NMDA mediated contextual conditioning changes miRNA expression

Growth-suppressor microRNAs mediate synaptic overgrowth and behavioral deficits in Fragile X mental retardation protein deficiency

Abnormal neuronal and synapse growth is a core pathology resulting from deficiency of the Fragile X mental retardation protein (FMRP), but molecular links underlying the excessive synthesis of key synaptic proteins remain incompletely defined. We find that basal brain levels of the growth suppressor let-7 microRNA (miRNA) family are selectively lowered in FMRP-deficient mice and activity-dependent let-7 downregulation is abrogated. Primary let-7 miRNA transcripts are not altered in FMRP-deficiency and posttranscriptional misregulation occurs downstream of MAPK pathway induction and elevation of Lin28a, a let-7 biogenesis inhibitor. Neonatal restoration of brain let-7 miRNAs corrects hallmarks of FMRP-deficiency, including dendritic spine overgrowth and social and cognitive behavioral deficits, in adult mice. Blockade of MAPK hyperactivation normalizes let-7 miRNA levels in both brain and peripheral blood plasma from Fmr1 KO mice. These results implicate dysregulated let-7 miRNA biogenesis in the pathogenesis of FMRP-deficiency, and highlight let-7 miRNA-based strategies for future biomarker and therapeutic development.

Endocannabinoid signaling and epigenetics modifications in the neurobiology of stress-related disorders

Stress exposure is associated with psychiatric conditions, such as depression, anxiety, and post-traumatic stress disorder (PTSD). It is also a vulnerability factor to developing or reinstating substance use disorder. Stress causes several changes in the neuro-immune-endocrine axis, potentially resulting in prolonged dysfunction and diseases. Changes in several transmitters, including serotonin, dopamine, glutamate, gamma-aminobutyric acid (GABA), glucocorticoids, and cytokines, are associated with psychiatric disorders or behavioral alterations in preclinical studies. Complex and interacting mechanisms make it very difficult to understand the physiopathology of psychiatry conditions; therefore, studying regulatory mechanisms that impact these alterations is a good approach. In the last decades, the impact of stress on biology through epigenetic markers, which directly impact gene expression, is under intense investigation; these mechanisms are associated with behavioral alterations in animal models after stress or drug exposure, for example. The endocannabinoid (eCB) system modulates stress response, reward circuits, and other physiological functions, including hypothalamus-pituitary-adrenal axis activation and immune response. eCBs, for example, act retrogradely at presynaptic neurons, limiting the release of neurotransmitters, a mechanism implicated in the antidepressant and anxiolytic effects after stress. Epigenetic mechanisms can impact the expression of eCB system molecules, which in turn can regulate epigenetic mechanisms. This review will present evidence of how the eCB system and epigenetic mechanisms interact and the consequences of this interaction in modulating behavioral changes after stress exposure in preclinical studies or psychiatric conditions. Moreover, evidence that correlates the involvement of the eCB system and epigenetic mechanisms in drug abuse contexts will be discussed.

The Expression of miRNAs Involved in Long-Term Memory Formation in the CNS of the Mollusk Helix lucorum

Mollusks are unique animals with a relatively simple central nervous system (CNS) containing giant neurons with identified functions. With such simple CNS, mollusks yet display sufficiently complex behavior, thus ideal for various studies of behavioral processes, including long-term memory (LTM) formation. For our research, we use the formation of the fear avoidance reflex in the terrestrial mollusk Helix lucorum as a learning model. We have shown previously that LTM formation in Helix requires epigenetic modifications of histones leading to both activation and inactivation of the specific genes. It is known that microRNAs (miRNAs) negatively regulate the expression of genes; however, the role of miRNAs in behavioral regulation has been poorly investigated. Currently, there is no miRNAs sequencing data being published on Helix lucorum, which makes it impossible to investigate the role of miRNAs in the memory formation of this mollusk. In this study, we have performed sequencing and comparative bioinformatics analysis of the miRNAs from the CNS of Helix lucorum. We have identified 95 different microRNAs, including microRNAs belonging to the MIR-9, MIR-10, MIR-22, MIR-124, MIR-137, and MIR-153 families, known to be involved in various CNS processes of vertebrates and other species, particularly, in the fear behavior and LTM. We have shown that in the CNS of Helix lucorum MIR-10 family (26 miRNAs) is the most representative one, including Hlu-Mir-10-S5-5p and Hlu-Mir-10-S9-5p as top hits. Moreover, we have shown the involvement of the MIR-10 family in LTM formation in Helix. The expression of 17 representatives of MIR-10 differentially changes during different periods of LTM consolidation in the CNS of Helix. In addition, using comparative analysis of microRNA expression upon learning in normal snails and snails with deficient learning abilities with dysfunction of the serotonergic system, we identified a number of microRNAs from several families, including MIR-10, which expression changes only in normal animals. The obtained data can be used for further fundamental and applied behavioral research.

MicroRNAs and Synaptic Plasticity: From Their Molecular Roles to Response to Therapy

Synaptic plasticity is the ability of synapses to weaken or strengthen over time, in response to changes in the activity of the neurons. It is orchestrated by a variety of genes, proteins, and external and internal factors, especially epigenetic factors. MicroRNAs (miRNAs) are well-acknowledged epigenetic modulators that regulate the translation and degradation of target genes in the nervous system. Increasing evidence has suggested that a number of miRNAs play important roles in modulating various aspects of synaptic plasticity. The deregulation of miRNAs could be associated with pathological alterations in synaptic plasticity, which could lead to different CNS-related diseases. Herein, we provide an update on the role of miRNAs in governing synaptic plasticity. In addition, we also summarize recent researches on the role of miRNAs in drug addiction, and their targets and mechanism of action. Understanding of the way in which miRNAs contribute to synaptic plasticity provides rational clues in establishing the novel biomarkers and new therapeutic strategies for the diagnosis and treatment of plasticity-related diseases and drug addiction.

Sporadic Alzheimer's Disease- and Neurotoxicity-Related microRNAs Affecting Key Events of Tau-Driven Adverse Outcome Pathway Toward Memory Loss

BACKGROUND

A complex network of aging-related homeostatic pathways that are sensitive to further deterioration in the presence of genetic, systemic, and environmental risk factors, and lifestyle, is implicated in the pathogenesis of progressive neurodegenerative diseases, such as sporadic (late-onset) Alzheimer's disease (sAD).

OBJECTIVE

Since sAD pathology and neurotoxicity share microRNAs (miRs) regulating common as well as overlapping pathological processes, environmental neurotoxic compounds are hypothesized to exert a risk for sAD initiation and progression.

METHODS

Literature search for miRs associated with human sAD and environmental neurotoxic compounds was conducted. Functional miR analysis using PathDip was performed to create miR-target interaction networks.

RESULTS

The identified miRs were successfully linked to the hypothetical starting point and key events of the earlier proposed tau-driven adverse outcome pathway toward memory loss. Functional miR analysis confirmed most of the findings retrieved from literature and revealed some interesting findings. The analysis identified 40 miRs involved in both sAD and neurotoxicity that dysregulated processes governing the plausible adverse outcome pathway for memory loss.

CONCLUSION

Creating miR-target interaction networks related to pathological processes involved in sAD initiation and progression, and environmental chemical-induced neurotoxicity, respectively, provided overlapping miR-target interaction networks. This overlap offered an opportunity to create an alternative picture of the mechanisms underlying sAD initiation and early progression. Looking at initiation and progression of sAD from this new angle may open for new biomarkers and novel drug targets for sAD before the appearance of the first clinical symptoms.

AP-2 reduces amyloidogenesis by promoting BACE1 trafficking and degradation in neurons

Cleavage of amyloid precursor protein (APP) by BACE-1 (β-site APP cleaving enzyme 1) is the rate-limiting step in amyloid-β (Aβ) production and a neuropathological hallmark of Alzheimer's disease (AD). Despite decades of research, mechanisms of amyloidogenic APP processing remain highly controversial. Here, we show that in neurons, APP processing and Aβ production are controlled by the protein complex-2 (AP-2), an endocytic adaptor known to be required for APP endocytosis. Now, we find that AP-2 prevents amyloidogenesis by additionally functioning downstream of BACE1 endocytosis, regulating BACE1 endosomal trafficking and its delivery to lysosomes. AP-2 is decreased in iPSC-derived neurons from patients with late-onset AD, while conditional AP-2 knockout (KO) mice exhibit increased Aβ production, resulting from accumulation of BACE1 within late endosomes and autophagosomes. Deletion of BACE1 decreases amyloidogenesis and mitigates synapse loss in neurons lacking AP-2. Taken together, these data suggest a mechanism for BACE1 intracellular trafficking and degradation via an endocytosis-independent function of AP-2 and reveal a novel role for endocytic proteins in AD.

A microRNA signature of toxic extrasynaptic N-methyl-D-aspartate (NMDA) receptor signaling

The cellular consequences of N-Methyl-D-Aspartate receptor (NMDAR) stimulation depend on the receptors' subcellular localization. Synaptic NMDARs promote plasticity and survival whereas extrasynaptic NMDARs mediate excitotoxicity and contribute to cell death in neurodegenerative diseases. The mechanisms that couple activation of extrasynaptic NMDARs to cell death remain incompletely understood. We here show that activation of extrasynaptic NMDARs by bath application of NMDA or L-glutamate leads to the upregulation of a group of 19 microRNAs in cultured mouse hippocampal neurons. In contrast, none of these microRNAs is induced upon stimulation of synaptic activity. Increased microRNA expression depends on the pri-miRNA processing enzyme Drosha, but not on de novo gene transcription. These findings suggest that toxic NMDAR signaling involves changes in the expression levels of particular microRNAs.

MicroRNAs are Necessary for BMP-7-induced Dendritic Growth in Cultured Rat Sympathetic Neurons

Neuronal connectivity is dependent on size and shape of the dendritic arbor. However, mechanisms controlling dendritic arborization, especially in the peripheral nervous system, are not completely understood. Previous studies have shown that bone morphogenetic proteins (BMPs) are important initiators of dendritic growth in peripheral neurons. In this study, we examined the hypothesis that post-transcriptional regulation mediated by microRNAs (miRNAs) is necessary for BMP-7-induced dendritic growth in these neurons. To examine the role of miRNAs in BMP-7-induced dendritic growth, microarray analyses was used to profile miRNA expression in cultured sympathetic neurons from the superior cervical ganglia of embryonic day 21 rat pups at 6 and 24 h after treatment with BMP-7 (50 ng/mL). Our data showed that BMP-7 significantly regulated the expression of 43 of the 762 miRNAs. Of the 43 miRNAs, 22 showed robust gene expression; 14 were upregulated by BMP-7 and 8 were downregulated by BMP-7. The expression profile for miR-335, miR-664-1*, miR-21, and miR-23b was confirmed using qPCR analyses. Functional studies using morphometric analyses of dendritic growth in cultured sympathetic neurons transfected with miRNA mimics and inhibitors indicated that miR-664-1*, miR-23b, and miR-21 regulated early stages of BMP-7-induced dendritic growth. In summary, our data provide evidence for miRNA-mediated post-transcriptional regulation as important downstream component of BMP-7 signaling during early stages of dendritic growth in sympathetic neurons.

Neuronal activity regulates DROSHA via autophagy in spinal muscular atrophy

Dysregulated miRNA expression and mutation of genes involved in miRNA biogenesis have been reported in motor neuron diseases including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). Therefore, identifying molecular mechanisms governing miRNA expression is important to understand these diseases. Here, we report that expression of DROSHA, which is a critical enzyme in the microprocessor complex and essential for miRNA biogenesis, is reduced in motor neurons from an SMA mouse model. We show that DROSHA is degraded by neuronal activity induced autophagy machinery, which is also dysregulated in SMA. Blocking neuronal activity or the autophagy-lysosome pathway restores DROSHA levels in SMA motor neurons. Moreover, reducing DROSHA levels enhances axonal growth. As impaired axonal growth is a well described phenotype of SMA motor neurons, these data suggest that DROSHA reduction by autophagy may mitigate the phenotype of SMA. In summary, these findings suggest that autophagy regulates RNA metabolism and neuronal growth via the DROSHA/miRNA pathway and this pathway is dysregulated in SMA.

A CREB2-targeting microRNA is required for long-term memory after single-trial learning

Although single-trial induced long-term memories (LTM) have been of major interest in neuroscience, how LTM can form after a single episode of learning remains largely unknown. We hypothesized that the removal of molecular inhibitory constraints by microRNAs (miRNAs) plays an important role in this process. To test this hypothesis, first we constructed small non-coding RNA (sncRNA) cDNA libraries from the CNS of Lymnaea stagnalis subjected to a single conditioning trial. Then, by next generation sequencing of these libraries, we identified a specific pool of miRNAs regulated by training. Of these miRNAs, we focussed on Lym-miR-137 whose seed region shows perfect complementarity to a target sequence in the 3' UTR of the mRNA for CREB2, a well-known memory repressor. We found that Lym-miR-137 was transiently up-regulated 1 h after single-trial conditioning, preceding a down-regulation of Lym-CREB2 mRNA. Furthermore, we discovered that Lym-miR-137 is co-expressed with Lym-CREB2 mRNA in an identified neuron with an established role in LTM. Finally, using an in vivo loss-of-function approach we demonstrated that Lym-miR-137 is required for single-trial induced LTM.

Interplay between TETs and microRNAs in the adult brain for memory formation

5-hydroxymethylation (5-hmC) is an epigenetic modification on DNA that results from the conversion of 5-methylcytosine by Ten-Eleven Translocation (TET) proteins. 5-hmC is widely present in the brain and is subjected to dynamic regulation during development and upon neuronal activity. It was recently shown to be involved in memory processes but currently, little is known about how it is controlled in the brain during memory formation. Here, we show that Tet3 is selectively up-regulated by activity in hippocampal neurons in vitro, and after formation of fear memory in the hippocampus. This is accompanied by a decrease in miR-29b expression that, through complementary sequences, regulates the level of Tet3 by preferential binding to its 3′UTR. We newly reveal that SAM68, a nuclear RNA-binding protein known to regulate splicing, acts upstream of miR-29 by modulating its biogenesis. Together, these findings identify novel players in the adult brain necessary for the regulation of 5-hmC during memory formation.

Activity-dependent spatially localized miRNA maturation in neuronal dendrites

MicroRNAs (miRNAs) regulate gene expression by binding to target messenger RNAs (mRNAs) and preventing their translation. In general, the number of potential mRNA targets in a cell is much greater than the miRNA copy number, complicating high-fidelity miRNA-target interactions. We developed an inducible fluorescent probe to explore whether the maturation of a miRNA could be regulated in space and time in neurons. A precursor miRNA (pre-miRNA) probe exhibited an activity-dependent increase in fluorescence, suggesting the stimulation of miRNA maturation. Single-synapse stimulation resulted in a local maturation of miRNA that was associated with a spatially restricted reduction in the protein synthesis of a target mRNA. Thus, the spatially and temporally regulated maturation of pre-miRNAs can be used to increase the precision and robustness of miRNA-mediated translational repression.

Physical exercise as an epigenetic modulator of brain plasticity and cognition

A large amount of evidence has demonstrated the power of exercise to support cognitive function, the effects of which can last for considerable time. An emerging line of scientific evidence indicates that the effects of exercise are longer lasting than previously thought up to the point to affect future generations. The action of exercise on epigenetic regulation of gene expression seem central to building an "epigenetic memory" to influence long-term brain function and behavior. In this review article, we discuss new developments in the epigenetic field connecting exercise with changes in cognitive function, including DNA methylation, histone modifications and microRNAs (miRNAs). The understanding of how exercise promotes long-term cognitive effects is crucial for directing the power of exercise to reduce the burden of neurological and psychiatric disorders.

mTOR referees memory and disease through mRNA repression and competition

Mammalian target of rapamycin (mTOR) activity is required for memory and is dysregulated in disease. Activation of mTOR promotes protein synthesis; however, new studies are demonstrating that mTOR activity also represses the translation of mRNAs. Almost three decades ago, Kandel and colleagues hypothesised that memory was due to the induction of positive regulators and removal of negative constraints. Are these negative constraints repressed mRNAs that code for proteins that block memory formation? Herein, we will discuss the mRNAs coded by putative memory suppressors, how activation/inactivation of mTOR repress protein expression at the synapse, how mTOR activity regulates RNA binding proteins, mRNA stability, and translation, and what the possible implications of mRNA repression are to memory and neurodegenerative disorders.

A microRNA negative feedback loop downregulates vesicle transport and inhibits fear memory

The SNARE-mediated vesicular transport pathway plays major roles in synaptic remodeling associated with formation of long-term memories, but the mechanisms that regulate this pathway during memory acquisition are not fully understood. Here we identify miRNAs that are up-regulated in the rodent hippocampus upon contextual fear-conditioning and identify the vesicular transport and synaptogenesis pathways as the major targets of the fear-induced miRNAs. We demonstrate that miR-153, a member of this group, inhibits the expression of key components of the vesicular transport machinery, and down-regulates Glutamate receptor A1 trafficking and neurotransmitter release. MiR-153 expression is specifically induced during LTP induction in hippocampal slices and its knockdown in the hippocampus of adult mice results in enhanced fear memory. Our results suggest that miR-153, and possibly other fear-induced miRNAs, act as components of a negative feedback loop that blocks neuronal hyperactivity at least partly through the inhibition of the vesicular transport pathway.

DOI:http://dx.doi.org/10.7554/eLife.22467.001

MicroRNA-139-5p negatively regulates NR2A-containing NMDA receptor in the rat pilocarpine model and patients with temporal lobe epilepsy

OBJECTIVES

Regulation of N-methyl-d-aspartate (NMDA) subunits NR2A and NR2B expression during status epilepticus (SE) remains incompletely understood. Here we explored the role of brain-enriched microRNA (miR)-139-5p in this process.

METHODS

miRNA microarray was performed to examine changes in miRNA expression in the rat pilocarpine model following NMDA-receptor blockade. The dynamic expression patterns of miR-139-5p, NR2A, and NR2B levels were measured in rats during the three phases of temporal lobe epilepsy (TLE) development using quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Western blot. Similar expression methods were applied to hippocampi obtained from patients with TLE and from normal controls. Moreover, miR-139-5p agomir and antagomir were utilized to explore the role of miR-139-5p in determining NMDA-receptor subunit expression patterns.

RESULTS

We identified 18 miRNAs that were significantly altered in the rat pilocarpine model following NMDA-receptor blockade. Of these, miR-139-5p was significantly up-regulated and Grin2A was predicted as its potential putative target. In patients with TLE, miR-139-5p expression was significantly down-regulated, whereas NR2A and NR2B levels were significantly up-regulated. In the rat model of SE, miR-139-5p expression was down-regulated while NR2A was up-regulated in the acute and chronic phases, but not in the latent phase. NR2B expression was up-regulated during the three phases of TLE development. Overexpression of miR-139-5p decreased, whereas depletion of miR-139-5p enhanced the expression levels of NR2A, but not NR2B, induced by pilocarpine treatment. Of interest, NMDA nonselective antagonist and NR2A selective antagonist enhanced miR-139-5p levels suppressed by pilocarpine treatment, whereas the NR2B selective antagonist was ineffective.

SIGNIFICANCE

These findings elucidate the potential role of miR-139-5p in NMDA-receptor involvement in TLE development and may provide novel therapeutic targets for the future treatment of TLE.

MicroRNAs: Key Regulators in the Central Nervous System and Their Implication in Neurological Diseases

MicroRNAs (miRNAs) are a class of small, well-conserved noncoding RNAs that regulate gene expression post-transcriptionally. They have been demonstrated to regulate a lot of biological pathways and cellular functions. Many miRNAs are dynamically regulated during central nervous system (CNS) development and are spatially expressed in adult brain indicating their essential roles in neural development and function. In addition, accumulating evidence strongly suggests that dysfunction of miRNAs contributes to neurological diseases. These observations, together with their gene regulation property, implicated miRNAs to be the key regulators in the complex genetic network of the CNS. In this review, we first focus on the ways through which miRNAs exert the regulatory function and how miRNAs are regulated in the CNS. We then summarize recent findings that highlight the versatile roles of miRNAs in normal CNS physiology and their association with several types of neurological diseases. Subsequently we discuss the limitations of miRNAs research based on current studies as well as the potential therapeutic applications and challenges of miRNAs in neurological disorders. We endeavor to provide an updated description of the regulatory roles of miRNAs in normal CNS functions and pathogenesis of neurological diseases.

Epigenetic pathways through which experiences become linked with biology

This article highlights the defining principles, progress, and future directions in epigenetics research in relation to this Special Issue. Exciting studies in the fields of neuroscience, psychology, and psychiatry have provided new insights into the epigenetic factors (e.g., DNA methylation) that are responsive to environmental input and serve as biological pathways in behavioral development. Here we highlight the experimental evidence, mainly from animal models, that factors such as psychosocial stress and environmental adversity can become encoded within epigenetic factors with functional consequences for brain plasticity and behavior. We also highlight evidence that epigenetic marking of genes in one generation can have consequences for future generations (i.e., inherited), and work with humans linking epigenetics, cognitive dysfunction, and psychiatric disorder. Though epigenetics has offered more of a beginning than an answer to the centuries-old nature-nurture debate, continued research is certain to yield substantial information regarding biological determinants of central nervous system changes and behavior with relevance for the study of developmental psychopathology.

Role of MicroRNA in Governing Synaptic Plasticity

Although synaptic plasticity in neural circuits is orchestrated by an ocean of genes, molecules, and proteins, the underlying mechanisms remain poorly understood. Recently, it is well acknowledged that miRNA exerts widespread regulation over the translation and degradation of target gene in nervous system. Increasing evidence suggests that quite a few specific miRNAs play important roles in various respects of synaptic plasticity including synaptogenesis, synaptic morphology alteration, and synaptic function modification. More importantly, the miRNA-mediated regulation of synaptic plasticity is not only responsible for synapse development and function but also involved in the pathophysiology of plasticity-related diseases. A review is made here on the function of miRNAs in governing synaptic plasticity, emphasizing the emerging regulatory role of individual miRNAs in synaptic morphological and functional plasticity, as well as their implications in neurological disorders. Understanding of the way in which miRNAs contribute to synaptic plasticity provides rational clues in establishing the novel therapeutic strategy for plasticity-related diseases.

Targeted deletion of miR-132/-212 impairs memory and alters the hippocampal transcriptome

miR-132 and miR-212 are structurally related microRNAs that have been found to exert powerful modulatory effects within the central nervous system (CNS). Notably, these microRNAs are tandomly processed from the same noncoding transcript, and share a common seed sequence: thus it has been difficult to assess the distinct contribution of each microRNA to gene expression within the CNS. Here, we employed a combination of conditional knockout and transgenic mouse models to examine the contribution of the miR-132/-212 gene locus to learning and memory, and then to assess the distinct effects that each microRNA has on hippocampal gene expression. Using a conditional deletion approach, we show that miR-132/-212 double-knockout mice exhibit significant cognitive deficits in spatial memory, recognition memory, and in tests of novel object recognition. Next, we utilized transgenic miR-132 and miR-212 overexpression mouse lines and the miR-132/-212 double-knockout line to explore the distinct effects of these two miRNAs on the transcriptional profile of the hippocampus. Illumina sequencing revealed that miR-132/-212 deletion increased the expression of 1138 genes; Venn analysis showed that 96 of these genes were also downregulated in mice overexpressing miR-132. Of the 58 genes that were decreased in animals overexpressing miR-212, only four of them were also increased in the knockout line. Functional gene ontology analysis of downregulated genes revealed significant enrichment of genes related to synaptic transmission, neuronal proliferation, and morphogenesis, processes known for their roles in learning, and memory formation. These data, coupled with previous studies, firmly establish a role for the miR-132/-212 gene locus as a key regulator of cognitive capacity. Further, although miR-132 and miR-212 share a seed sequence, these data indicate that these miRNAs do not exhibit strongly overlapping mRNA targeting profiles, thus indicating that these two genes may function in a complex, nonredundant manner to shape the transcriptional profile of the CNS. The dysregulation of miR-132/-212 expression could contribute to signaling mechanisms that are involved in an array of cognitive disorders.

Micro-RNAs in cognition and cognitive disorders: Potential for novel biomarkers and therapeutics

Micro-RNAs (miRNAs) are small regulatory non-coding RNAs involved in the regulation of many biological functions. In the brain, they have distinct expression patterns depending on region, cell-type and developmental stage. Their expression profile is altered by neuronal activation in response to behavioral training or chemical/electrical stimulation. The dynamic changes in miRNA level regulate the expression of genes required for cognitive processes such as learning and memory. In addition, in cognitive dysfunctions such as dementias, expression levels of many miRNAs are perturbed, not only in brain areas affected by the pathology, but also in peripheral body fluids such as serum and cerebrospinal fluid. This presents an opportunity to utilize miRNAs as biomarkers for early detection and assessment of cognitive dysfunctions. Further, since miRNAs target many genes and pathways, they may represent key molecular signatures that can help understand the mechanisms of cognitive disorders and the development of potential therapeutic agents.

miR-191 and miR-135 are required for long-lasting spine remodelling associated with synaptic long-term depression

Activity-dependent modification of dendritic spines, subcellular compartments accommodating postsynaptic specializations in the brain, is an important cellular mechanism for brain development, cognition and synaptic pathology of brain disorders. NMDA receptor-dependent long-term depression (NMDAR-LTD), a prototypic form of synaptic plasticity, is accompanied by prolonged remodeling of spines. The mechanisms underlying long-lasting spine remodeling in NMDAR-LTD, however, are largely unclear. Here we show that LTD induction causes global changes in miRNA transcriptomes affecting many cellular activities. Specifically, we show that expression changes of miR-191 and miR-135 are required for maintenance but not induction of spine restructuring. Moreover, we find that actin depolymerization and AMPA receptor exocytosis are regulated for extended periods of time by miRNAs to support long-lasting spine plasticity. These findings reveal a novel miRNA mediated-mechanism and a new role of AMPA receptor exocytosis in long-lasting spine plasticity, and identify a number of candidate miRNAs involved in LTD.

Infralimbic EphB2 Modulates Fear Extinction in Adolescent Rats

Adolescent rats are prone to impaired fear extinction, suggesting that mechanistic differences in extinction could exist in adolescent and adult rats. Since the infralimbic cortex (IL) is critical for fear extinction, we used PCR array technology to identify gene expression changes in IL induced by fear extinction in adolescent rats. Interestingly, the ephrin type B receptor 2 (EphB2), a tyrosine kinase receptor associated with synaptic development, was downregulated in IL after fear extinction. Consistent with the PCR array results, EphB2 levels of mRNA and protein were reduced in IL after fear extinction compared with fear conditioning, suggesting that EphB2 signaling in IL regulates fear extinction memory in adolescents. Finally, reducing EphB2 synthesis in IL with shRNA accelerated fear extinction learning in adolescent rats, but not in adult rats. These findings identify EphB2 in IL as a key regulator of fear extinction during adolescence, perhaps due to the increase in synaptic remodeling occurring during this developmental phase.

microRNAs Modulate Spatial Memory in the Hippocampus and in the Ventral Striatum in a Region-Specific Manner

MicroRNAs are endogenous, noncoding RNAs crucial for the post-transcriptional regulation of gene expression. Their role in spatial memory formation, however, is poorly explored. In this study, we analyzed learning-induced microRNA expression in the hippocampus and in the ventral striatum. Among miRNAs specifically downregulated by spatial training, we focused on the hippocampus-specific miR-324-5p and the ventral striatum-specific miR-24. In vivo overexpression of the two miRNAs demonstrated that miR-324-5p is able to impair memory if administered in the hippocampus but not in the ventral striatum, while the opposite is true for miR-24. Overall, these findings demonstrate a causal relationship between miRNA expression changes and spatial memory formation. Furthermore, they provide support for a regional dissociation in the post-transcriptional processes underlying spatial memory in the two brain structures analyzed.

Association of N-cadherin levels and downstream effectors of Rho GTPases with dendritic spine loss induced by chronic stress in rat hippocampal neurons

Chronic stress promotes cognitive impairment and dendritic spine loss in hippocampal neurons. In this animal model of depression, spine loss probably involves a weakening of the interaction between pre- and postsynaptic cell adhesion molecules, such as N-cadherin, followed by disruption of the cytoskeleton. N-cadherin, in concert with catenin, stabilizes the cytoskeleton through Rho-family GTPases. Via their effector LIM kinase (LIMK), RhoA and ras-related C3 botulinum toxin substrate 1 (RAC) GTPases phosphorylate and inhibit cofilin, an actin-depolymerizing molecule, favoring spine growth. Additionally, RhoA, through Rho kinase (ROCK), inactivates myosin phosphatase through phosphorylation of the myosin-binding subunit (MYPT1), producing actomyosin contraction and probable spine loss. Some micro-RNAs negatively control the translation of specific mRNAs involved in Rho GTPase signaling. For example, miR-138 indirectly activates RhoA, and miR-134 reduces LIMK1 levels, resulting in spine shrinkage; in contrast, miR-132 activates RAC1, promoting spine formation. We evaluated whether N-cadherin/β-catenin and Rho signaling is sensitive to chronic restraint stress. Stressed rats exhibit anhedonia, impaired associative learning, and immobility in the forced swim test and reduction in N-cadherin levels but not β-catenin in the hippocampus. We observed a reduction in spine number in the apical dendrites of CA1 pyramidal neurons, with no effect on the levels of miR-132 or miR-134. Although the stress did not modify the RAC-LIMK-cofilin signaling pathway, we observed increased phospho-MYPT1 levels, probably mediated by RhoA-ROCK activation. Furthermore, chronic stress raises the levels of miR-138 in accordance with the observed activation of the RhoA-ROCK pathway. Our findings suggest that a dysregulation of RhoA-ROCK activity by chronic stress could potentially underlie spine loss in hippocampal neurons.

MicroRNAs and synaptic plasticity--a mutual relationship

MicroRNAs (miRNAs) are rapidly emerging as central regulators of gene expression in the postnatal mammalian brain. Initial studies mostly focused on the function of specific miRNAs during the development of neuronal connectivity in culture, using classical gain- and loss-of-function approaches. More recently, first examples have documented important roles of miRNAs in plastic processes in intact neural circuits in the rodent brain related to higher cognitive abilities and neuropsychiatric disease. At the same time, evidence is accumulating that miRNA function itself is subjected to sophisticated control mechanisms engaged by the activity of neural circuits. In this review, we attempt to pay tribute to this mutual relationship between miRNAs and synaptic plasticity. In particular, in the first part, we summarize how neuronal activity influences each step in the lifetime of miRNAs, including the regulation of transcription, maturation, gene regulatory function and turnover in mammals. In the second part, we discuss recent examples of miRNA function in synaptic plasticity in rodent models and their implications for higher cognitive function and neurological disorders, with a special emphasis on epilepsy as a disorder of abnormal nerve cell activity.

The RNA-centred view of the synapse: non-coding RNAs and synaptic plasticity

If mRNAs were the only RNAs made by a neuron, there would be a simple mapping of mRNAs to proteins. However, microRNAs and other non-coding RNAs (ncRNAs; endo-siRNAs, piRNAs, BC1, BC200, antisense and long ncRNAs, repeat-related transcripts, etc.) regulate mRNAs via effects on protein translation as well as transcriptional and epigenetic mechanisms. Not only are genes ON or OFF, but their ability to be translated can be turned ON or OFF at the level of synapses, supporting an enormous increase in information capacity. Here, I review evidence that ncRNAs are expressed pervasively within dendrites in mammalian brain; that some are activity-dependent and highly enriched near synapses; and that synaptic ncRNAs participate in plasticity responses including learning and memory. Ultimately, ncRNAs can be viewed as the post-it notes of the neuron. They have no literal meaning of their own, but derive their functions from where (and to what) they are stuck. This may explain, in part, why ncRNAs differ so dramatically from protein-coding genes, both in terms of the usual indicators of functionality and in terms of evolutionary constraints. ncRNAs do not appear to be direct mediators of synaptic transmission in the manner of neurotransmitters or receptors, yet they orchestrate synaptic plasticity—and may drive species-specific changes in cognition.

Noradrenergic activation of the basolateral amygdala enhances object recognition memory and induces chromatin remodeling in the insular cortex

It is well established that arousal-induced memory enhancement requires noradrenergic activation of the basolateral complex of the amygdala (BLA) and modulatory influences on information storage processes in its many target regions. While this concept is well accepted, the molecular basis of such BLA effects on neural plasticity changes within other brain regions remains to be elucidated. The present study investigated whether noradrenergic activation of the BLA after object recognition training induces chromatin remodeling through histone post-translational modifications in the insular cortex (IC), a brain region that is importantly involved in object recognition memory. Male Sprague—Dawley rats were trained on an object recognition task, followed immediately by bilateral microinfusions of norepinephrine (1.0 μg) or saline administered into the BLA. Saline-treated control rats exhibited poor 24-h retention, whereas norepinephrine treatment induced robust 24-h object recognition memory. Most importantly, this memory-enhancing dose of norepinephrine induced a global reduction in the acetylation levels of histone H3 at lysine 14, H2B and H4 in the IC 1 h later, whereas it had no effect on the phosphorylation of histone H3 at serine 10 or tri-methylation of histone H3 at lysine 27. Norepinephrine administered into the BLA of non-trained control rats did not induce any changes in the histone marks investigated in this study. These findings indicate that noradrenergic activation of the BLA induces training-specific effects on chromatin remodeling mechanisms, and presumably gene transcription, in its target regions, which may contribute to the understanding of the molecular mechanisms of stress and emotional arousal effects on memory consolidation.

Plasticity-related microRNA and their potential contribution to the maintenance of long-term potentiation

Long-term potentiation (LTP) is a form of synaptic plasticity that is an excellent model for the molecular mechanisms that underlie memory. LTP, like memory, is persistent, and both are widely believed to be maintained by a coordinated genomic response. Recently, a novel class of non-coding RNA, microRNA, has been implicated in the regulation of LTP. MicroRNA negatively regulate protein synthesis by binding to specific messenger RNA response elements. The aim of this review is to summarize experimental evidence for the proposal that microRNA play a major role in the regulation of LTP. We discuss a growing body of research which indicates that specific microRNA regulate synaptic proteins relevant to LTP maintenance, as well as studies that have reported differential expression of microRNA in response to LTP induction. We conclude that microRNA are ideally suited to contribute to the regulation of LTP-related gene expression; microRNA are pleiotropic, synaptically located, tightly regulated, and function in response to synaptic activity. The potential impact of microRNA on LTP maintenance as regulators of gene expression is enormous.

Synaptic adaptations by alcohol and drugs of abuse: changes in microRNA expression and mRNA regulation

Local translation of mRNAs is a mechanism by which cells can rapidly remodel synaptic structure and function. There is ample evidence for a role of synaptic translation in the neuroadaptations resulting from chronic drug use and abuse. Persistent and coordinated changes of many mRNAs, globally and locally, may have a causal role in complex disorders such as addiction. In this review we examine the evidence that translational regulation by microRNAs drives synaptic remodeling and mRNA expression, which may regulate the transition from recreational to compulsive drug use. microRNAs are small, non-coding RNAs that control the translation of mRNAs in the cell and within spatially restricted sites such as the synapse. microRNAs typically repress the translation of mRNAs into protein by binding to the 3′UTR of their targets. As ‘master regulators’ of many mRNAs, changes in microRNAs could account for the systemic alterations in mRNA and protein expression observed with drug abuse and dependence. Recent studies indicate that manipulation of microRNAs affects addiction-related behaviors such as the rewarding properties of cocaine, cocaine-seeking behavior, and self-administration rates of alcohol. There is limited evidence, however, regarding how synaptic microRNAs control local mRNA translation during chronic drug exposure and how this contributes to the development of dependence. Here, we discuss research supporting microRNA regulation of local mRNA translation and how drugs of abuse may target this process. The ability of synaptic microRNAs to rapidly regulate mRNAs provides a discrete, localized system that could potentially be used as diagnostic and treatment tools for alcohol and other addiction disorders.

SMN regulates axonal local translation via miR-183/mTOR pathway

Reduced expression of SMN protein causes spinal muscular atrophy (SMA), a neurodegenerative disorder leading to motor neuron dysfunction and loss. However, the molecular mechanisms by which SMN regulates neuronal dysfunction are not fully understood. Here, we report that reduced SMN protein level alters miRNA expression and distribution in neurons. In particular, miR-183 levels are increased in neurites of SMN-deficient neurons. We demonstrate that miR-183 regulates translation of mTor via direct binding to its 3' UTR. Interestingly, local axonal translation of mTor is reduced in SMN-deficient neurons, and this can be recovered by miR-183 inhibition. Finally, inhibition of miR-183 expression in the spinal cord of an SMA mouse model prolongs survival and improves motor function of Smn-mutant mice. Together, these observations suggest that axonal miRNAs and the mTOR pathway are previously unidentified molecular mechanisms contributing to SMA pathology.

Neuronal activity-dependent regulation of MicroRNAs

MicroRNAs are non-coding short (~23 nucleotides) RNAs that mediate post-transcriptional regulation through sequence-specific gene silencing. The role of miRNAs in neuronal development, synapse formation and synaptic plasticity has been highlighted. However, the role of neuronal activity on miRNA regulation has been less focused. Neuronal activity-dependent regulation of miRNA may fine-tune gene expression in response to synaptic plasticity and memory formation. Here, we provide an overview of miRNA regulation by neuronal activity including high-throughput screening studies. We also discuss the possible molecular mechanisms of activity-dependent induction and turnover of miRNAs.

MicroRNA 135 is essential for chronic stress resiliency, antidepressant efficacy, and intact serotonergic activity

The link between dysregulated serotonergic activity and depression and anxiety disorders is well established, yet the molecular mechanisms underlying these psychopathologies are not fully understood. Here, we explore the role of microRNAs in regulating serotonergic (5HT) neuron activity. To this end, we determined the specific microRNA "fingerprint" of 5HT neurons and identified a strong microRNA-target interaction between microRNA 135 (miR135), and both serotonin transporter and serotonin receptor-1a transcripts. Intriguingly, miR135a levels were upregulated after administration of antidepressants. Genetically modified mouse models, expressing higher or lower levels of miR135, demonstrated major alterations in anxiety- and depression-like behaviors, 5HT levels, and behavioral response to antidepressant treatment. Finally, miR135a levels in blood and brain of depressed human patients were significantly lower. The current results suggest a potential role for miR135 as an endogenous antidepressant and provide a venue for potential treatment and insights into the onset, susceptibility, and heterogeneity of stress-related psychopathologies.

Genes and signaling pathways involved in memory enhancement in mutant mice

Mutant mice have been used successfully as a tool for investigating the mechanisms of memory at multiple levels, from genes to behavior. In most cases, manipulating a gene expressed in the brain impairs cognitive functions such as memory and their underlying cellular mechanisms, including synaptic plasticity. However, a remarkable number of mutations have been shown to enhance memory in mice. Understanding how to improve a system provides valuable insights into how the system works under normal conditions, because this involves understanding what the crucial components are. Therefore, more can be learned about the basic mechanisms of memory by studying mutant mice with enhanced memory. This review will summarize the genes and signaling pathways that are altered in the mutants with enhanced memory, as well as their roles in synaptic plasticity. Finally, I will discuss how knowledge of memory-enhancing mechanisms could be used to develop treatments for cognitive disorders associated with impaired plasticity.

miR-8 controls synapse structure by repression of the actin regulator enabled

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play important roles in nervous system development and physiology. However, our understanding of the strategies by which miRNAs control synapse development is limited. We find that the highly conserved miRNA miR-8 regulates the morphology of presynaptic arbors at the Drosophila neuromuscular junction (NMJ) through a postsynaptic mechanism. Developmental analysis shows that miR-8 is required for presynaptic expansion that occurs in response to larval growth of the postsynaptic muscle targets. With an in vivo sensor, we confirm our hypothesis that the founding member of the conserved Ena/VASP (Enabled/Vasodilator Activated Protein) family is regulated by miR-8 through a conserved site in the Ena 3′ untranslated region (UTR). Synaptic marker analysis and localization studies suggest that Ena functions within the subsynaptic reticulum (SSR) surrounding presynaptic terminals. Transgenic lines that express forms of a conserved mammalian Ena ortholog further suggest that this localization and function of postsynaptic Ena/VASP family protein is dependent on conserved C-terminal domains known to mediate actin binding and assembly while antagonizing actin-capping proteins. Ultrastructural analysis demonstrates that miR-8 is required for SSR morphogenesis. As predicted by our model, we find that Ena is both sufficient and necessary to account for miR-8-mediated regulation of SSR architecture, consistent with its localization in this compartment. Finally, electrophysiological analysis shows that miR-8 is important for spontaneous neurotransmitter release frequency and quantal content. However, unlike the structural phenotypes, increased expression of Ena fails to mimic the functional defects observed in miR-8-null animals. Together, these findings suggest that miR-8 limits the expansion of presynaptic terminals during larval synapse development through regulation of postsynaptic actin assembly that is independent of changes in synapse physiology.

Soluble beta-amyloid peptides, but not insoluble fibrils, have specific effect on neuronal microRNA expression

Recent studies indicate that soluble β-amyloid (sAβ) oligomers, rather than their fibrillar aggregates, contribute to the pathogenesis of Alzheimer's disease (AD), though the mechanisms of their neurotoxicity are still elusive. Here, we demonstrate that sAβ derived from 7PA2 cells exert a much stronger effect on the regulation of a set of functionally validated microRNAs (miRNAs) in primary cultured neurons than the synthetic insoluble Aβ fibrils (fAβ). Synthetic sAβ peptides at a higher concentration present comparable effect on these miRNAs in our neuronal model. Further, the sAβ-induced miR-134, miR-145 and miR-210 expressions are fully reversed by two selective N-methyl-d-aspartate (NMDA) receptor inhibitors, but are neither reversed by insulin nor by forskolin, suggesting an NMDA receptor-dependent, rather than PI3K/AKT or PKA/CREB signaling dependent regulatory mechanism. In addition, the repression of miR-107 expression by the sAβ containing 7PA2 CM is likely involved multiple mechanisms and multiple players including NMDA receptor, N-terminally truncated Aβ and reactive oxygen species (ROS).

Neuroepigenetics of memory formation and impairment: the role of microRNAs

MicroRNAs (miRNAs) are a class of short non-coding RNAs that primarily regulate protein synthesis through reversible translational repression or mRNA degradation. MiRNAs can act by translational control of transcription factors or via direct action on the chromatin, and thereby contribute to the non-genetic control of gene-environment interactions. MiRNAs that regulate components of pathways required for learning and memory further modulate the influence of epigenetics on cognition in the normal and diseased brain. This review summarizes recent data exemplifying the known roles of miRNAs in memory formation in different model organisms, and describes how neuronal plasticity regulates miRNA biogenesis, activity and degradation. It also examines the relevance of miRNAs for memory impairment in human, using recent clinical observations related to neurodevelopmental and neurodegenerative diseases, and discusses the potential mechanisms by which these miRNAs may contribute to memory disorders.

MicroRNAs in neuronal communication

MicroRNAs (miRNAs) are short nucleotides sequences that regulate the expression of genes in different eukaryotic cell types. A tremendous amount of knowledge on miRNAs has rapidly accumulated over the last few years, revealing the growing interest in this field of research. On the other hand, clarifying the physiological regulation of gene expression in the central nervous system is important for establishing a reference for comparison to the diseased state. It is well known that the fine tuning of neuronal networks relies on intricate molecular mechanisms, such as the adjustment of the synaptic transmission. As determined by recent studies, regulation of neuronal interactions by miRNAs has critical consequences in the development, adaptation to ambient demands, and degeneration of the nervous system. In contrast, activation of synaptic receptors triggers downstream signaling cascades that generate a vast array of effects, which includes the regulation of novel genes involved in the control of the miRNA life cycle. In this review, we have examined the hot topics on miRNA gene-regulatory activities in the broad field of neuronal communication-related processes. Furthermore, in addition to indicating the newly described effect of miRNAs on the regulation of specific neurotransmitter systems, we have pointed out how these systems affect the expression, transport, and stability of miRNAs. Moreover, we discuss newly described and under-investigation mechanisms involving the intercellular transfer of miRNAs, aided by exosomes and gap junctions. Thus, in the current review, we were able to highlight recent findings related to miRNAs that indisputably contributed towards the understanding of the nervous system in health and disease.

Developmental and activity-dependent miRNA expression profiling in primary hippocampal neuron cultures

MicroRNAs (miRNAs) are evolutionarily conserved non-coding RNAs of ∼22 nucleotides that regulate gene expression at the level of translation and play vital roles in hippocampal neuron development, function and plasticity. Here, we performed a systematic and in-depth analysis of miRNA expression profiles in cultured hippocampal neurons during development and after induction of neuronal activity. MiRNA profiling of primary hippocampal cultures was carried out using locked nucleic-acid-based miRNA arrays. The expression of 264 different miRNAs was tested in young neurons, at various developmental stages (stage 2-4) and in mature fully differentiated neurons (stage 5) following the induction of neuronal activity using chemical stimulation protocols. We identified 210 miRNAs in mature hippocampal neurons; the expression of most neuronal miRNAs is low at early stages of development and steadily increases during neuronal differentiation. We found a specific subset of 14 miRNAs with reduced expression at stage 3 and showed that sustained expression of these miRNAs stimulates axonal outgrowth. Expression profiling following induction of neuronal activity demonstrates that 51 miRNAs, including miR-134, miR-146, miR-181, miR-185, miR-191 and miR-200a show altered patterns of expression after NMDA receptor-dependent plasticity, and 31 miRNAs, including miR-107, miR-134, miR-470 and miR-546 were upregulated by homeostatic plasticity protocols. Our results indicate that specific miRNA expression profiles correlate with changes in neuronal development and neuronal activity. Identification and characterization of miRNA targets may further elucidate translational control mechanisms involved in hippocampal development, differentiation and activity-depended processes.

Prenatal stress changes the glycoprotein GPM6A gene expression and induces epigenetic changes in rat offspring brain

Prenatal stress (PS) exerts strong impact on fetal brain development and on adult offspring brain functions. Previous work demonstrated that chronic stress alters the mRNA expression of GPM6A, a neuronal glycoprotein involved in filopodium extension. In this work, we analyzed the effect of PS on gpm6a expression and the epigenetic mechanisms involved. Pregnant Wistar rats received restraint stress during the last week of gestation. Male offspring were sacrificed on postnatal days 28 and 60. Hippocampus and prefrontal cortex samples were analyzed for gene expression (qPCR for mRNAs and microRNAs), methylation status (bisulfite conversion) and protein levels. Hippocampal neurons in culture were used to analyze microRNA overexpression effects. Prenatal stress induced changes in gpm6a levels in both tissues and at both ages analyzed, indicating a persistent effect. Two CpG islands in the gpm6a gene were identified. Variations in the methylation pattern at three specific CpGs were found in hippocampus, but not in PFC samples from PS offspring. microRNAs predicted to target gpm6a were identified in silico. qPCR measurements showed that PS modified the expression of several microRNAs in both tissues, being microRNA-133b the most significantly altered. Further studies overexpressing this microRNA in neuronal cultures showed a reduction in gmp6a mRNA and protein level. Moreover filopodium density was also reduced, suggesting that GPM6A function was affected. Gestational stress affected gpm6a gene expression in offspring likely through changes in methylation status and in posttranscriptional regulation by microRNAs. Thus, our findings propose gpm6a as a novel target for epigenetic regulation during prenatal stress.

MicroRNA-276a functions in ellipsoid body and mushroom body neurons for naive and conditioned olfactory avoidance in Drosophila

MicroRNA (miRNA)-mediated gene regulation plays a key role in brain development and function. But there are few cases in which the roles of individual miRNAs have been elucidated in behaving animals. We report a miR-276a::DopR regulatory module in Drosophila that functions in distinct circuits for naive odor responses and conditioned odor memory. Drosophila olfactory aversive memory involves convergence of the odors (conditioned stimulus) and the electric shock (unconditioned stimulus) in mushroom body (MB) neurons. Dopamine receptor DopR mediates the unconditioned stimulus inputs onto MB. Distinct dopaminergic neurons also innervate ellipsoid body (EB), where DopR function modulates arousal to external stimuli. We demonstrate that miR-276a is required in MB neurons for memory formation and in EB for naive responses to odors. Both roles of miR-276a are mediated by tuning DopR expression. The dual role of this miR-276a::DopR genetic module in these two neural circuits highlights the importance of miRNA-mediated gene regulation within distinct circuits underlying both naive behavioral responses and memory.

MicroRNA-182 regulates amygdala-dependent memory formation

De novo protein synthesis supports long-lasting functional and structural plasticity and is a molecular requirement for new memory formation. Recent evidence has suggested that microRNAs may be involved in regulating the molecular mechanisms underlying neural plasticity. MicroRNAs are endogenous, noncoding RNAs capable of post-transcriptional repression of their mRNA targets. To explore the potential for microRNA-mediated regulation of amygdala-dependent memory formation, we performed expression profiling of microRNAs in the lateral amygdala of rats 1 h after auditory fear conditioning. Microarray analysis revealed that over half of all known microRNAs are endogenously expressed in the lateral amygdala, with 7 microRNAs upregulated and 32 downregulated by auditory fear training. Bioinformatic analysis identified several of the downregulated microRNAs as potential repressors of actin-regulating proteins known to be involved in plasticity and memory. Downregulation of one of these microRNAs by auditory fear conditioning, miR-182, was confirmed by quantitative real-time PCR. Overexpression of miR-182 within the lateral amygdala resulted in decreased expression of the protein but not mRNA of two synapse-enriched regulators of actin known to modulate structural plasticity, cortactin and Rac1. The overexpression of miR-182 also disrupted long-term but not short-term auditory fear memory. These data indicate that learning-induced suppression of miR-182, a microRNA previously uncharacterized in the brain, supports long-term memory formation in the amygdala and suggests it does so, at least in part, through the derepression of key actin-regulating proteins. These findings further indicate that microRNAs may represent a previously underappreciated mechanism for regulating protein synthesis during memory consolidation.

Differences in microRNAs and their expressions between foraging and dancing honey bees, Apis mellifera L

Many studies have established that microRNAs (miRNAs) regulate gene expression in various biological processes in mammals and insects including honey bees. Dancing behavior is a form of communication unique to honey bees. However, it remains unclear which miRNAs regulate the dancing behavior in honey bees, and how. In the present study, total small RNAs (sRNAs) in Apis mellifera foragers and dancers were extracted and analyzed by a Solexa Sequencer to determine differentially expressed miRNAs. A small percentage (12.62%) of the unique sRNAs (the number of sequence types) were shared between foragers and dancers, but their expression accounted for 92.92% of the total sRNAs (the number of all sequence reads), and the length of them centered around 22nt. Out of 58 previously identified miRNAs, 54 were present in both foragers and dancers and most of them were down-regulated in dancers. The fold-changes of ame-miR-34, ame-miR-210, ame-miR-278 and ame-miR-282 were higher than 2. 86 and 104 novel miRNAs were detected in foragers and dancers, respectively. Furthermore, two known miRNAs (ame-miR-278 and ame-miR-282) were confirmed, by qPCR, to have lower expressions in dancers. The target genes of ame-miR-278 and ame-miR-282 were associated with kinase, neural function, synaptotagmin and energy. These results indicate that miRNAs are substantially different between the foraging and dancing stages, and suggest that miRNAs might play important roles in regulating dancing behaviors in honey bees.

Consolidation and translation regulation

mRNA translation, or protein synthesis, is a major component of the transformation of the genetic code into any cellular activity. This complicated, multistep process is divided into three phases: initiation, elongation, and termination. Initiation is the step at which the ribosome is recruited to the mRNA, and is regarded as the major rate-limiting step in translation, while elongation consists of the elongation of the polypeptide chain; both steps are frequent targets for regulation, which is defined as a change in the rate of translation of an mRNA per unit time. In the normal brain, control of translation is a key mechanism for regulation of memory and synaptic plasticity consolidation, i.e., the off-line processing of acquired information. These regulation processes may differ between different brain structures or neuronal populations. Moreover, dysregulation of translation leads to pathological brain function such as memory impairment. Both normal and abnormal function of the translation machinery is believed to lead to translational up-regulation or down-regulation of a subset of mRNAs. However, the identification of these newly synthesized proteins and determination of the rates of protein synthesis or degradation taking place in different neuronal types and compartments at different time points in the brain demand new proteomic methods and system biology approaches. Here, we discuss in detail the relationship between translation regulation and memory or synaptic plasticity consolidation while focusing on a model of cortical-dependent taste learning task and hippocampal-dependent plasticity. In addition, we describe a novel systems biology perspective to better describe consolidation.

MicroRNAs shape the neuronal landscape

The nervous system equips us with capability to adapt to many conditions and circumstances. We rely on an armamentarium of intricately formed neural circuits for many of our adaptive strategies. However, this capability also depends on a well-conserved toolkit of different molecular mechanisms that offer not only compensatory responses to a changing world, but also provide plasticity to achieve changes in cellular state that underlie a broad range of processes from early developmental transitions to life-long memory. Among the molecular tools that mediate changes in cellular state, our understanding of posttranscriptional regulation of gene expression is expanding rapidly. Part of the "epigenetic landscape" that shapes the deployment and robust regulation of gene networks during the construction and the remodeling of the brain is the microRNA system controlling both levels and translation of messenger RNA. Here we consider recent advances in the study of microRNA-mediated regulation of synaptic form and function.