MicroRNAs shape the neuronal landscape

Clinical Insights into MicroRNAs in Depression: Bridging Molecular Discoveries and Therapeutic Potential

Depression is a major contributor to the overall global burden of disease. The discovery of biomarkers for diagnosis or prediction of treatment responses and as therapeutic agents is a current priority. Previous studies have demonstrated the importance of short RNA molecules in the etiology of depression. The most extensively researched of these are microRNAs, a major component of cellular gene regulation and function. MicroRNAs function in a temporal and tissue-specific manner to regulate and modify the post-transcriptional expression of target mRNAs. They can also be shuttled as cargo of extracellular vesicles between the brain and the blood, thus informing about relevant mechanisms in the CNS through the periphery. In fact, studies have already shown that microRNAs identified peripherally are dysregulated in the pathological phenotypes seen in depression. Our article aims to review the existing evidence on microRNA dysregulation in depression and to summarize and evaluate the growing body of evidence for the use of microRNAs as a target for diagnostics and RNA-based therapies.

The role of glial cells in mental illness: a systematic review on astroglia and microglia as potential players in schizophrenia and its cognitive and emotional aspects

Schizophrenia is a complex and severe mental disorder that affects approximately 1% of the global population. It is characterized by a wide range of symptoms, including delusions, hallucinations, disorganized speech and behavior, and cognitive impairment. Recent research has suggested that the immune system dysregulation may play a significant role in the pathogenesis of schizophrenia, and glial cells, such as astroglia and microglia known to be involved in neuroinflammation and immune regulation, have emerged as potential players in this process. The aim of this systematic review is to summarize the glial hallmarks of schizophrenia, choosing as cellular candidate the astroglia and microglia, and focusing also on disease-associated psychological (cognitive and emotional) changes. We conducted a systematic review following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched PubMed, Scopus, and Web of Science for articles that investigated the differences in astroglia and microglia in patients with schizophrenia, published in the last 5 years. The present systematic review indicates that changes in the density, morphology, and functioning of astroglia and microglia may be involved in the development of schizophrenia. The glial alterations may contribute to the pathogenesis of schizophrenia by dysregulating neurotransmission and immune responses, worsening cognitive capabilities. The complex interplay of astroglial and microglial activation, genetic/epigenetic variations, and cognitive assessments underscores the intricate relationship between biological mechanisms, symptomatology, and cognitive functioning in schizophrenia.

Five-minute exposure to a novel appetitive food substance is sufficient time for a microRNA-dependent long-term memory to form

The Garcia effect is a unique form of conditioned taste aversion which requires that a novel food stimulus be followed sometime later by a sickness state associated with the novel food stimulus. The long-lasting associative memory resulting from the Garcia effect ensures that organisms avoid toxic foods in their environment. Considering its ecological relevance, we sought to investigate whether a brief encounter (5 min) with a novel, appetitive food stimulus can cause a persisting long-term memory (LTM) to form that would in turn block the Garcia effect in Lymnaea stagnalis. Furthermore, we wanted to explore whether that persisting LTM could be modified by the alteration of microRNAs via an injection of poly-L-lysine (PLL), an inhibitor of Dicer-mediated microRNA biogenesis. The Garcia effect procedure involved two observations of feeding behavior in carrot separated by a heat stress (30 °C for 1 h). Exposing snails to carrot for 5 min caused a LTM to form and persist for 1 week, effectively preventing the Garcia effect in snails. In contrast, PLL injection following the 5-min carrot exposure impaired LTM formation, allowing the Garcia effect to occur. These results provide more insight into LTM formation and the Garcia effect, an important survival mechanism.

MicroRNA-92a-CPEB3 axis protects neurons against inflammatory neurodegeneration

Neuroinflammation causes neuronal injury in multiple sclerosis (MS) and other neurological diseases. MicroRNAs (miRNAs) are important modulators of neuronal stress responses, but knowledge about their contribution to neuronal protection or damage during inflammation is limited. Here, we constructed a regulatory miRNA-mRNA network of inflamed motor neurons by leveraging cell type-specific miRNA and mRNA sequencing of mice undergoing experimental autoimmune encephalomyelitis (EAE). We found robust induction of miR-92a in inflamed spinal cord neurons and identified cytoplasmic polyadenylation element-binding protein 3 (Cpeb3) as a key target of miR-92a-mediated posttranscriptional silencing. We detected CPEB3 repression in inflamed neurons in murine EAE and human MS. Moreover, both miR-92a delivery and Cpeb3 deletion protected neuronal cultures against excitotoxicity. Supporting a detrimental effect of Cpeb3 in vivo, neuron-specific deletion in conditional Cpeb3 knockout animals led to reduced inflammation-induced clinical disability in EAE. Together, we identified a neuroprotective miR-92a-Cpeb3 axis in neuroinflammation that might serve as potential treatment target to limit inflammation-induced neuronal damage.

MicroRNA-218 instructs proper assembly of hippocampal networks

The assembly of the mammalian brain is orchestrated by temporally coordinated waves of gene expression. Post-transcriptional regulation by microRNAs (miRNAs) is a key aspect of this program. Indeed, deletion of neuron-enriched miRNAs induces strong developmental phenotypes, and miRNA levels are altered in patients with neurodevelopmental disorders. However, the mechanisms used by miRNAs to instruct brain development remain largely unexplored. Here, we identified miR-218 as a critical regulator of hippocampal assembly. MiR-218 is highly expressed in the hippocampus and enriched in both excitatory principal neurons (PNs) and GABAergic inhibitory interneurons (INs). Early life inhibition of miR-218 results in an adult brain with a predisposition to seizures. Changes in gene expression in the absence of miR-218 suggest that network assembly is impaired. Indeed, we find that miR-218 inhibition results in the disruption of early depolarizing GABAergic signaling, structural defects in dendritic spines, and altered intrinsic membrane excitability. Conditional knockout of Mir218-2 in INs, but not PNs, is sufficient to recapitulate long-term instability. Finally, de-repressing Kif21b and Syt13, two miR-218 targets, phenocopies the effects on early synchronous network activity induced by miR-218 inhibition. Taken together, the data suggest that miR-218 orchestrates formative events in PNs and INs to produce stable networks.

Reduced cerebral cortex thickness is related to overexpression of exosomal miR-146a-5p in medication-free patients with major depressive disorder

BACKGROUND

Previous studies have confirmed that miR-146a-5p overexpression suppresses neurogenesis, thereby enhancing depression-like behaviors. However, it remains unclear how miR-146a-5p dysregulation produces in vivo brain structural abnormalities in patients with major depressive disorder (MDD).

METHODS

In this case-control study, we combined cortical morphology analysis of magnetic resonance imaging (MRI) and miR-146a-5p quantification to investigate the neuropathological effect of miR-146a-5p on cortical thickness in MDD patients. Serum-derived exosomes that were considered to readily cross the blood-brain barrier and contain miR-146a-5p were isolated for miRNA quantification. Moreover, follow-up MRI scans were performed in the MDD patients after 6 weeks of antidepressant treatment to further validate the clinical relevance of the relationship between miR-146a-5p and brain structural abnormalities.

RESULTS

In total, 113 medication-free MDD patients and 107 matched healthy controls were included. Vertex-vise general linear model revealed miR-146a-5p-dependent cortical thinning in MDD patients compared with healthy individuals, i.e., overexpression of miR-146a-5p was associated with reduced cortical thickness in the left orbitofrontal cortex (OFC), anterior cingulate cortex, bilateral lateral occipital cortices (LOCs), etc. Moreover, this relationship between baseline miR-146a-5p and cortical thinning was nonsignificant for all regions in the patients who had received antidepressant treatment, and higher baseline miR-146a-5p expression was found to be related to greater longitudinal cortical thickening in the left OFC and right LOC.

CONCLUSIONS

The findings of this study reveal a relationship between miR-146a-5p overexpression and cortical atrophy and thus may help specify the in vivo mediating effect of miR-146a-5p dysregulation on brain structural abnormalities in patients with MDD.

MicroRNAs are necessary for the emergence of Purkinje cell identity

Brain computations are dictated by the unique morphology and connectivity of neuronal subtypes, features established by closely timed developmental events. MicroRNAs (miRNAs) are critical for brain development, but current technologies lack the spatiotemporal resolution to determine how miRNAs instruct the steps leading to subtype identity. Here, we developed new tools to tackle this major gap. Fast and reversible miRNA loss-of-function revealed that miRNAs are necessary for cerebellar Purkinje cell (PC) differentiation, which previously appeared miRNA-independent, and resolved distinct miRNA critical windows in PC dendritogenesis and climbing fiber synaptogenesis, key determinants of PC identity. To identify underlying mechanisms, we generated a mouse model, which enables precise mapping of miRNAs and their targets in rare cell types. With PC-specific maps, we found that the PC-enriched miR-206 drives exuberant dendritogenesis and modulates synaptogenesis. Our results showcase vastly improved approaches for dissecting miRNA function and reveal that many critical miRNA mechanisms remain largely unexplored.

Highlights

Fast miRNA loss-of-function with T6B impairs postnatal Purkinje cell developmentReversible T6B reveals critical miRNA windows for dendritogenesis and synaptogenesisConditional Spy3-Ago2 mouse line enables miRNA-target network mapping in rare cellsPurkinje cell-enriched miR-206 regulates its unique dendritic and synaptic morphology.

Gestational dexamethasone exposure impacts hippocampal excitatory synaptic transmission and learning and memory function with transgenerational effects

The formation of learning and memory is regulated by synaptic plasticity in hippocampal neurons. Here we explored how gestational exposure to dexamethasone, a synthetic glucocorticoid commonly used in clinical practice, has lasting effects on offspring's learning and memory. Adult offspring rats of prenatal dexamethasone exposure (PDE) displayed significant impairments in novelty recognition and spatial learning memory, with some phenotypes maintained transgenerationally. PDE impaired synaptic transmission of hippocampal excitatory neurons in offspring of F1 to F3 generations, and abnormalities of neurotransmitters and receptors would impair synaptic plasticity and lead to impaired learning and memory, but these changes failed to carry over to offspring of F5 and F7 generations. Mechanistically, altered hippocampal miR-133a-3p-SIRT1-CDK5-NR2B signaling axis in PDE multigeneration caused inhibition of excitatory synaptic transmission, which might be related to oocyte-specific high expression and transmission of miR-133a-3p. Together, PDE affects hippocampal excitatory synaptic transmission, with lasting consequences across generations, and CDK5 in offspring's peripheral blood might be used as an early-warning marker for fetal-originated learning and memory impairment.

GABAergic miR-34a regulates Dorsal Raphè inhibitory transmission in response to aversive, but not rewarding, stimuli

The brain employs distinct circuitries to encode positive and negative valence stimuli, and dysfunctions of these neuronal circuits have a key role in the etiopathogenesis of many psychiatric disorders. The Dorsal Raphè Nucleus (DRN) is involved in various behaviors and drives the emotional response to rewarding and aversive experiences. Whether specific subpopulations of neurons within the DRN encode these behaviors with different valence is still unknown. Notably, microRNA expression in the mammalian brain is characterized by tissue and neuronal specificity, suggesting that it might play a role in cell and circuit functionality. However, this specificity has not been fully exploited. Here, we demonstrate that microRNA-34a (miR-34a) is selectively expressed in a subpopulation of GABAergic neurons of the ventrolateral DRN. Moreover, we report that acute exposure to both aversive (restraint stress) and rewarding (chocolate) stimuli reduces GABA release in the DRN, an effect prevented by the inactivation of DRN miR-34a or its genetic deletion in GABAergic neurons in aversive but not rewarding conditions. Finally, miR-34a inhibition selectively reduced passive coping with severe stressors. These data support a role of miR-34a in regulating GABAergic neurotransmitter activity and behavior in a context-dependent manner and suggest that microRNAs could represent a functional signature of specific neuronal subpopulations with valence-specific activity in the brain.

Combining inflammatory miRNA molecules as diagnostic biomarkers for depression: a clinical study

Background

Inflammation has been implicated in core features of depression pathophysiology and treatment resistance. Therefore, new challenges in the discovery of inflammatory mediators implicated in depression have emerged. MicroRNAs (miRNAs) have been found aberrantly expressed in several pathologies, increasing their potential as biomarkers and therapeutical targets. In this study, the aim was to assess the changes and biomarker potential of inflammation-related miRNAs in depression patients.

Methods

Depression diagnosis was performed according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). 40 healthy controls and 32 depression patients were included in the study. The levels of inflammatory cytokines were measured in plasma, and expression levels of cytokines and inflammation-related miRNAs were evaluated in peripheral blood mononuclear cells (PBMCs).

Results

Depression patients were found to have a pro-inflammatory profile in plasma, with significantly higher levels of TNF-α and CCL2 compared with controls. In PBMCs of depression patients, TNF-α and IL-6 expression levels were significantly up and downregulated, respectively. Moreover, miR-342 levels were found upregulated, while miR-146a and miR-155 were significantly downregulated. miR-342 expression levels were positively correlated with TNF-α. Importantly, when analyzed as a diagnostic panel, receiver operating characteristics (ROC) analysis of miR-342, miR-146a, miR-155 in combination, showed to be highly specific and sensitive in distinguishing between depression patients and healthy controls.

Conclusion

In summary, these findings suggest that inflammation-related miRNAs are aberrantly expressed in depression patients. Moreover, we show evidences on the potential of the combination of dysregulated miRNAs as a powerful diagnostic tool for depression.

Non-coding RNA in the wiring and remodeling of neural circuits

The brain constantly adapts to changes in the environment, a capability that underlies memory and behavior. Long-term adaptations require the remodeling of neural circuits that are mediated by activity-dependent alterations in gene expression. Over the last two decades, it has been shown that the expression of protein-coding genes is significantly regulated by a complex layer of non-coding RNA (ncRNA) interactions. The aim of this review is to summarize recent discoveries regarding the functional involvement of ncRNAs during different stages of neural circuit development, activity-dependent circuit remodeling, and circuit maladapations underlying neurological and neuropsychiatric disorders. In addition to the intensively studied microRNA (miRNA) family, we focus on more recently added ncRNA classes, such as long ncRNAs (lncRNAs) and circular RNAs (circRNAs), and discuss the complex regulatory interactions between these different RNAs. We conclude by discussing the potential relevance of ncRNAs for cell-type and -state-specific regulation in the context of memory formation, the evolution of human cognitive abilities, and the development of new diagnostic and therapeutic tools in brain disorders.

Remember the poke: microRNAs are required for long-term memory formation following operant conditioning in Lymnaea

MicroRNAs (miRNAs) play an important role in learning and memory formation by controlling the expression of genes through epigenetic processes. Although miRNAs unquestionably play a role in memory, past literature focusing on whether miRNAs play key roles in the consolidation of associative long-term memory in Lymnaea contained confounding variables. Using operant conditioning of aerial respiratory behaviour, we investigated long-term memory (LTM) formation after injection of poly-L-lysine (PLL), an inhibitor of Dicer-mediated miRNA biogenesis, in Lymnaea stagnalis. Homeostatic breathing experiments were also performed to test whether PLL affects breathing. Homeostatic breathing was significantly suppressed 45 min but not 24 h after PLL injection. The operant conditioning procedure involved two 30-min training sessions separated by 1 h to cause LTM. Using this operant conditioning procedure, LTM formation was significantly impaired when snails were injected with PLL 15 min after the second training session. In contrast, when snails were injected with PLL 24 h before the first training session, LTM formation was not impaired. These results are consistent with past literature and highlight an important role for miRNAs in LTM formation.

Detection of MicroRNAs in Brain Slices Using In Situ Hybridization

MicroRNAs are key posttranscriptional regulators of protein levels in cells. The brain is particularly enriched in microRNAs, and important roles have been demonstrated for these noncoding RNAs in various neurological disorders. To this end, visualization of microRNAs in specific cell types and subcellular compartments within tissue sections provides researchers with essential insights that support understanding of the cell and molecular mechanisms of microRNAs in brain diseases. In this chapter we describe an in situ hybridization protocol for the detection of microRNAs in mouse brain sections, which provides cellular resolution of the expression of microRNAs in the brain.

microRNA-dependent regulation of gene expression in GABAergic interneurons

Information processing within neuronal circuits relies on their proper development and a balanced interplay between principal and local inhibitory interneurons within those circuits. Gamma-aminobutyric acid (GABA)ergic inhibitory interneurons are a remarkably heterogeneous population, comprising subclasses based on their morphological, electrophysiological, and molecular features, with differential connectivity and activity patterns. microRNA (miRNA)-dependent post-transcriptional control of gene expression represents an important regulatory mechanism for neuronal development and plasticity. miRNAs are a large group of small non-coding RNAs (21-24 nucleotides) acting as negative regulators of mRNA translation and stability. However, while miRNA-dependent gene regulation in principal neurons has been described heretofore in several studies, an understanding of the role of miRNAs in inhibitory interneurons is only beginning to emerge. Recent research demonstrated that miRNAs are differentially expressed in interneuron subclasses, are vitally important for migration, maturation, and survival of interneurons during embryonic development and are crucial for cognitive function and memory formation. In this review, we discuss recent progress in understanding miRNA-dependent regulation of gene expression in interneuron development and function. We aim to shed light onto mechanisms by which miRNAs in GABAergic interneurons contribute to sculpting neuronal circuits, and how their dysregulation may underlie the emergence of numerous neurodevelopmental and neuropsychiatric disorders.

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.

microRNA Biology on Brain Development and Neuroimaging Approach

Proper brain development requires the precise coordination and orchestration of various molecular and cellular processes and dysregulation of these processes can lead to neurological diseases. In the past decades, post-transcriptional regulation of gene expression has been shown to contribute to various aspects of brain development and function in the central nervous system. MicroRNAs (miRNAs), short non-coding RNAs, are emerging as crucial players in post-transcriptional gene regulation in a variety of tissues, such as the nervous system. In recent years, miRNAs have been implicated in multiple aspects of brain development, including neurogenesis, migration, axon and dendrite formation, and synaptogenesis. Moreover, altered expression and dysregulation of miRNAs have been linked to neurodevelopmental and psychiatric disorders. Magnetic resonance imaging (MRI) is a powerful imaging technology to obtain high-quality, detailed structural and functional information from the brains of human and animal models in a non-invasive manner. Because the spatial expression patterns of miRNAs in the brain, unlike those of DNA and RNA, remain largely unknown, a whole-brain imaging approach using MRI may be useful in revealing biological and pathological information about the brain affected by miRNAs. In this review, we highlight recent advancements in the research of miRNA-mediated modulation of neuronal processes that are important for brain development and their involvement in disease pathogenesis. Also, we overview each MRI technique, and its technological considerations, and discuss the applications of MRI techniques in miRNA research. This review aims to link miRNA biological study with MRI analytical technology and deepen our understanding of how miRNAs impact brain development and pathology of neurological diseases.

Drosophila FMRP controls miR-276-mediated regulation of nejire mRNA for space-filling dendrite development

MicroRNAs are enriched in neurons and play important roles in dendritic spine development and synaptic plasticity. MicroRNA activity is controlled by a wide range of RNA-binding proteins. FMRP, a highly conserved RNA-binding protein, has been linked to microRNA-mediated gene regulation in axonal development and dendritic spine formation. FMRP also participates in dendritic arbor morphogenesis, but whether and how microRNAs contribute to its function in this process remains to be elucidated. Here, using Drosophila larval sensory neurons, we show that a FMRP-associated microRNA, miR-276, functions in FMRP-mediated space-filling dendrite morphogenesis. Using EGFP microRNA sensors, we demonstrate that FMRP likely acts by regulating miR-276a RNA targeting rather than by modulating microRNA levels. Supporting this conclusion, miR-276a coimmunoprecipitated with FMRP and this association was dependent on the FMRP KH domains. By testing putative targets of the FMRP–miR-276a regulatory axis, we identified nejire as a FMRP-associated mRNA and, using EGFP reporters, showed that the nejire 3′ untranslated region is a target of miR-276a in vivo. Genetic analysis places nejire downstream of the FMRP–miR-276a pathway in regulating dendrite patterning. Together, our findings support a model in which FMRP facilitates miR-276a-mediated control of nejire for proper dendrite space-filling morphology and shed light on microRNA-dependent dendrite developmental pathology of fragile X syndrome.

A New Player in Depression: MiRNAs as Modulators of Altered Synaptic Plasticity

Depression is a psychiatric disorder that presents with a persistent depressed mood as the main clinical feature and is accompanied by cognitive impairment. Changes in neuroplasticity and neurogenesis greatly affect depression. Without genetic changes, epigenetic mechanisms have been shown to function by regulating gene expression during the body’s adaptation to stress. Studies in recent years have shown that as important regulatory factors in epigenetic mechanisms, microRNAs (miRNAs) play important roles in the development and progression of depression through the regulation of protein expression. Herein, we review the mechanisms of miRNA-mediated neuroplasticity in depression and discus synaptic structural plasticity, synaptic functional plasticity, and neurogenesis. Furthermore, we found that miRNAs regulate neuroplasticity through several signalling pathways to affect cognitive functions. However, these pathways do not work independently. Therefore, we try to identify synergistic correlations between miRNAs and multiple signalling pathways to broaden the potential pathogenesis of depression. In addition, in the future, dual-function miRNAs (protection/injury) are promising candidate biomarkers for the diagnosis of depression, and their regulated genes can potentially be used as target genes for the treatment of depression.

Integrated Analysis of the miRNA-mRNA Regulatory Network Involved in HIV-Associated Neurocognitive Disorder

HIV-associated neurocognitive disorder (HAND) is an array of neurocognitive changes associated with HIV infection, and the roles of microRNAs in HAND have not yet been completely revealed. Based on published data and publicly available databases, we constructed an integrated miRNA-mRNA network involved in HAND. Bioinformatics analyses, including gene ontology, network analysis, and KEGG pathway analysis, were applied for further study of the network and the genes of the network. The axon guidance KEGG pathway, three genes NTNG1, EFNB2, CXCL12, and 17 miRNAs which regulate these genes are spotlighted in our study. This study provides new perspectives to the knowledge of miRNAs’ roles in the progression of HAND, and our findings provide potential therapeutic targets and clues of HAND.

microRNA-138 controls hippocampal interneuron function and short-term memory in mice

The proper development and function of neuronal circuits rely on a tightly regulated balance between excitatory and inhibitory (E/I) synaptic transmission, and disrupting this balance can cause neurodevelopmental disorders, for example, schizophrenia. MicroRNA-dependent gene regulation in pyramidal neurons is important for excitatory synaptic function and cognition, but its role in inhibitory interneurons is poorly understood. Here, we identify miR138-5p as a regulator of short-term memory and inhibitory synaptic transmission in the mouse hippocampus. Sponge-mediated miR138-5p inactivation specifically in mouse parvalbumin (PV)-expressing interneurons impairs spatial recognition memory and enhances GABAergic synaptic input onto pyramidal neurons. Cellular and behavioral phenotypes associated with miR138-5p inactivation are paralleled by an upregulation of the schizophrenia (SCZ)-associated Erbb4, which we validated as a direct miR138-5p target gene. Our findings suggest that miR138-5p is a critical regulator of PV interneuron function in mice, with implications for cognition and SCZ. More generally, they provide evidence that microRNAs orchestrate neural circuit development by fine-tuning both excitatory and inhibitory synaptic transmission.

Microglia secrete miR-146a-5p-containing exosomes to regulate neurogenesis in depression

Enhancing neurogenesis within the hippocampal dentate gyrus (DG) is critical for maintaining brain development and function in many neurological diseases. However, the neural mechanisms underlying neurogenesis in depression remain unclear. Here, we show that microglia transfer a microglia-enriched microRNA, miR-146a-5p, via secreting exosomes to inhibit neurogenesis in depression. Overexpression of miR-146a-5p in hippocampal DG suppresses neurogenesis and spontaneous discharge of excitatory neurons by directly targeting Krüppel-like factor 4 (KLF4). Downregulation of miR-146a-5p expression ameliorates adult neurogenesis deficits in DG regions and depression-like behaviors in rats. Intriguingly, circular RNA ANKS1B acts as a miRNA sequester for miR-146a-5p to mediate post-transcriptional regulation of KLF4 expression. Collectively, these results indicate that miR-146a-5p can function as a critical factor regulating neurogenesis under conditions of pathological processes resulting from depression and suggest that microglial exosomes generate new crosstalk channels between glial cells and neurons.

Systematic Review: microRNAs as Potential Biomarkers in Mild Cognitive Impairment Diagnosis

The rate of progression from Mild Cognitive Impairment (MCI) to Alzheimer's disease (AD) is estimated at >10% per year, reaching up to 80-90% after 6 years. MCI is considered an indicator of early-stage AD. In this context, the diagnostic screening of MCI is crucial for detecting individuals at high risk of AD before they progress and manifest further severe symptoms. Typically, MCI has been determined using neuropsychological assessment tools such as the Montreal Cognitive Assessment (MoCA) or Mini-Mental Status Examination (MMSE). Unfortunately, other diagnostic methods are not available or are unable to identify MCI in its early stages. Therefore, identifying new biomarkers for MCI diagnosis and prognosis is a significant challenge. In this framework, miRNAs in serum, plasma, and other body fluids have emerged as a promising source of biomarkers for MCI and AD-related cognitive impairments. Interestingly, miRNAs can regulate several signaling pathways via multiple and diverse targets in response to pathophysiological stimuli. This systematic review aims to describe the current state of the art regarding AD-related target genes modulated by differentially expressed miRNAs in peripheral fluids samples in MCI subjects to identify potential miRNA biomarkers in the early stages of AD. We found 30 articles that described five miRNA expression profiles from peripheral fluid in MCI subjects, showing possible candidates for miRNA biomarkers that may be followed up as fluid biomarkers or therapeutic targets of early-stage AD. However, additional research is needed to validate these miRNAs and characterize the precise neuropathological mechanisms.

A change in taste: The role of MicroRNAs in altering hedonic value

The mechanisms associated with neophobia, and anhedonia remain largely unknown. Neuropsychological disorders such as depression and schizophrenia are associated with excessive fear and anhedonia and have been linked to microRNA 137. We hypothesized that microRNAs (miRNAs) in the snail Lymnaea stagnalis are important for regulating feeding behaviour through either preventing neophobia or establishing hedonic value. To test these hypotheses, we used an injection of Poly-L-Lysine (PLL) to inhibit miRNA biogenesis and observed its effects on feeding behaviour. We repeated these experiments with pre-exposure to novel stimuli capable of eliciting neophobia to disentangle the processes predicted to regulate feeding behaviour. Next, we exposed snails to food stimuli of high hedonic value after PLL injection to reset their hedonic value for that food. Finally, we consolidated our results with previous research by examining the effect of PLL injection on a one trial appetitive classical conditioning procedure (1TT) to induce long term memory (LTM). We found that miRNAs are likely not required for preventing neophobia. Moreover, we discovered that snails experienced anhedonia in response to inhibition of miRNA biogenesis, resulting in diminished feeding behaviour for food stimuli with a previously high hedonic value. Snails showed diminished feeding behaviour for multiple food stimuli of high hedonic value post 1TT with PLL injection. This finding suggested that PLL causes anhedonia rather than an impairment of LTM formation following the 1TT procedure. This is the first evidence suggesting that inhibiting the biogenesis of miRNAs contributes to anhedonia in Lymnaea.

Real-Time Quantitative PCR and Fluorescence In Situ Hybridization for Subcellular Localization of miRNAs in Neurons

Neuronal miRNAs play major roles in regulation of synaptic development and plasticity. The small size of miRNAs and, in some cases, their low level of expression make their quantification and detection challenging. Here, we outline methods to quantify steady state levels of miRNAs in neurons and the brain by using real-time quantitative PCR (RT-qPCR) and to determine miRNA subcellular localization in primary neurons by a sensitive fluorescence in situ hybridization (FISH) method.

Prophylactic treatment of curcumin in a rat model of depression by attenuating hippocampal synaptic loss

Curcumin is a polyphenol substance considered to be effective in the treatment of a number of neurodegenerative diseases. However, details regarding the exact mechanisms for the protective effects of curcumin in neuropsychiatric disorders, like depression, remain unknown. In the pathogenesis of major depressive disorder (MDD) it appears that dysregulation of oxidative stress and immune systems, particularly within the hippocampal region, may play a critical role. Here, we show that pre-treatment with curcumin (40 mg kg^-1) alleviates depression-like behaviors in a LPS-induced rat model of depression, effects which were accompanied with suppression of oxidative stress and inflammation and an inhibition of neuronal apoptosis in the hippocampal CA1 region, and results from ultramicrostructure electrophysiological experiments revealed that the curcumin pre-treatment significantly prevented excessive synaptic loss and enhanced synaptic functioning in this LPS-induced rat model. In addition, curcumin attenuated the increases in levels of miR-146a-5p and decreases in the expression of p-ERK signaling that would normally occur within CA1 regions of these depressed rats. Taken together, these results demonstrated that curcumin exerts neuroprotective and antidepressant activities by suppressing oxidative stress, neural inflammation and their related effects upon synaptic dysregulation. One of the mechanisms for these beneficial effects of curcumin appears to involve the miR-146a-5p/ERK signaling pathway within the hippocampal CA1 region. These findings not only elucidated some of the mechanisms underlying the neuroprotective/antidepressant effects of curcumin, but also suggested a role of curcumin as a potential therapeutic strategy for depression.

Potential of different cells-derived exosomal microRNA cargos for treating spinal cord injury

Spinal cord injury (SCI) is a disastrous situation that affects many patients worldwide. A profound understanding of the pathology and etiology of SCI is of great importance in inspiring new therapeutic concepts and treatment. In recent years, exosomes, which are complex lipid membrane structures secreted nearly by all kinds of plants and animal cells, can transport their valuable cargoes (e.g., proteins, lipids, RNAs) to the targeted cells and exert their communication and regulation functions, which open up a new field of treatment of SCI. Notably, the exosome's advantage is transporting the carried material to the target cells across the blood-brain barrier and exerting regulatory functions. Among the cargoes of exosomes, microRNAs, through the modulation of their mRNA targets, emerges with great potentiality in the pathological process, diagnosis and treatment of SCI. In this review, we discuss the role of miRNAs transported by different cell-derived exosomes in SCI that are poised to enhance SCI-specific therapeutic capabilities of exosomes.

Distinct temporal expression of the GW182 paralog TNRC6A in neurons regulates dendritic arborization

Precise development of the dendritic architecture is a critical determinant of mature neuronal circuitry. MicroRNA (miRNA)-mediated regulation of protein synthesis plays a crucial role in dendritic morphogenesis, but the role of miRNA-induced silencing complex (miRISC) protein components in this process is less studied. Here, we show an important role of a key miRISC protein, the GW182 paralog TNRC6A, in the regulation of dendritic growth. We identified a distinct brain region-specific spatiotemporal expression pattern of GW182 during rat postnatal development. We found that the window of peak GW182 expression coincides with the period of extensive dendritic growth, both in the hippocampus and cerebellum. Perturbation of GW182 function during a specific temporal window resulted in reduced dendritic growth of cultured hippocampal neurons. Mechanistically, we show that GW182 modulates dendritic growth by regulating global somatodendritic translation and actin cytoskeletal dynamics of developing neurons. Furthermore, we found that GW182 affects dendritic architecture by regulating the expression of actin modulator LIMK1. Taken together, our data reveal a previously undescribed neurodevelopmental expression pattern of GW182 and its role in dendritic morphogenesis, which involves both translational control and actin cytoskeletal rearrangement. This article has an associated First Person interview with the first author of the paper.

MicroRNA-26a-3p rescues depression-like behaviors in male rats via preventing hippocampal neuronal anomalies

Depression is a neuropsychiatric disease associated with neuronal anomalies within specific brain regions. In the present study, we screened microRNA (miRNA) expression profiles in the dentate gyrus (DG) of the hippocampus and found that miR-26a-3p was markedly downregulated in a rat model of depression, whereas upregulation of miR-26a-3p within DG regions rescued the neuronal deterioration and depression-like phenotypes resulting from stress exposure, effects that appear to be mediated by the PTEN pathway. The knockdown of miR-26a-3p in DG regions of normal control rats induced depression-like behaviors, effects that were accompanied by activation of the PTEN/PI3K/Akt signaling pathway and neuronal deterioration via suppression of autophagy, impairments in synaptic plasticity, and promotion of neuronal apoptosis. In conclusion, these results suggest that miR-26a-3p deficits within the hippocampal DG mediated the neuronal anomalies contributing to the display of depression-like behaviors. This miRNA may serve as a potential therapeutic target for the treatment of depression.

Activation of a hippocampal CREB-pCREB-miRNA-MEF2 axis modulates individual variation of spatial learning and memory capability

Phenotypic variation is a fundamental prerequisite for cell and organism evolution by natural selection. Whereas the role of stochastic gene expression in phenotypic diversity of genetically identical cells is well studied, not much is known regarding the relationship between stochastic gene expression and individual behavioral variation in animals. We demonstrate that a specific miRNA (miR-466f-3p) is upregulated in the hippocampus of a portion of individual inbred mice upon a Morris water maze task. Significantly, miR-466f-3p positively regulates the neuron morphology, function and spatial learning, and memory capability of mice. Mechanistically, miR-466f-3p represses translation of MEF2A, a negative regulator of learning/memory. Finally, we show that varied upregulation of hippocampal miR-466f-3p results from randomized phosphorylation of hippocampal cyclic AMP (cAMP)-response element binding (CREB) in individuals. This finding of modulation of spatial learning and memory via a randomized hippocampal signaling axis upon neuronal stimulation represents a demonstration of how variation in tissue gene expression lead to varied animal behavior.

Regulation of AMPAR expression by microRNAs

AMPA receptors (AMPARs) are the major excitatory neurotransmitter receptor in the brain, and their expression at synapses is a critical determinant of synaptic transmission and therefore brain function. Synaptic plasticity involves increases or decreases in synaptic strength, caused by changes in the number or subunit-specific subtype of AMPARs expressed at synapses, and resulting in modifications of functional connectivity of neuronal circuits, a process which is thought to underpin learning and the formation or loss of memories. Furthermore, numerous neurological disorders involve dysregulation of excitatory synaptic transmission or aberrant recruitment of plasticity processes. MicroRNAs (miRNAs) repress the translation of target genes by partial complementary base pairing with mRNAs, and are the core component of a mechanism widely used in a range of cell processes for regulating protein translation. MiRNA-dependent translational repression can occur locally in neuronal dendrites, close to synapses, and can also result in relatively rapid changes in protein expression. MiRNAs are therefore well-placed to regulate synaptic plasticity via the local control of AMPAR subunit synthesis, and can also result in synaptic dysfunction in the event of dysregulation in disease. Here, I will review the miRNAs that have been identified as playing a role in physiological or pathological changes in AMPAR subunit expression at synapses, focussing on miRNAs that target mRNAs encoding AMPAR subunits, and on miRNAs that target AMPAR accessory proteins involved in AMPAR trafficking and hence the regulation of AMPAR synaptic localisation.

The Coordination of Local Translation, Membranous Organelle Trafficking, and Synaptic Plasticity in Neurons

Neurons are highly complex polarized cells, displaying an extraordinary degree of spatial compartmentalization. At presynaptic and postsynaptic sites, far from the cell body, local protein synthesis is utilized to continually modify the synaptic proteome, enabling rapid changes in protein production to support synaptic function. Synapses undergo diverse forms of plasticity, resulting in long-term, persistent changes in synapse strength, which are paramount for learning, memory, and cognition. It is now well-established that local translation of numerous synaptic proteins is essential for many forms of synaptic plasticity, and much work has gone into deciphering the strategies that neurons use to regulate activity-dependent protein synthesis. Recent studies have pointed to a coordination of the local mRNA translation required for synaptic plasticity and the trafficking of membranous organelles in neurons. This includes the co-trafficking of RNAs to their site of action using endosome/lysosome “transports,” the regulation of activity-dependent translation at synapses, and the role of mitochondria in fueling synaptic translation. Here, we review our current understanding of these mechanisms that impact local translation during synaptic plasticity, providing an overview of these novel and nuanced regulatory processes involving membranous organelles in neurons.

miR-8b is involved in brain and eye regeneration of Dugesia japonica in head regeneration

MicroRNAs (miRNAs) are a class of evolutionarily conserved small non-coding RNAs that regulate gene expression at the translation level in cell growth, proliferation and differentiation. In addition, some types of miRNAs have been proven to be key modulators of both CNS development and plasticity, such as let-7, miR-9 and miR-124. In this research, we found miR-8b acts as an important regulator involved in brain and eyespot regeneration in Dugesia japonica. miR-8b was highly conserved among species and was abundantly expressed in central nervous system. Here, we detected the expression dynamics of miR-8b by qPCR during the head regeneration of D. japonica. Knockdown miR-8b by anti-MIRs method caused severe defects of eyes and CNS. Our study revealed the evolutionary conserved role of miR-8b in the planarian regeneration process, and further provided more research ideas and available information for planarian miRNAs.

Summary: Most miRNAs in planarians are homologous to humans and other mammals, and may also play a similar regulatory role. Knockdown miR-8b planarian miR-8b induces brain and eyespot defects during head regeneration.

miR-128 as a Regulator of Synaptic Properties in 5xFAD Mice Hippocampal Neurons

Alzheimer's disease (AD) is characterized by progressive synaptic dysfunction, deterioration of neuronal transmission, and consequently neuronal death. Although there is no treatment for AD, exposure to enriched environment (EE) in mice, as well as physical and mental activity in human subjects have been shown to have a protective effect by slowing the disease's progression and reducing AD-like cognitive impairment. However, the molecular mechanism of this mitigating effect is still not understood. One of the mechanisms that has recently been shown to be involved in neuronal degeneration is microRNAs (miRNAs) regulation, which act as a post-transcriptional regulators of gene expression. miR-128 has been shown to be significantly altered in individuals with AD and in mice following exposure to EE. Here, we focused on elucidating the possible role of miR-128 in AD pathology and found that miR-128 regulates the expression of two proteins essential for synaptic transmission, SNAP-25, and synaptotagmin1 (Syt1). Clinically relevant, in 5xFAD mouse model for AD, this miRNA's expression was found as downregulated, resembling the alteration found in the hippocampi of individuals with AD. Interestingly, exposing WT mice to EE also resulted in downregulation of miR-128 expression levels, although EE and AD conditions demonstrate opposing effects on neuronal functioning and synaptic plasticity. We also found that miR-128 expression downregulation in primary hippocampal cultures from 5xFAD mice results in increased neuronal network activity and neuronal excitability. Altogether, our findings place miR-128 as a synaptic player that may contribute to synaptic functioning and plasticity through regulation of synaptic protein expression and function.

Making sense of mRNA landscapes: Translation control in neurodevelopment

Like all other parts of the central nervous system, the mammalian neocortex undergoes temporally ordered set of developmental events, including proliferation, differentiation, migration, cellular identity, synaptogenesis, connectivity formation, and plasticity changes. These neurodevelopmental mechanisms have been characterized by studies focused on transcriptional control. Recent findings, however, have shown that the spatiotemporal regulation of post-transcriptional steps like alternative splicing, mRNA traffic/localization, mRNA stability/decay, and finally repression/derepression of protein synthesis (mRNA translation) have become just as central to the neurodevelopment as transcriptional control. A number of dynamic players act post-transcriptionally in the neocortex to regulate these steps, as RNA binding proteins (RBPs), ribosomal proteins (RPs), long non-coding RNAs, and/or microRNA. Remarkably, mutations in these post-transcriptional regulators have been associated with neurodevelopmental, neurodegenerative, inherited, or often co-morbid disorders, such as microcephaly, autism, epilepsy, intellectual disability, white matter diseases, Rett-syndrome like phenotype, spinocerebellar ataxia, and amyotrophic lateral sclerosis. Here, we focus on the current state, advanced methodologies and pitfalls of this exciting and upcoming field of RNA metabolism with vast potential in understanding fundamental neurodevelopmental processes and pathologies. This article is categorized under: Translation > Translation Regulation RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Local miRNA-Dependent Translational Control of GABAAR Synthesis during Inhibitory Long-Term Potentiation

Molecular mechanisms underlying plasticity at brain inhibitory synapses remain poorly characterized. Increased postsynaptic clustering of GABA_A receptors (GABA_ARs) rapidly strengthens inhibition during inhibitory long-term potentiation (iLTP). However, it is unclear how synaptic GABA_AR clustering is maintained to sustain iLTP. Here, we identify a role for miR376c in regulating the translation of mRNAs encoding the synaptic α1 and γ2 GABA_AR subunits, GABRA1 and GABRG2, respectively. Following iLTP induction, transcriptional repression of miR376c is induced through a calcineurin-NFAT-HDAC signaling pathway and promotes increased translation and clustering of synaptic GABA_ARs. This pathway is essential for the long-term expression of iLTP and is blocked by miR376c overexpression, specifically impairing inhibitory synaptic strength. Finally, we show that local de novo synthesis of synaptic GABA_ARs occurs exclusively in dendrites and in a miR376c-dependent manner following iLTP. Together, this work describes a local post-transcriptional mechanism that regulates inhibitory synaptic plasticity via miRNA control of dendritic protein synthesis.

Clustering of GABA_ARs at inhibitory synapses is crucial for synaptic inhibition. Rajgor et al. discover that synaptic GABA_AR expression is controlled by their local translation, regulated by miR376c. During inhibitory synaptic potentiation, miR376c is downregulated, relieving its translational repression of GABA_AR mRNAs and leading to de novo synthesis of dendritic GABA_ARs.

A Sex Perspective in Neurodegenerative Diseases: microRNAs as Possible Peripheral Biomarkers

Sex is a significant variable in the prevalence and incidence of neurological disorders. Sex differences exist in neurodegenerative disorders (NDs), where sex dimorphisms play important roles in the development and progression of Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. In the last few years, some sex specific biomarkers for the identification of NDs have been described and recent studies have suggested that microRNA (miRNA) could be included among these, as influenced by the hormonal and genetic background. Failing to consider the possible differences between males and females in miRNA evaluation could introduce a sex bias in studies by not considering some of these sex-related biomarkers. In this review, we recapitulate what is known about the sex-specific differences in peripheral miRNA levels in neurodegenerative diseases. Several studies have reported sex-linked disparities, and from the literature analysis miR-206 particularly has been shown to have a sex-specific involvement. Hopefully, in the near future, patient stratification will provide important additional clues in diagnosis, prognosis, and tailoring of the best therapeutic approaches for each patient. Sex-specific biomarkers, such as miRNAs, could represent a useful tool for characterizing subgroups of patients.

MicroRNAs in central nervous system disorders: current advances in pathogenesis and treatment

MicroRNAs (miRNAs) are a class of short, non-coding, regulatory RNA molecules that function as post transcriptional regulators of gene expression. Altered expression of multiple miRNAs was found to be extensively involved in the pathogenesis of different neurological disorders including Alzheimer’s disease, Parkinson’s disease, stroke, epilepsy, multiple sclerosis, amyotrophic lateral sclerosis, and Huntington’s disease. miRNAs are implicated in the pathogenesis of excitotoxicity, apoptosis, oxidative stress, inflammation, neurogenesis, angiogenesis, and blood–brain barrier protection. Consequently, miRNAs can serve as biomarkers for different neurological disorders. In recent years, advances in the miRNA field led to identification of potentially novel prospects in the development of new therapies for incurable CNS disorders. MiRNA-based therapeutics include miRNA mimics and inhibitors that can decrease or increase the expression of target genes. Better understanding of the mechanisms by which miRNAs are implicated in the pathogenesis of neurological disorders may provide novel targets to researchers for innovative therapeutic strategies.

Systemic delivery of antagomirs during blood-brain barrier disruption is disease-modifying in experimental epilepsy

Oligonucleotide therapies offer precision treatments for a variety of neurological diseases, including epilepsy, but their deployment is hampered by the blood-brain barrier (BBB). Previous studies showed that intracerebroventricular injection of an antisense oligonucleotide (antagomir) targeting microRNA-134 (Ant-134) reduced evoked and spontaneous seizures in animal models of epilepsy. In this study, we used assays of serum protein and tracer extravasation to determine that BBB disruption occurring after status epilepticus in mice was sufficient to permit passage of systemically injected Ant-134 into the brain parenchyma. Intraperitoneal and intravenous injection of Ant-134 reached the hippocampus and blocked seizure-induced upregulation of miR-134. A single intraperitoneal injection of Ant-134 at 2 h after status epilepticus in mice resulted in potent suppression of spontaneous recurrent seizures, reaching a 99.5% reduction during recordings at 3 months. The duration of spontaneous seizures, when they occurred, was also reduced in Ant-134-treated mice. In vivo knockdown of LIM kinase-1 (Limk-1) increased seizure frequency in Ant-134-treated mice, implicating de-repression of Limk-1 in the antagomir mechanism. These studies indicate that systemic delivery of Ant-134 reaches the brain and produces long-lasting seizure-suppressive effects after systemic injection in mice when timed with BBB disruption and may be a clinically viable approach for this and other disease-modifying microRNA therapies.

The role of microRNAs in learning and long-term memory

The mechanisms of long-term memory formation and ways to improve it (in the case of its impairment) remain an extremely difficult problem yet to be solved. Over the recent years, much attention has been paid to microRNAs in this regard. MicroRNAs are unique endogenous non-coding RNAs about 22 nucleotides in length; each can regulate translation of hundreds of messenger RNA targets, thereby controlling entire gene networks. MicroRNAs are widely represented in the central nervous system. A large number of studies are currently being conducted to investigate the role of microRNAs in the brain functioning. A number of microRNAs have been shown to be involved in the process of synaptic plasticity, as well as in the long-term memory formation. Disruption of microRNA biogenesis leads to significant cognitive dysfunctions. Moreover, impaired microRNA biogenesis is one of the causes of the pathogenesis of mental disorders, neurodegenerative illnesses and senile dementia, which are often accompanied by deterioration in the learning ability and by memory impairment. Optimistic predictions are made that microRNAs can be used as targets for therapeutic treatment and for diagnosing the above pathologies. The importance of applications related to microRNAs significantly raises interest in studying their functions in the brain. Thus, this review is focused on the role of microRNAs in cognitive processes. It describes microRNA biogenesis and the role of miRNAs in the regulation of gene expression, as well as the latest achievements in studying the functional role of microRNAs in learning and in long-term memory formation, depending on the activation or inhibition of their expression. The review presents summarized data on the effect of impaired microRNA biogenesis on long-term memory formation, including those associated with sleep deprivation. In addition, analysis is provided of the current literature related to the prospects of improving cognitive processes by influencing microRNA biogenesis via the use of CRISPR/Cas9 technologies and active mental and physical exercises.

Social Isolation and Enrichment Induce Unique miRNA Signatures in the Prefrontal Cortex and Behavioral Changes in Mice

An extensive body of evidence supports the notion that exposure to an enriched/impoverished environment alters brain functions via epigenetic changes. However, how specific modifications of social environment modulate brain functions remains poorly understood. To address this issue, we investigate the molecular and behavioral consequences of briefly manipulating social settings in young and middle-aged wild-type mice. We observe that, modifications of the social context, only affect the performance in socially related tasks. Social enrichment increases sociability whereas isolation leads to the opposite effect. Our work also pointed out specific miRNA signatures associated to each social environment. These miRNA alterations are reversible and found selectively in the medial prefrontal cortex. Finally, we show that miRNA modifications linked to social enrichment or isolation might target rather different intracellular pathways. Together, these observations suggest that the prefrontal cortex may be a key brain area integrating social information via the modification of precise miRNA networks.

Neurotrophic Factor BDNF, Physiological Functions and Therapeutic Potential in Depression, Neurodegeneration and Brain Cancer

Brain-derived neurotrophic factor (BDNF) is one of the most distributed and extensively studied neurotrophins in the mammalian brain. BDNF signals through the tropomycin receptor kinase B (TrkB) and the low affinity p75 neurotrophin receptor (p75NTR). BDNF plays an important role in proper growth, development, and plasticity of glutamatergic and GABAergic synapses and through modulation of neuronal differentiation, it influences serotonergic and dopaminergic neurotransmission. BDNF acts as paracrine and autocrine factor, on both pre-synaptic and post-synaptic target sites. It is crucial in the transformation of synaptic activity into long-term synaptic memories. BDNF is considered an instructive mediator of functional and structural plasticity in the central nervous system (CNS), influencing dendritic spines and, at least in the hippocampus, the adult neurogenesis. Changes in the rate of adult neurogenesis and in spine density can influence several forms of learning and memory and can contribute to depression-like behaviors. The possible roles of BDNF in neuronal plasticity highlighted in this review focus on the effect of antidepressant therapies on BDNF-mediated plasticity. Moreover, we will review data that illustrate the role of BDNF as a potent protective factor that is able to confer protection against neurodegeneration, in particular in Alzheimer’s disease. Finally, we will give evidence of how the involvement of BDNF in the pathogenesis of brain glioblastoma has emerged, thus opening new avenues for the treatment of this deadly cancer.

Differential Expression of microRNA Profiles and Wnt Signals in Stem Cell-Derived Exosomes During Dopaminergic Neuron Differentiation

The role of secreted exosomes during dopaminergic (DA) neuron differentiation is still unknown. To investigate the roles of exosomes in DA neuron fate specification, we profiled exosomal microRNAs (miRNAs) during DA neuron differentiation of epiblast-derived stem cells (EpiSCs). There were 26 miRNAs differentially expressed (relative fold >2, p < 0.05) in EpiSC-derived exosomes at 0, 2, 4, 6, 8, 10, 12, and 14 days of DA epiblast differentiation. Among them, 23 exosomic miRNAs were significantly increased, including miR-124, miR-132, miR-133b, miR-218, miR-9, miR-34b, miR-34c, and miR-135a2, while three exosomic miRNAs (miR-214, miR-7a, and miR-302b) were decreased, when compared with control samples. Bioinformatics analysis by DIANA-mirPath demonstrated that extracellular matrix-receptor interaction, signaling pathways regulating pluripotency of stem cells, FoxO signaling pathway, DA synapse, Wnt signaling pathway, GABAergic synapse, and neurotrophin signaling pathway were significantly enriched in DA differentiation-related miRNA signature (all p-values <0.012). Furthermore, messenger RNAs for nine DA neuronal markers tyrosine hydroxylase (TH), Nr4a2, Pitx3, Drd1a, Lmx1a, Lmx1b, Foxa1, Dmrt5, and Slc18a2 were significantly increased expressed over time in exosomes derived from differentiated EpiSCs. Interestingly, adding with exosomes derived from EpiSC induction experiment resulted in a twofold increase of TH-positive neurons production (35% vs. 17%, p < 0.01) during DA neuronal differentiation from mouse embryonic stem cells (ESCs). In summary, our results suggested exosomal miRNAs are potential regulators of DA neuron differentiation. More importantly, EpiSC-derived exosomes could promote the generation of DA neuron differentiation from ESCs.

Supraspinal Mechanisms of Intestinal Hypersensitivity

Gut inflammation or injury causes intestinal hypersensitivity (IHS) and hyperalgesia, which can persist after the initiating pathology resolves, are often referred to somatic regions and exacerbated by psychological stress, anxiety or depression, suggesting the involvement of both the spinal cord and the brain. The supraspinal mechanisms of IHS remain to be fully elucidated, however, over the last decades the series of intestinal pathology-associated neuroplastic changes in the brain has been revealed, being potentially responsible for the phenomenon. This paper reviews current clinical and experimental data, including the authors' own findings, on these functional, structural, and neurochemical/molecular changes within cortical, subcortical and brainstem regions processing and modulating sensory signals from the gut. As concluded in the review, IHS can develop and maintain due to the bowel inflammation/injury-induced persistent hyperexcitability of viscerosensory brainstem and thalamic nuclei and sensitization of hypothalamic, amygdala, hippocampal, anterior insular, and anterior cingulate cortical areas implicated in the neuroendocrine, emotional and cognitive modulation of visceral sensation and pain. An additional contribution may come from the pathology-triggered dysfunction of the brainstem structures inhibiting nociception. The mechanism underlying IHS-associated regional hyperexcitability is enhanced NMDA-, AMPA- and group I metabotropic receptor-mediated glutamatergic neurotransmission in association with altered neuropeptide Y, corticotropin-releasing factor, and cannabinoid 1 receptor signaling. These alterations are at least partially mediated by brain microglia and local production of cytokines, especially tumor necrosis factor α. Studying the IHS-related brain neuroplasticity in greater depth may enable the development of new therapeutic approaches against chronic abdominal pain in inflammatory bowel disease.

A miRNome analysis of drug-free manic psychotic bipolar patients versus healthy controls

The lifetime presence of psychotic symptoms is associated with more clinical severity, poorer outcome and biological changes in patients affected by bipolar disorder (BD). Epigenetic mechanisms have been evoked to explain the onset of psychotic symptoms in BD as well as the associated biological changes. The main objective of the present study was to evaluate the expression profiles of circulating microRNAs (miRNAs) in drug-free manic psychotic bipolar patients versus healthy controls (HC), to identify possible non-invasive molecular markers of the disorder. 15 drug-free manic psychotic bipolar patients and 9 HC were enrolled and 800 miRNAs expression profile was measured by Nanostring nCounter technology on plasma samples and validated through qPCR. Overall, twelve miRNAs showed a significantly altered expression between the two groups (p < 0.05). Functional annotation of predicted miRNAs targets by MultiMIR R tool showed repression in bipolar patients of genes with a role in neurodevelopment and neurogenesis, and upregulation of genes involved in metabolism regulation. We identified a signature of circulating miRNA characteristic of manic psychotic bipolar patients, suggesting a possible role in neurodevelopment and metabolic processes regulation.

The conserved microRNA miR-210 regulates lipid metabolism and photoreceptor maintenance in the Drosophila retina

Increasing evidence suggests that miRNAs play important regulatory roles in the nervous system. However, the molecular mechanisms of how specific miRNAs affect neuronal development and functions remain less well understood. In the present study, we provide evidence that the conserved microRNA miR-210 regulates lipid metabolism and prevents neurodegeneration in the Drosophila retina. miR-210 is specifically expressed in the photoreceptor neurons and other sensory organs. Genetic deletion of miR-210 leads to lipid droplet accumulation and photoreceptor degeneration in the retina. These effects are associated with abnormal activation of the Drosophila sterol regulatory element-binding protein signaling. We further identify the acetyl-coenzyme A synthetase (ACS) as one functionally important target of miR-210 in this context. Reduction of ACS in the miR-210 mutant background suppresses the neurodegeneration defects, suggesting that miR-210 acts through regulation of the ACS transcript. Together, these results reveal an unexpected role of miR-210 in controlling lipid metabolism and neuronal functions.

Noncoding RNAs and Midbrain DA Neurons: Novel Molecular Mechanisms and Therapeutic Targets in Health and Disease

Midbrain dopamine neurons have crucial functions in motor and emotional control and their degeneration leads to several neurological dysfunctions such as Parkinson’s disease, addiction, depression, schizophrenia, and others. Despite advances in the understanding of specific altered proteins and coding genes, little is known about cumulative changes in the transcriptional landscape of noncoding genes in midbrain dopamine neurons. Noncoding RNAs—specifically microRNAs and long noncoding RNAs—are emerging as crucial post-transcriptional regulators of gene expression in the brain. The identification of noncoding RNA networks underlying all stages of dopamine neuron development and plasticity is an essential step to deeply understand their physiological role and also their involvement in the etiology of dopaminergic diseases. Here, we provide an update about noncoding RNAs involved in dopaminergic development and metabolism, and the related evidence of these biomolecules for applications in potential treatments for dopaminergic neurodegeneration.

Elevated exosomal secretion of miR-124-3p from spinal neurons positively associates with disease severity in ALS

MicroRNAs (miRs) are powerful regulators of CNS development and diseases. Plasma and cerebrospinal fluid (CSF) miRs have recently been implicated as potential new sources for biomarker development. Previously we showed that miR-124-3p, an essential miR for neuronal identity, is highly abundant in neuronal exosomes and its expression decreases in spinal cord of ALS model SOD1G93A mice. In the current study, we found a disease associated reduction of miR-124-3p levels specifically in spinal neurons using in situ hybridization. By employing our recently developed exosome reporter mice in combination with sciatic nerve injections, we observed an increased association of miR-124-3p with spinal motor neuron-derived exosomes in SOD1G93A mice, even at the pre-symptomatic stage. Sciatic nerve injection delivered miR-124-3p is also more frequently localized outside of spinal motor neurons in SOD1G93A mice. Subsequent quantitative analysis of miR-124-3p levels in CSF exosomes from ALS patients found a significant correlation between CSF exosomal miR-124-3p levels and disease stage (indicated by the ALSFRS-R score) of (male) ALS patients. These results provide preliminary evidence to support the potential use of CSF exosomal miR-124-3p as a disease stage indicator in ALS.

Molecular Regulation in Dopaminergic Neuron Development. Cues to Unveil Molecular Pathogenesis and Pharmacological Targets of Neurodegeneration

The relatively few dopaminergic neurons in the mammalian brain are mostly located in the midbrain and regulate many important neural functions, including motor integration, cognition, emotive behaviors and reward. Therefore, alteration of their function or degeneration leads to severe neurological and neuropsychiatric diseases. Unraveling the mechanisms of midbrain dopaminergic (mDA) phenotype induction and maturation and elucidating the role of the gene network involved in the development and maintenance of these neurons is of pivotal importance to rescue or substitute these cells in order to restore dopaminergic functions. Recently, in addition to morphogens and transcription factors, microRNAs have been identified as critical players to confer mDA identity. The elucidation of the gene network involved in mDA neuron development and function will be crucial to identify early changes of mDA neurons that occur in pre-symptomatic pathological conditions, such as Parkinson’s disease. In addition, it can help to identify targets for new therapies and for cell reprogramming into mDA neurons. In this essay, we review the cascade of transcriptional and posttranscriptional regulation that confers mDA identity and regulates their functions. Additionally, we highlight certain mechanisms that offer important clues to unveil molecular pathogenesis of mDA neuron dysfunction and potential pharmacological targets for the treatment of mDA neuron dysfunction.

A motor neuron protective role of miR-969 mediated by the transcription factor kay

Maintenance of motor neuron structure and function is crucial in development and motor behaviour. However, the genetic regulatory mechanism of motor neuron function remains less well understood. In the present study, we identify a novel neuroprotective role of the microRNA miR-969 in Drosophila motor neurons. miR-969 is highly expressed in motor neurons. Loss of miR-969 results in early-onset and age-progressive locomotion impairment. Flies lacking miR-969 also exhibit shortened lifespan. Moreover, miR-969 is required in motor neurons. We further identify kay as a functionally important target of miR-969. Together, our results indicate that miR-969 can protect motor neuron function by limiting kay activity in Drosophila.

Potential of activated microglia as a source of dysregulated extracellular microRNAs contributing to neurodegeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common form of motor neuron degeneration in adults, and several mechanisms underlying the disease pathology have been proposed. It has been shown that glia communicate with other cells by releasing extracellular vesicles containing proteins and nucleic acids, including microRNAs (miRNAs), which play a role in the post-transcriptional regulation of gene expression. Dysregulation of miRNAs is commonly observed in ALS patients, together with inflammation and an altered microglial phenotype. However, the role of miRNA-containing vesicles in microglia-to-neuron communication in the context of ALS has not been explored in depth. This review summarises the evidence for the presence of inflammation, pro-inflammatory microglia and dysregulated miRNAs in ALS, then explores how microglia may potentially be responsible for this miRNA dysregulation. The possibility of pro-inflammatory ALS microglia releasing miRNAs which may then enter neuronal cells to contribute to degeneration is also explored. Based on the literature reviewed here, microglia are a likely source of dysregulated miRNAs and potential mediators of neurodegenerative processes. Therefore, dysregulated miRNAs may be promising candidates for the development of therapeutic strategies.