and we showed recently that its 3'-UTR is sufficient for axonal localization of heterologous transcripts (Donnelly et al., 2013). Here, we show that amphoterin mRNA is transported constitutively into axons of adult DRG neurons. A preconditioning nerve injury increases the levels of amphoterin protein in axons without a corresponding increase in amphoterin mRNA in the axons. A 60 nucleotide region of the amphoterin mRNA 3'-UTR is necessary and sufficient for its localization into axons of cultured sensory neurons. Amphoterin mRNA 3'-UTR is also sufficient for axonal localization in distal axons of DRG neurons in vivo. Overexpression of axonally targeted amphoterin mRNA increases axon outgrowth in cultured sensory neurons, but axon growth is not affected when the overexpressed mRNA is restricted to the cell body.

Single-molecule insights into mRNA dynamics in neurons

Targeting of mRNAs to neuronal dendrites and axons plays an integral role in intracellular signaling, development, and synaptic plasticity. Single-molecule imaging of mRNAs in neurons and brain tissue has led to enhanced understanding of mRNA dynamics. Here we discuss aspects of mRNA regulation as revealed by single-molecule detection, which has led to quantitative analyses of mRNA diversity, localization, transport, and translation. These exciting new discoveries propel our understanding of the life of an mRNA in a neuron and how its activity is regulated at the single-molecule level.

From Learning to Memory: What Flies Can Tell Us about Intellectual Disability Treatment

Intellectual disability (ID), previously known as mental retardation, affects 3% of the population and remains without pharmacological treatment. ID is characterized by impaired general mental abilities associated with defects in adaptive function in which onset occurs before 18 years of age. Genetic factors are increasing and being recognized as the causes of severe ID due to increased use of genome-wide screening tools. Unfortunately drug discovery for treatment of ID has not followed the same pace as gene discovery, leaving clinicians, patients, and families without the ability to ameliorate symptoms. Despite this, several model organisms have proven valuable in developing and screening candidate drugs. One such model organism is the fruit fly Drosophila. First, we review the current understanding of memory in human and its model in Drosophila. Second, we describe key signaling pathways involved in ID and memory such as the cyclic adenosine 3',5'-monophosphate (cAMP)-cAMP response element binding protein (CREB) pathway, the regulation of protein synthesis, the role of receptors and anchoring proteins, the role of neuronal proliferation, and finally the role of neurotransmitters. Third, we characterize the types of memory defects found in patients with ID. Finally, we discuss how important insights gained from Drosophila learning and memory could be translated in clinical research to lead to better treatment development.

Mammalian Target of Rapamycin (mTOR) Tagging Promotes Dendritic Branch Variability through the Capture of Ca2+/Calmodulin-dependent Protein Kinase II α (CaMKIIα) mRNAs by the RNA-binding Protein HuD

The fate of a memory, whether stored or forgotten, is determined by the ability of an active or tagged synapse to undergo changes in synaptic efficacy requiring protein synthesis of plasticity-related proteins. A synapse can be tagged, but without the "capture" of plasticity-related proteins, it will not undergo long lasting forms of plasticity (synaptic tagging and capture hypothesis). What the "tag" is and how plasticity-related proteins are captured at tagged synapses are unknown. Ca(2+)/calmodulin-dependent protein kinase II α (CaMKIIα) is critical in learning and memory and is synthesized locally in neuronal dendrites. The mechanistic (mammalian) target of rapamycin (mTOR) is a protein kinase that increases CaMKIIα protein expression; however, the mechanism and site of dendritic expression are unknown. Herein, we show that mTOR activity mediates the branch-specific expression of CaMKIIα, favoring one secondary, daughter branch over the other in a single neuron. mTOR inhibition decreased the dendritic levels of CaMKIIα protein and mRNA by shortening its poly(A) tail. Overexpression of the RNA-stabilizing protein HuD increased CaMKIIα protein levels and preserved its selective expression in one daughter branch over the other when mTOR was inhibited. Unexpectedly, deleting the third RNA recognition motif of HuD, the domain that binds the poly(A) tail, eliminated the branch-specific expression of CaMKIIα when mTOR was active. These results provide a model for one molecular mechanism that may underlie the synaptic tagging and capture hypothesis where mTOR is the tag, preventing deadenylation of CaMKIIα mRNA, whereas HuD captures and promotes its expression in a branch-specific manner.

Postnatal loss of hap1 reduces hippocampal neurogenesis and causes adult depressive-like behavior in mice

Depression is a serious mental disorder that affects a person’s mood, thoughts, behavior, physical health, and life in general. Despite our continuous efforts to understand the disease, the etiology of depressive behavior remains perplexing. Recently, aberrant early life or postnatal neurogenesis has been linked to adult depressive behavior; however, genetic evidence for this is still lacking. Here we genetically depleted the expression of huntingtin-associated protein 1 (Hap1) in mice at various ages or in selective brain regions. Depletion of Hap1 in the early postnatal period, but not later life, led to a depressive-like phenotype when the mice reached adulthood. Deletion of Hap1 in adult mice rendered the mice more susceptible to stress-induced depressive-like behavior. Furthermore, early Hap1 depletion impaired postnatal neurogenesis in the dentate gyrus (DG) of the hippocampus and reduced the level of c-kit, a protein expressed in neuroproliferative zones of the rodent brain and that is stabilized by Hap1. Importantly, stereotaxically injected adeno-associated virus (AAV) that directs the expression of c-kit in the hippocampus promoted postnatal hippocampal neurogenesis and ameliorated the depressive-like phenotype in conditional Hap1 KO mice, indicating a link between postnatal-born hippocampal neurons and adult depression. Our results demonstrate critical roles for Hap1 and c-kit in postnatal neurogenesis and adult depressive behavior, and also suggest that genetic variations affecting postnatal neurogenesis may lead to adult depression.

Although the majority of the neurons in the brain are generated during embryonic stage, new neurons are continuously being produced postnatally, and at a much lower rate in adulthood. As postnatal neurogenesis is a key component of the brain maturation process that creates dynamic ‘wirings’ in the brain necessary for an individual to grow, learn, and cope with the external world, attenuated postnatal neurogenesis may affect an individual’s mental stability, rendering a higher susceptibility to depression later in life. In the current study, we genetically ablated the expression of huntingtin-associated protein 1 (Hap1) in mice at various ages or in selective brain regions, and found that early loss of Hap1 significantly reduces postnatal hippocampal neurogenesis, and leads to adult depressive-like behavior. We also found c-kit as an effector to mediate the neurogenesis defect and adult depressive-like phenotype in mice lacking Hap1. The results provide the first genetic evidence to demonstrate the importance of postnatal neurogenesis in adult depression, and may offer new avenues in the prevention and treatment of depression. Our study also has potential implications to other adult-onset mental disorders.

Direct visualization of newly synthesized target proteins in situ

Protein synthesis is a dynamic process that tunes the cellular proteome in response to internal and external demands. Metabolic labeling approaches identify the general proteomic response but cannot visualize specific newly synthesized proteins within cells. Here we describe a technique that couples noncanonical amino acid tagging or puromycylation with the proximity ligation assay to visualize specific newly synthesized proteins and monitor their origin, redistribution and turnover in situ.

Neural circular RNAs are derived from synaptic genes and regulated by development and plasticity

Circular RNAs (circRNAs) have re-emerged as an interesting RNA species. Here, by deep RNA profiling in different mouse tissues, we observed that circRNAs are significantly enriched in brain.and a disproportionate fraction of them is derived from host genes that code for synaptic proteins. Moreover, based on separate profiling of the RNAs localized in neuronal cell bodies and neuropil, on average, circRNAs are more enriched in the neuropil than their host gene mRNA isoforms. Using high resolution in situ hybridization we, for the first time, visualized circRNA punctae in the dendrites of neurons. Consistent with the idea that circRNAs might regulate synaptic function, during development, many circRNAs change their abundance abruptly at a time corresponding to synaptogenesis. In addition, following a homeostatic downscaling of neuronal activity many circRNAs exhibit significant up or down-regulation. Together, our data indicate that brain circRNAs are positioned to respond to and regulate synaptic function.

Aging alters long-term potentiation--related gene networks and impairs synaptic protein synthesis in the rat hippocampus

During aging, memory retention and persistence of long-term potentiation (LTP) are impaired, suggesting an aging-related deterioration in mechanisms regulating information storage. Late-phase LTP requires synthesis of proteins at synapses as well as integrated regulation of gene networks. Because aging diminishes the persistence of LTP, primarily by affecting the transition between early and late phases, we assessed whether this was reflected in perturbation of gene networks. Using DNA microarray analysis, we compared LTP-associated gene expression in young (5 months), middle-aged (15 months), and old (22 months) male Sprague-Dawley rats. As expected, we found no significant difference in LTP measured 20 minutes postinduction; however, we found that overall more genes were regulated in the young group. Bioinformatics predicted not only dysregulation of activator protein-1 and nuclear factor kB transcription factor activity and epigenetic modifications but also dysregulation of protein synthesis. Notably, we confirmed an age-related impairment in metabotropic and ionotropic receptor-mediated synaptic protein synthesis. Together, these results demonstrate that LTP-specific gene expression is altered with aging and suggest that dysregulation of synaptic protein synthesis also contributes to the age-dependent reduction in LTP persistence.

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.

Immune dysregulation and cognitive vulnerability in the aging brain: Interactions of microglia, IL-1β, BDNF and synaptic plasticity

Older individuals often experience declines in cognitive function after events (e.g. infection, or injury) that trigger activation of the immune system. This occurs at least in part because aging sensitizes the response of microglia (the brain's resident immune cells) to signals triggered by an immune challenge. In the aging brain, microglia respond to these signals by producing more pro-inflammatory cytokines (e.g. interleukin-1beta or IL-1β) and producing them for longer than microglia in younger brains. This exaggerated inflammatory response can compromise processes critical for optimal cognitive functioning. Interleukin-1β is central to the inflammatory response and is a key mediator and modulator of an array of associated biological functions; thus its production and release is usually very tightly regulated. This review will focus on the impact of dysregulated production of IL-1β on hippocampus dependent-memory systems and associated synaptic plasticity processes. The neurotrophin brain-derived neurotrophic factor (BNDF) helps to protect neurons from damage caused by infection or injury, and it plays a critical role in many of the same memory and hippocampal plasticity processes compromised by dysregulated production of IL-1β. This suggests that an exaggerated brain inflammatory response, arising from aging and a secondary immune challenge, may erode the capacity to provide the BDNF needed for memory-related plasticity processes at hippocampal synapses. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.