Reversible inhibition of PSD-95 mRNA translation by miR-125a, FMRP phosphorylation, and mGluR signaling

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

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

MicroRNAs and synapse turnover in Alzheimer's disease

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder characterized by the accumulation of amyloid-beta plaques and neurofibrillary tangles in the brain, leading to synaptic dysfunction and cognitive decline. Healthy synapses are the crucial for normal brain function, memory restoration and other neurophysiological function. Synapse loss and synaptic dysfunction are two primary events that occur during AD initiation. Synapse lifecycle and/or synapse turnover is divided into five key stages and several sub-stages such as synapse formation, synapse assembly, synapse maturation, synapse transmission and synapse termination. In normal state, the synapse turnover is regulated by various biological and molecular factors for a healthy neurotransmission. In AD, the different stages of synapse turnover are affected by AD-related toxic proteins. MicroRNAs (miRNAs) have emerged as critical regulators of gene expression and have been implicated in various neurological diseases, including AD. Deregulation of miRNAs modulate the synaptic proteins and affect the synapse turnover at different stages. In this review, we discussed the key milestones of synapse turnover and how they are affected in AD. Further, we discussed the involvement of miRNAs in synaptic turnover, focusing specifically on their role in AD pathogenesis. We also emphasized the regulatory mechanisms by which miRNAs modulate the synaptic turnover stages in AD. Current studies will help to understand the synaptic life-cycle and role of miRNAs in each stage that is deregulated in AD, further allowing for a better understanding of the pathogenesis of devastating disease.

Role of fragile X messenger ribonucleoprotein 1 in the pathophysiology of brain disorders: a glia perspective

Fragile X messenger ribonucleoprotein 1 (FMRP) is a widely expressed RNA binding protein involved in several steps of mRNA metabolism. Mutations in the FMR1 gene encoding FMRP are responsible for fragile X syndrome (FXS), a leading genetic cause of intellectual disability and autism spectrum disorder, and fragile X-associated tremor-ataxia syndrome (FXTAS), a neurodegenerative disorder in aging men. Although FMRP is mainly expressed in neurons, it is also present in glial cells and its deficiency or altered expression can affect functions of glial cells with implications for the pathophysiology of brain disorders. The present review focuses on recent advances on the role of glial subtypes, astrocytes, oligodendrocytes and microglia, in the pathophysiology of FXS and FXTAS, and describes how the absence or reduced expression of FMRP in these cells can impact on glial and neuronal functions. We will also briefly address the role of FMRP in radial glial cells and its effects on neural development, and gliomas and will speculate on the role of glial FMRP in other brain disorders.

Translational modulator ISRIB alleviates synaptic and behavioral phenotypes in Fragile X syndrome

Fragile X syndrome (FXS) is caused by the loss of fragile X messenger ribonucleoprotein (FMRP), a translational regulator that binds the transcripts of proteins involved in synaptic function and plasticity. Dysregulated protein synthesis is a central effect of FMRP loss, however, direct translational modulation has not been leveraged in the treatment of FXS. Thus, we examined the effect of the translational modulator integrated stress response inhibitor (ISRIB) in treating synaptic and behavioral symptoms of FXS. We show that FMRP loss dysregulates synaptic protein abundance, stabilizing dendritic spines through increased PSD-95 levels while preventing spine maturation through reduced glutamate receptor accumulation, thus leading to the formation of dense, immature dendritic spines, characteristic of FXS patients and Fmr1 knockout (KO) mice. ISRIB rescues these deficits and improves social recognition in Fmr1 KO mice. These findings highlight the therapeutic potential of targeting core translational mechanisms in FXS and neurodevelopmental disorders more broadly.

KIF5B plays important roles in dendritic spine plasticity and dendritic localization of PSD95 and FMRP in the mouse cortex in vivo

Kinesin 1 (KIF5) is one major type of motor protein in neurons, but its members' function in the intact brain remains less studied. Using in vivo two-photon imaging, we find that conditional knockout of Kif5b (KIF5B cKO) in CaMKIIα-Cre-expressing neurons shows heightened turnover and lower stability of dendritic spines in layer 2/3 pyramidal neurons with reduced spine postsynaptic density protein 95 acquisition in the mouse cortex. Furthermore, the RNA-binding protein fragile X mental retardation protein (FMRP) is translocated to the proximity of newly formed spines several hours before the spine formation events in vivo in control mice, but this preceding transport of FMRP is abolished in KIF5B cKO mice. We further find that FMRP is localized closer to newly formed spines after fear extinction, but this learning-dependent localization is disrupted in KIF5B cKO mice. Our findings provide the crucial in vivo evidence that KIF5B is involved in the dendritic targeting of synaptic proteins that underlies dendritic spine plasticity.

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.

The multifaceted role of Fragile X-Related Protein 1 (FXR1) in cellular processes: an updated review on cancer and clinical applications

RNA-binding proteins (RBPs) modulate the expression level of several target RNAs (such as mRNAs) post-transcriptionally through interactions with unique binding sites in the 3'-untranslated region. There is mounting information that suggests RBP dysregulation plays a significant role in carcinogenesis. However, the function of FMR1 autosomal homolog 1(FXR1) in malignancies is just beginning to be unveiled. Due to the diversity of their RNA-binding domains and functional adaptability, FXR1 can regulate diverse transcript processing. Changes in FXR1 interaction with RNA networks have been linked to the emergence of cancer, although the theoretical framework defining these alterations in interaction is insufficient. Alteration in FXR1 expression or localization has been linked to the mRNAs of cancer suppressor genes, cancer-causing genes, and genes involved in genomic expression stability. In particular, FXR1-mediated gene regulation involves in several cellular phenomena related to cancer growth, metastasis, epithelial-mesenchymal transition, senescence, apoptosis, and angiogenesis. FXR1 dysregulation has been implicated in diverse cancer types, suggesting its diagnostic and therapeutic potential. However, the molecular mechanisms and biological effects of FXR1 regulation in cancer have yet to be understood. This review highlights the current knowledge of FXR1 expression and function in various cancer situations, emphasizing its functional variety and complexity. We further address the challenges and opportunities of targeting FXR1 for cancer diagnosis and treatment and propose future directions for FXR1 research in oncology. This work intends to provide an in-depth review of FXR1 as an emerging oncotarget with multiple roles and implications in cancer biology and therapy.

Unraveling the role of miRNAs in the diagnosis, progression, and therapeutic intervention of Alzheimer's disease

Alzheimer's disease (AD) is a multifaceted, advancing neurodegenerative illness that is responsible for most cases of neurological impairment and dementia in the aged population. As the disease progresses, affected individuals may experience cognitive decline, linguistic problems, affective instability, and behavioral changes. The intricate nature of AD reflects the altered molecular mechanisms participating in the affected human brain. MicroRNAs (miRNAs, miR) are essential for the intricate control of gene expression in neurobiology. miRNAs exert their influence by modulating the transcriptome of brain cells, which typically exhibit substantial genetic activity, encompassing gene transcription and mRNA production. Presently, comprehensive studies are being conducted on AD to identify miRNA-based signatures that are indicative of the disease pathophysiology. These findings can contribute to the advancement of our understanding of the mechanisms underlying this disorder and can inform the development of therapeutic interventions based on miRNA and related RNA molecules. Therefore, this comprehensive review provides a detailed holistic analysis of the latest advances discussing the emerging role of miRNAs in the progression of AD and their possible application as potential biomarkers and targets for therapeutic interventions in future studies.

Overview: Research on the Genetic Architecture of the Developing Cerebral Cortex in Norms and Diseases

The human brain is characterized by high cell numbers, diverse cell types with diverse functions, and intricate connectivity with an exceedingly broad surface of the cortex. Human-specific brain development was accomplished by a long timeline for maturation from the prenatal period to the third decade of life. The long timeline makes complicated architecture and circuits of human cerebral cortex possible, and it makes human brain vulnerable to intrinsic and extrinsic insults resulting in the development of variety of neuropsychiatric disorders. Unraveling the molecular and cellular processes underlying human brain development under the elaborate regulation of gene expression in a spatiotemporally specific manner, especially that of the cortex will provide a biological understanding of human cognition and behavior in health and diseases. Global research consortia and the advancing technologies in brain science including functional genomics equipped with emergent neuroinformatics such as single-cell multiomics, novel human models, and high-volume databases with high-throughput computation facilitate the biological understanding of the development of the human brain cortex. Knowing the process of interplay of the genome and the environment in cortex development will lead us to understand the human-specific cognitive function and its individual diversity. Thus, it is worthwhile to overview the recent progress in neurotechnology to foresee further understanding of the human brain and norms and diseases.

Utilities of Isolated Nerve Terminals in Ex Vivo Analyses of Protein Translation in (Patho)physiological Brain States: Focus on Alzheimer's Disease

Synapses are the cellular substrates of higher-order brain functions, and their dysfunction is an early and primary pathogenic mechanism across several neurological disorders. In particular, Alzheimer's disease (AD) is categorized by prodromal structural and functional synaptic deficits, prior to the advent of classical behavioral and pathological features. Recent research has shown that the development, maintenance, and plasticity of synapses depend on localized protein translation. Synaptosomes and synaptoneurosomes are biochemically isolated synaptic terminal preparations which have long been used to examine a variety of synaptic processes ex vivo in both healthy and pathological conditions. These ex vivo preparations preserve the mRNA species and the protein translational machinery. Hence, they are excellent in organello tools for the study of alterations in mRNA levels and protein translation in neuropathologies. Evaluation of synapse-specific basal and activity-driven de novo protein translation activity can be conveniently performed in synaptosomal/synaptoneurosomal preparations from both rodent and human brain tissue samples. This review gives a quick overview of the methods for isolating synaptosomes and synaptoneurosomes before discussing the studies that have utilized these preparations to study localized synapse-specific protein translation in (patho)physiological situations, with an emphasis on AD. While the review is not an exhaustive accumulation of all the studies evaluating synaptic protein translation using the synaptosomal model, the aim is to assemble the most relevant studies that have done so. The hope is to provide a suitable research platform to aid neuroscientists to utilize the synaptosomal/synaptoneurosomal models to evaluate the molecular mechanisms of synaptic dysfunction within the specific confines of mRNA localization and protein translation research.

Mimicking hypomethylation of FUS requires liquid-liquid phase separation to induce synaptic dysfunctions

The hypomethylation of fused in sarcoma (FUS) in frontotemporal lobar degeneration promotes the formation of irreversible condensates of FUS. However, the mechanisms by which these hypomethylated FUS condensates cause neuronal dysfunction are unknown. Here we report that expression of FUS constructs mimicking hypomethylated FUS causes aberrant dendritic FUS condensates in CA1 neurons. These hypomethylated FUS condensates exhibit spontaneous, and activity induced movement within the dendrite. They impair excitatory synaptic transmission, postsynaptic density-95 expression, and dendritic spine plasticity. These neurophysiological defects are dependent upon both the dendritic localisation of the condensates, and their ability to undergo liquid-liquid phase separation. These results indicate that the irreversible liquid-liquid phase separation is a key component of hypomethylated FUS pathophysiology in sporadic FTLD, and this can cause synapse dysfunction in sporadic FTLD.

Phosphorylation alters FMRP granules and determines their transport or protein synthesis abilities

Fragile X messenger ribonucleoprotein (FMRP) is an RNA-binding protein implicated in autism that suppresses translation and forms granules. While FMRP function has been well-studied, how phosphorylation regulates granule binding and function remains limited. Here, we found that Fragile X patient-derived I304N mutant FMRP could not stably bind granules, underscoring the essential nature of FMRP granule association for function. Next, phosphorylation on serine 499 (S499) led to differences in puncta size, intensity, contrast, and transport as shown by phospho-deficient (S499A) and phospho-mimic (S499D) mutant FMRP granules. Additionally, S499D exchanged slowly on granules relative to S499A, suggesting that phosphorylated FMRP can attenuate translation. Furthermore, the S499A mutant enhanced translation in presynaptic boutons of the mouse hippocampus. Thus, the phospho-state of FMRP altered the structure of individual granules with changes in transport and translation to achieve spatiotemporal regulation of local protein synthesis.

Teaser

The phosphorylation-state of S499 on FMRP can change FMRP granule structure and function to facilitate processive transport or local protein synthesis.

How dendritic spines shape is determined by MMP-9 activity in FXS

Matrix metalloproteinase-9 (MMP-9) belongs to the family of endopeptidases expressed in neurons and secreted at the synapse in response to neuronal activity. It regulates the pericellular environment by cleaving its protein components. MMP9 is involved in activity-dependent reorganization of spine architecture. In the mouse model of fragile X syndrome (FXS), the most common inherited intellectual disability and the most common single-gene cause of autism, increased synaptic expression of MMP-9 is responsible for the observed dendritic spine abnormalities. In this chapter, I summarize the current data on the molecular regulatory pathways responsible for synaptic MMP-9 expression and discuss the fact that MMP-9 is extracellularly localized, making it a particularly attractive potential target for therapeutic pharmacological intervention in FXS.

Age-Dependent Regulation of Dendritic Spine Density and Protein Expression in Mir324 KO Mice

Dendritic spines are small, dynamic protrusions along the dendrite that comprise more than 90% of excitatory connections in the brain, making them essential sites for neuronal communication. These synaptic sites change throughout the process of development, reducing in density and shifting morphology as synapses are refined. One important class of dendritic spine regulators is microRNA (miRNA), small-noncoding RNAs that post-transcriptionally regulate gene expression. Several studies suggest that miRNA-324-5p regulates dendritic spine formation. In addition, we have previously shown that miR-324-5p plays a role in seizure and long-term potentiation, both of which involve dendritic spine changes. In this study, we aimed to characterize the role of miRNA-324-5p in developmental spine regulation by assessing the effect of Mir324 knockout (KO) on dendritic spine density and expression of a subset of dendritic proteins at select developmental time points. We show that miR-324-5p expression is developmentally regulated and peaks at 4 weeks of age. We demonstrate that loss of miR-324-5p expression leads to differential changes in both target protein expression and spine density at different time points during development, disrupting the pattern of spine density changes and leading to a premature loss of dendritic spines in KO mice, which is compensated later. Our findings indicate that miR-324-5p plays a role in synaptic refinement across development. Additionally, our data illustrate the importance of context in the study of miRNA, as regulation by and/or of miRNA can vary dramatically across development and in disease.

The potential role of ribonucleic acid methylation in the pathological mechanisms of fragile X syndrome

Fragile X syndrome (FXS) is a common inherited cause of intellectual disabilities and single-gene cause of autism spectrum disorder (ASD), resulting from the loss of functional fragile X messenger ribonucleoprotein (FMRP), an RNA-binding protein (RBP) encoded by the fragile X messenger ribonucleoprotein 1 (FMR1) gene. Ribonucleic acid (RNA) methylation can lead to developmental diseases, including FXS, through various mechanisms mediated by 5-hydroxymethylcytosine, 5-methylcytosine, N6-methyladenosine, etc. Emerging evidence suggests that modifications of some RNA species have been linked to FXS. However, the underlying pathological mechanism has yet to be elucidated. In this review, we reviewed the implication of RNA modification in FXS and summarized its specific characteristics for facilitating the identification of new therapeutic targets.

Age-dependent regulation of dendritic spine density and protein expression in Mir324 KO mice

Dendritic spines are small, dynamic protrusions along the dendrite that comprise more than 90% of excitatory connections in the brain, making them essential sites for neuronal communication. These synaptic sites change throughout the process of development, reducing in density and shifting morphology as synapses are refined. One important class of dendritic spine regulators is microRNA (miRNA), small noncoding RNAs that post-transcriptionally regulate gene expression. Several studies suggest that miRNA-324-5p regulates dendritic spine formation. In addition, we have previously shown that miR-324-5p plays a role in seizure and long-term potentiation, both of which involve dendritic spine changes. In this study, we aimed to characterize the role of miRNA-324-5p in developmental spine regulation by assessing the effect of Mir324 knockout (KO) on dendritic spine density and expression of a subset of dendritic proteins at select developmental time points. We show that miR-324-5p expression is developmentally regulated and peaks at four weeks of age. We demonstrate that loss of miR-324-5p expression leads to differential changes in both target protein expression and spine density at different time points during development, disrupting the pattern of spine density changes and leading to a premature loss of dendritic spines in KO mice, which is compensated later. Our findings indicate that miR-324-5p plays a role in synaptic refinement across development. Additionally, our data illustrate the importance of context in the study of miRNA, as regulation by and/or of miRNA can vary dramatically across development and in disease.

Massively parallel identification of mRNA localization elements in primary cortical neurons

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

Adaptive control of synaptic plasticity integrates micro- and macroscopic network function

Synaptic plasticity configures interactions between neurons and is therefore likely to be a primary driver of behavioral learning and development. How this microscopic-macroscopic interaction occurs is poorly understood, as researchers frequently examine models within particular ranges of abstraction and scale. Computational neuroscience and machine learning models offer theoretically powerful analyses of plasticity in neural networks, but results are often siloed and only coarsely linked to biology. In this review, we examine connections between these areas, asking how network computations change as a function of diverse features of plasticity and vice versa. We review how plasticity can be controlled at synapses by calcium dynamics and neuromodulatory signals, the manifestation of these changes in networks, and their impacts in specialized circuits. We conclude that metaplasticity-defined broadly as the adaptive control of plasticity-forges connections across scales by governing what groups of synapses can and can't learn about, when, and to what ends. The metaplasticity we discuss acts by co-opting Hebbian mechanisms, shifting network properties, and routing activity within and across brain systems. Asking how these operations can go awry should also be useful for understanding pathology, which we address in the context of autism, schizophrenia and Parkinson's disease.

The RGG motif proteins: Interactions, functions, and regulations

Proteins with motifs rich in arginines and glycines were discovered decades ago and are functionally involved in a staggering range of essential processes in the cell. Versatile, specific, yet adaptable molecular interactions enabled by the unique combination of arginine and glycine, combined with multiplicity of molecular recognition conferred by repeated di-, tri-, and multiple peptide motifs, allow RGG motif proteins to interact with a broad range of proteins and nucleic acids. Furthermore, posttranslational modifications at the arginines in the motif extend the RGG protein's capacity for a fine-tuned regulation. In this review, we focus on the biochemical properties of the RGG motif, its molecular interactions with RNAs and proteins, and roles of the posttranslational modification in modulating their interactions. We discuss current knowledge of the RGG motif proteins involved in mRNA transport and translation, highlight our merging understanding of their molecular functions in translational regulation and summarize areas of research in the future critical in understanding this important family of proteins. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Mechanisms.

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.

Function of FMRP Domains in Regulating Distinct Roles of Neuronal Protein Synthesis

The Fragile-X Mental Retardation Protein (FMRP) is an RNA binding protein that regulates translation of mRNAs essential for synaptic development and plasticity. FMRP interacts with a specific set of mRNAs, aids in their microtubule-dependent transport and regulates their translation through its association with ribosomes. However, the biochemical role of FMRP's domains in forming neuronal granules and associating with microtubules and ribosomes is currently undefined. We report that the C-terminus domain of FMRP is sufficient to bind to ribosomes akin to the full-length protein. Furthermore, the C-terminus domain alone is essential and responsible for FMRP-mediated neuronal translation repression. However, dendritic distribution of FMRP and its microtubule association is favored by the synergistic combination of FMRP domains rather than individual domains. Interestingly, we show that the phosphorylation of hFMRP at Serine-500 is important in modulating the dynamics of translation by controlling ribosome association. This is a fundamental mechanism governing the size and number of FMRP puncta that contain actively translating ribosomes. Finally through the use of pathogenic mutations, we emphasize the hierarchical contribution of FMRP's domains in translation regulation.

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

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

Development of single-molecule ubiquitination mediated fluorescence complementation to visualize protein ubiquitination dynamics in dendrites

The ubiquitination/proteasome system is important for the spatiotemporal control of protein synthesis and degradation at synapses, while dysregulation may underlie autism spectrum disorders (ASDs). However, methods allowing direct visualization of the subcellular localization and temporal dynamics of protein ubiquitination are lacking. Here we report the development of Single-Molecule Ubiquitin Mediated Fluorescence Complementation (SM-UbFC) as a method to visualize and quantify the dynamics of protein ubiquitination in dendrites of live neurons in culture. Using SM-UbFC, we demonstrate that the rate of PSD-95 ubiquitination is elevated in dendrites of FMR1 KO neurons compared with wild-type controls. We further demonstrate the rapid ubiquitination of the fragile X messenger ribonucleoprotein, FMRP, and the AMPA receptor subunit, GluA1, which are known to be key events in the regulation of synaptic protein synthesis and plasticity. SM-UbFC will be useful for future studies on the regulation of synaptic protein homeostasis.

Therapeutic Implications of microRNAs in Depressive Disorders: A Review

MicroRNAs are hidden players in complex psychophysical phenomena such as depression and anxiety related disorders though the activation and deactivation of multiple proteins in signaling cascades. Depression is classified as a mood disorder and described as feelings of sadness, loss, or anger that interfere with a person’s everyday activities. In this review, we have focused on exploration of the significant role of miRNAs in depression by affecting associated target proteins (cellular and synaptic) and their signaling pathways which can be controlled by the attachment of miRNAs at transcriptional and translational levels. Moreover, miRNAs have potential role as biomarkers and may help to cure depression through involvement and interactions with multiple pharmacological and physiological therapies. Taken together, miRNAs might be considered as promising novel therapy targets themselves and may interfere with currently available antidepressant treatments.

Increased Serum Levels of miR-125b and miR-132 in Fragile X Syndrome: A Preliminary Study

Background and Objectives

Fragile X syndrome (FXS) is a neurodevelopmental disorder, identified as the most common cause of hereditary intellectual disability and monogenic cause of autism spectrum disorders (ASDs), caused by the loss of fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein, a regulator of translation that plays an important role in neurodevelopment, and its loss causes cognitive and behavioral deficits. MicroRNAs (miRNAs) are small molecules that regulate gene expression in diverse biological processes. Previous studies found that the interaction of FMRP with miR-125b and miR-132 regulates the maturation and synaptic plasticity in animal models and miRNA dysregulation plays a role in the pathophysiology of FXS. The present study aimed to analyze the expression of miR-125b-5p and miR-132-3p in the serum of patients with FXS.

Methods

The expressions of circulating miRNAs were studied in the serum of 10 patients with FXS and 20 controls using the real-time quantitative retrotranscribed method analyzed by relative quantification. Receiver operating characteristic (ROC) curves and the area under the ROC curve (AUC) were generated to assess the diagnostic values of the miRNAs.

Results

We found that both miR-125b and miR-132 were increased in the serum of patients with FXS compared with controls and likely involved with FMRP loss. The AUC (95% confidence interval) of miR-125b and miR-132 was 0.94 (0.86–1.0) and 0.89 (0.77–1.0), respectively. Databases allowed for the identification of possible target genes for miR-125b and miR-132, whose products play an important role in the homeostasis of the nervous system.

Discussion

Our results indicate that serum miR-125b and miR-132 may serve as potential biomarkers for FXS. The increased expression of circulating miR-125b and miR-132 seems to be associated with the genotype of FXS. Predicted gene targets of the differentially regulated miRNAs are involved in cognitive performance and ASD phenotype.

Classification of Evidence

This study provides Class III evidence that miR-125b and miR-132 distinguish men with FXS from normal controls.

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.

Presynaptic FMRP and local protein synthesis support structural and functional plasticity of glutamatergic axon terminals

Learning and memory rely on long-lasting, synapse-specific modifications. Although postsynaptic forms of plasticity typically require local protein synthesis, whether and how local protein synthesis contributes to presynaptic changes remain unclear. Here, we examined the mouse hippocampal mossy fiber (MF)-CA3 synapse, which expresses both structural and functional presynaptic plasticity and contains presynaptic fragile X messenger ribonucleoprotein (FMRP), an RNA-binding protein involved in postsynaptic protein-synthesis-dependent plasticity. We report that MF boutons contain ribosomes and synthesize protein locally. The long-term potentiation of MF-CA3 synaptic transmission (MF-LTP) was associated with the translation-dependent enlargement of MF boutons. Remarkably, increasing in vitro or in vivo MF activity enhanced the protein synthesis in MFs. Moreover, the deletion of presynaptic FMRP blocked structural and functional MF-LTP, suggesting that FMRP is a critical regulator of presynaptic MF plasticity. Thus, presynaptic FMRP and protein synthesis dynamically control presynaptic structure and function in the mature mammalian brain.

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.

RNA and neuronal function: the importance of post-transcriptional regulation

The brain represents an organ with a particularly high diversity of genes that undergo post-transcriptional gene regulation through multiple mechanisms that affect RNA metabolism and, consequently, brain function. This vast regulatory process in the brain allows for a tight spatiotemporal control over protein expression, a necessary factor due to the unique morphologies of neurons. The numerous mechanisms of post-transcriptional regulation or translational control of gene expression in the brain include alternative splicing, RNA editing, mRNA stability and transport. A large number of trans-elements such as RNA-binding proteins and micro RNAs bind to specific cis-elements on transcripts to dictate the fate of mRNAs including its stability, localization, activation and degradation. Several trans-elements are exemplary regulators of translation, employing multiple cofactors and regulatory machinery so as to influence mRNA fate. Networks of regulatory trans-elements exert control over key neuronal processes such as neurogenesis, synaptic transmission and plasticity. Perturbations in these networks may directly or indirectly cause neuropsychiatric and neurodegenerative disorders. We will be reviewing multiple mechanisms of gene regulation by trans-elements occurring specifically in neurons.

The Drosophila hnRNP F/H homolog Glorund recruits dFMRP to inhibit nanos translation elongation

Translational control of maternal mRNAs generates spatial and temporal patterns of protein expression necessary to begin animal development. Translational repression of unlocalized nanos (nos) mRNA in late-stage Drosophila oocytes by the hnRNP F/H homolog, Glorund (Glo), is important for embryonic body patterning. While previous work has suggested that repression occurs at both the translation initiation and elongation phases, the molecular mechanism by which Glo regulates nos translation remains elusive. Here, we have identified the Drosophila fragile X mental retardation protein, dFMRP, as a Glo interaction partner with links to the translational machinery. Using an oocyte-based in vitro translation system, we confirmed that Glo regulates both initiation and elongation of a nos translational reporter and showed that dFMRP specifically represses translation elongation and promotes ribosome stalling. Furthermore, we combined mutational analysis and in vivo and in vitro binding assays to show that Glo's qRRM2 domain specifically and directly interacts with dFMRP. Our findings suggest that Glo regulates nos translation elongation by recruiting dFMRP and that Glo's RNA-binding domains can also function as protein-protein interaction interfaces critical for its regulatory functions. Additionally, they reveal a mechanism for targeting dFMRP to specific transcripts.

The role of altered translation in intellectual disability and epilepsy

A very high proportion of cases of intellectual disability are genetic in origin and are associated with the occurrence of epileptic seizures during childhood. These two disorders together effect more than 5% of the world's population. One feature linking the two diseases is that learning and memory require the synthesis of new synaptic components and ion channels, while maintenance of overall excitability also requires synthesis of similar proteins in response to altered neuronal stimulation. Many of these disorders result from mutations in proteins that regulate mRNA processing, translation initiation, translation elongation, mRNA stability or upstream translation modulators. One theme that emerges on reviewing this field is that mutations in proteins that regulate changes in translation following neuronal stimulation are more likely to result in epilepsy with intellectual disability than general translation regulators with no known role in activity-dependent changes. This is consistent with the notion that activity-dependent translation in neurons differs from that in other cells types in that the changes in local cellular composition, morphology and connectivity that occur generally in response to stimuli are directly coupled to local synaptic activity and persist for months or years after the original stimulus.

Fragile X Mental Retardation Protein Mediates the Effects of Androgen on Hippocampal PSD95 Expression and Dendritic Spines Density/Morphology and Autism-Like Behaviors Through miR-125a

Dysregulated synaptic plasticity is a key feature of neurodevelopmental disorders, including autism. This study investigated whether Fragile X mental retardation protein (FMRP), a selective RNA-binding protein that regulates synaptic protein expression by interacting with miRNAs, mediates the effects of androgens that play an important role in regulating the synaptic plasticity in the hippocampus. Experiments using mouse hippocampal neuron HT22 cells demonstrated that dihydrotestosterone (DHT) increased the expression of postsynaptic density protein 95 (PSD95) by inhibiting FMRP expression. Administration of miR-125a inhibitor upregulated the PSD95 expression and significantly increased the DHT-induced upregulation of PSD95. FMRP knockdown in HT22 cells reduced the expression of miR-125a. Moreover, miR-125a inhibitor upregulated the PSD95 expression in the DHT-treated HT22 cells with FMRP knockdown. Subsequently, the effects of androgen-mediated via FMRP in regulating neural behaviors and PSD95 expression and dendritic spines density/morphology were investigated using Fmr1 knockout (KO) and wild-type littermate (WT) mice. The castration of WT mice reduced the androgen levels, aggravated anxiety and depression, and impaired learning and memory and sociability of mice. DHT supplementation post-castration reversed the alterations in density and maturity of dendritic spines of hippocampal neurons and behavioral disorders in WT mice; however, it did not reveal such effects in Fmr1 KO mice. Further, immunohistochemical staining and western blotting analyses after knocking down miR-125a revealed similar effects of castration and post-castration DHT supplementation on PSD95 protein expression. These findings clarified that FMRP mediated the effects of DHT through miR-125a in regulating the expression of hippocampal synaptic protein PSD95. This study provides evidence for the neuroprotective mechanism of androgen in PSD95 expression and dendritic spines density/morphology and suggests that treatment interventions with androgen could be helpful for the management of synaptic plasticity disorders.

A Proposed Role for Interactions between Argonautes, miRISC, and RNA Binding Proteins in the Regulation of Local Translation in Neurons and Glia

The first evidence of local translation in the CNS appeared nearly 40 years ago, when electron microscopic studies showed polyribosomes localized to the base of dendritic spines. Since then, local translation has been established as an important regulatory mechanism for gene expression in polarized or functionally compartmentalized cells. While much attention has been placed on characterizing the local transcriptome and regulatory "grammar" directing mRNA localization in neurons and glia, less is understood about how these cells subsequently de-repress mRNA translation in their peripheral processes to produce a rapid translational response to stimuli. MicroRNA-mediated translation regulation offers a possible solution to this question. Not only do miRNAs provide the specificity needed for targeted gene regulation, but association and dynamic interactions between Argonaute (AGO) with sequence-specific RNA-binding proteins may provide a molecular switch to allow for de-repression of target mRNAs. Here, we review the expression and activity of different AGO proteins in miRNA-induced silencing complexes in neurons and glia and discuss known pathways of miRNA-mediated regulation, including activity-dependent pre-miRNA maturation in dendrites. We further detail work on AGO and RNA-binding protein interactions that allow for the reversal of miRNA-mediated translational silencing, and we propose a model for how intercellular communication may play a role in the regulation of local translation.

Schizophrenia Risk Mediated by microRNA Target Genes Overlapped by Genome-Wide Rare Copy Number Variation in 22q11.2 Deletion Syndrome

The 22q11.2 deletion is associated with >20-fold increased risk for schizophrenia. The presence of gene DGCR8 in the 22q11.2 deletion region has suggested microRNA (miRNA) dysregulation as possibly contributing to this risk. We therefore investigated the role of miRNA target genes in the context of previously identified genome-wide risk for schizophrenia conveyed by additional copy number variation (CNV) in 22q11.2 deletion syndrome (22q11.2DS). Using a cohort of individuals with 22q11.2DS and documented additional rare CNVs overlapping protein coding genes, we compared those with schizophrenia (n = 100) to those with no psychotic illness (n = 118), assessing for rare CNVs that overlapped experimentally supported miRNA target genes. We further characterized the contributing miRNA target genes using gene set enrichment analyses and identified the miRNAs most implicated. Consistent with our hypothesis, we found a significantly higher proportion of individuals in the schizophrenia than in the non-psychotic group to have an additional rare CNV that overlapped one or more miRNA target genes (odds ratio = 2.12, p = 0.0138). Gene set analyses identified an enrichment of FMRP targets and genes involved in nervous system development and postsynaptic density amongst these miRNA target genes in the schizophrenia group. The miRNAs most implicated included miR-17-5p, miR-34a-5p and miR-124-3p. These results provide initial correlational evidence in support of a possible role for miRNA perturbation involving genes affected by rare genome-wide CNVs in the elevated risk for schizophrenia in 22q11.2DS, consistent with the multi-hit and multi-layered genetic mechanisms implicated in this and other forms of schizophrenia.

Identification of microRNAs associated with human fragile X syndrome using next-generation sequencing

Fragile X syndrome (FXS) is caused by a mutation in the FMR1 gene which can lead to a loss or shortage of the FMR1 protein. This protein interacts with specific miRNAs and can cause a range of neurological disorders. Therefore, miRNAs could act as a novel class of biomarkers for common CNS diseases. This study aimed to test this theory by exploring the expression profiles of various miRNAs in Iranian using deep sequencing-based technologies and validating the miRNAs affecting the expression of the FMR1 gene. Blood samples were taken from 15 patients with FXS (9 males, 6 females) and 12 controls. 25 miRNAs were differentially expressed in individuals with FXS compared to controls. Levels of 9 miRNAs were found to be significantly changed (3 upregulated and 6 downregulated). In Patients, the levels of hsa-miR-532-5p, hsa-miR-652-3p and hsa-miR-4797-3p were significantly upregulated while levels of hsa-miR-191-5p, hsa-miR-181-5p, hsa-miR-26a-5p, hsa-miR-30e-5p, hsa-miR-186-5p, and hsa-miR-4797-5p exhibited significant downregulation; and these dysregulations were confirmed by RT-qPCR. This study presents among the first evidence of altered miRNA expression in blood samples from patients with FXS, which could be used for diagnostic, prognostic, and treatment purposes. Larger studies are required to confirm these preliminary results.

Silencing of Ago-2 Interacting Protein SERBP1 Relieves KCC2 Repression by miR-92 in Neurons

RNA-binding proteins (RBPs) play important roles in modulating miRNA-mediated mRNA target repression. Argonaute2 (Ago2) is an essential component of the RNA-induced silencing complex (RISC) that plays a central role in silencing mechanisms via small non-coding RNA molecules known as siRNAs and miRNAs. Small RNAs loaded into Argonaute proteins catalyze endoribonucleolytic cleavage of target RNAs or recruit factors responsible for translational silencing and mRNA target destabilization. In previous studies we have shown that KCC2, a neuronal Cl (-) extruding K (+) Cl (-) co-transporter 2, is regulated by miR-92 in neuronal cells. Searching for Ago2 partners by immunoprecipitation and LC-MS/MS analysis, we isolated among other proteins the Serpine mRNA binding protein 1 (SERBP1) from SH-SY5Y neuroblastoma cells. Exploring the role of SERBP1 in miRNA-mediated gene silencing in SH-SY5Y cells and primary hippocampal neurons, we demonstrated that SERBP1 silencing regulates KCC2 expression through the 3' untranslated region (UTR). In addition, we found that SERBP1 as well as Ago2/miR-92 complex bind to KCC2 3'UTR. Finally, we demonstrated the attenuation of miR-92-mediated repression of KCC2 3'UTR by SERBP1 silencing. These findings advance our knowledge regarding the miR-92-mediated modulation of KCC2 translation in neuronal cells and highlight SERBP1 as a key component of this gene regulation.

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.

MicroRNAs as Biomarkers for Birth Defects

It is estimated that 2-4% of live births will have a birth defect (BD). The availability of biomarkers for the prenatal detection of BDs will facilitate early risk assessment, prompt medical intervention and ameliorating disease severity. miRNA expression levels are often found to be altered in many diseases. There is, thus, a growing interest in determining whether miRNAs, particularly extracellular miRNAs, can predict, diagnose, or monitor BDs. These miRNAs, typically encapsulated in exosomes, are released by cells (including those of the fetus and placenta) into the extracellular milieu, such as blood, urine, saliva and cerebrospinal fluid, thereby enabling interaction with target cells. Exosomal miRNAs are stable, protected from degradation, and retain functionality. The observation that placental and fetal miRNAs can be detected in maternal serum, provides a strong rationale for adopting miRNAs as noninvasive prenatal biomarkers for BDs. In this mini-review, we examine the current state of research involving the use of miRNAs as prognostic and diagnostic biomarkers for BD.

Rett Syndrome and Fragile X Syndrome: Different Etiology With Common Molecular Dysfunctions

Rett syndrome (RTT) and Fragile X syndrome (FXS) are two monogenetic neurodevelopmental disorders with complex clinical presentations. RTT is caused by mutations in the Methyl-CpG binding protein 2 gene (MECP2) altering the function of its protein product MeCP2. MeCP2 modulates gene expression by binding methylated CpG dinucleotides, and by interacting with transcription factors. FXS is caused by the silencing of the FMR1 gene encoding the Fragile X Mental Retardation Protein (FMRP), a RNA binding protein involved in multiple steps of RNA metabolism, and modulating the translation of thousands of proteins including a large set of synaptic proteins. Despite differences in genetic etiology, there are overlapping features in RTT and FXS, possibly due to interactions between MeCP2 and FMRP, and to the regulation of pathways resulting in dysregulation of common molecular signaling. Furthermore, basic physiological mechanisms are regulated by these proteins and might concur to the pathophysiology of both syndromes. Considering that RTT and FXS are disorders affecting brain development, and that most of the common targets of MeCP2 and FMRP are involved in brain activity, we discuss the mechanisms of synaptic function and plasticity altered in RTT and FXS, and we consider the similarities and the differences between these two disorders.

Epitranscriptomic regulation of cognitive development and decline

Functional genomics and systems biology have opened new doors to previously inaccessible genomic information and holistic approaches to study complex networks of genes and proteins in the central nervous system. The advances are revolutionizing our understanding of the genetic underpinning of cognitive development and decline by facilitating identifications of novel molecular regulators and physiological pathways underlying brain function, and by associating polymorphism and mutations to cognitive dysfunction and neurological diseases. However, our current understanding of these complex gene regulatory mechanisms has yet lacked sufficient mechanistic resolution for further translational breakthroughs. Here we review recent findings from the burgeoning field of epitranscriptomics in association of cognitive functions with a special focus on the epitranscritomic regulation in subcellular locations such as chromosome, synapse, and mitochondria. Although there are important gaps in knowledge, current evidence is suggesting that this layer of RNA regulation may be of particular interest for the spatiotemporally coordinated regulation of gene networks in developing and maintaining brain function that underlie cognitive changes.

Regulation and different functions of the animal microRNA-induced silencing complex

Among the different types of small RNAs, microRNAs (miRNAs) are key players in controlling gene expression at the mRNA level. To be active, they must associate with an Argonaute protein to form the miRNA induced silencing complex (miRISC) and binds to specific mRNA through complementarity sequences. The miRISC binding to an mRNA can lead to multiple outcomes, the most frequent being inhibition of the translation and/or deadenylation followed by decapping and mRNA decay. In the last years, several studies described different mechanisms modulating miRISC functions in animals. For instance, the regulation of the Argonaute protein through post-translational modifications can change the miRISC gene regulatory activity as well as modulate its binding to proteins, mRNA targets and miRISC stability. Furthermore, the presence of RNA binding proteins and multiple miRISCs at the targeted mRNA 3' untranslated region (3'UTR) can also affect its function through cooperation or competition mechanisms, underlying the importance of the 3'UTR environment in miRNA-mediated repression. Another way to regulate the miRISC function is by modulation of its interactors, forming different types of miRNA silencing complexes that affect gene regulation differently. It is also reported that the subcellular localization of several components of the miRNA pathway can modulate miRISC function, suggesting an important role for vesicular trafficking in the regulation of this essential silencing complex. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes Regulatory RNAs/RNAi/Riboswitches > RNAi: Mechanisms of Action Regulatory RNAs/RNAi/Riboswitches > Biogenesis of Effector Small RNAs.

Protein Binding to Cis-Motifs in mRNAs Coding Sequence Is Common and Regulates Transcript Stability and the Rate of Translation

Protein binding to the non-coding regions of mRNAs is relatively well characterized and its functionality has been described in many examples. New results obtained by high-throughput methods indicate that binding to the coding sequence (CDS) by RNA-binding proteins is also quite common, but the functions thereof are more obscure. As described in this review, CDS binding has a role in the regulation of mRNA stability, but it has also a more intriguing role in the regulation of translational efficiency. Global approaches, which suggest the significance of CDS binding along with specific examples of CDS-binding RBPs and their modes of action, are outlined here, pointing to the existence of a relatively less-known regulatory network controlling mRNA stability and translation on yet another level.

Targeting Epigenetic Mechanisms to Treat Alcohol Use Disorders (AUD)

BACKGROUND

The impact of abusive alcohol consumption on human health is remarkable. According to the World Health Organization (WHO), approximately 3.3 million people die annually because of harmful alcohol consumption (the figure represents around 5.9% of global deaths). Alcohol Use Disorder (AUD) is a chronic disease where individuals exhibit compulsive alcohol drinking and present negative emotional states when they do not drink. In the most severe manifestations of AUD, the individuals lose control over intake despite a decided will to stop drinking. Given the multiple faces and the specific forms of this disease, the term AUD often appears in the plural (AUDs). Since only a few approved pharmacological treatments are available to treat AUD and they do not apply to all individuals or AUD forms, the search for compounds that may help to eliminate the burden of the disease and complement other therapeutical approaches is necessary.

METHODS

This work reviews recent research focused on the involvement of epigenetic mechanisms in the pathophysiology of AUD. Excessive drinking leads to chronic and compulsive consumption that eventually damages the organism. The central nervous system is a key target and is the focus of this study. The search for the genetic and epigenetic mechanisms behind the intricated dysregulation induced by ethanol will aid researchers in establishing new therapy approaches.

CONCLUSION

Recent findings in the field of epigenetics are essential and offer new windows for observation and research. The study of small molecules that inhibit key epienzymes involved in nucleosome architecture dynamics is necessary in order to prove their action and specificity in the laboratory and to test their effectivity and safety in clinical trials with selected patients bearing defined alterations caused by ethanol.

Far from the nuclear crowd: Cytoplasmic lncRNA and their implications in synaptic plasticity and memory

A striking proportion of long non-coding RNAs are expressed specifically in the mammalian brain. Advances in genome-wide sequencing detected widespread diversity in neuronal lncRNAs based on their expression pattern, localization and function. A growing body of literature proposes that localization of lncRNAs is a critical determinant of their function. A rising number of recent findings documented distinct cytoplasmic functions of lncRNAs that are linked to activity-induced control of synaptic plasticity. However, the comprehensive role of cytoplasmic lncRNAs in neuronal functions is less understood. This review surveys our current understanding of lncRNAs that regulate the cytoplasmic life of mRNAs. We discuss the necessity of subcellular localization of lncRNAs in neuronal dendrites and the impact of their compartmentalized positioning on localized translation at the synapse. We have highlighted how lncRNAs modify a functional compartment to meet the demand for input-specific control of synaptic plasticity and memory.

Fragile X Syndrome: Lessons Learned and What New Treatment Avenues Are on the Horizon

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and the leading single-gene form of autism spectrum disorder, encompassing cognitive, behavioral, and physical forms of clinical involvement. FXS is caused by large expansions of a noncoding CGG repeat (>200 repeats) in the FMR1 gene, at which point the gene is generally silenced. Absence of FMR1 protein (FMRP), important for synaptic development and maintenance, gives rise to the neurodevelopmental disorder. There is, at present, no therapeutic approach that directly reverses the loss of FMRP; however, there is an increasing number of potential treatments that target the pathways dysregulated in FXS, including those that address the enhanced activity of the mGluR5 pathway and deficits in GABA pathways. Based on studies of targeted therapeutics to date, the prospects are good for one or more effective therapies for FXS in the near future. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

Sex-specific Regulation of Spine Density and Synaptic Proteins by G-protein-coupled Estrogen Receptor (GPER)1 in Developing Hippocampus

G-protein-coupled-estrogen-receptor 1 (GPER1) is a membrane-bound receptor that mediates estrogen signaling via intracellular signaling cascades. We recently showed that GPER1 promotes the distal dendritic enrichment of hyperpolarization activated and cyclic nucleotide-gated (HCN)1 channels in CA1 stratum lacunosum-moleculare (SLM), suggesting a role of GPER1-mediated signaling in neuronal plasticity. Here we studied whether this role involves processes of structural plasticity, such as the regulation of spine and synapse density in SLM. In organotypic entorhino-hippocampal cultures from mice expressing eGFP, we analyzed spine densities in SLM after treatment with GPER1 agonist G1 (20 nM). G1 significantly increased the density of "non-stubby" spines (maturing spines with a spine head and a neck), but did so only in cultures from female mice. In support of this finding, the expression of synaptic proteins was sex-specifically altered in the cultures: G1 increased the protein (but not mRNA) expression of PSD95 and reduced the p-/n-cofilin ratio only in cultures from females. Application of E2 (2 nM) reproduced the sex-specific effect on spine density in SLM, but only partially on the expression of synaptic proteins. Spine synapse density was, however, not altered after G1-treatment, suggesting that the increased spine density did not translate into an increased spine synapse density in the culture model. Taken together, our results support a role of GPER1 in mediating structural plasticity in CA1 SLM, but suggest that in developing hippocampus, this role is sex-specific.

The Genetic Control of Stoichiometry Underlying Autism

Autism is a common and complex neurologic disorder whose scientific underpinnings have begun to be established in the past decade. The essence of this breakthrough has been a focus on families, where genetic analyses are strongest, versus large-scale, case-control studies. Autism genetics has progressed in parallel with technology, from analyses of copy number variation to whole-exome sequencing (WES) and whole-genome sequencing (WGS). Gene mutations causing complete loss of function account for perhaps one-third of cases, largely detected through WES. This limitation has increased interest in understanding the regulatory variants of genes that contribute in more subtle ways to the disorder. Strategies combining biochemical analysis of gene regulation, WGS analysis of the noncoding genome, and machine learning have begun to succeed. The emerging picture is that careful control of the amounts of transcription, mRNA, and proteins made by key brain genes-stoichiometry-plays a critical role in defining the clinical features of autism.

Mitochondrial Structure and Polarity in Dendrites and the Axon Initial Segment Are Regulated by Homeostatic Plasticity and Dysregulated in Fragile X Syndrome

Mitochondrial dysfunction has long been overlooked in neurodevelopmental disorders, but recent studies have provided new links to genetic forms of autism, including Rett syndrome and fragile X syndrome (FXS). Mitochondria show plasticity in morphology and function in response to neuronal activity, and previous research has reported impairments in mitochondrial morphology and function in disease. We and others have previously reported abnormalities in distinct types of homeostatic plasticity in FXS. It remains unknown if or how activity deprivation triggering homeostatic plasticity affects mitochondria in axons and/or dendrites and whether impairments occur in neurodevelopmental disorders. Here, we test the hypothesis that mitochondria are structurally and functionally modified in a compartment-specific manner during homeostatic plasticity using a model of activity deprivation in cortical neurons from wild-type mice and that this plasticity-induced regulation is altered in Fmr1-knockout (KO) neurons. We uncovered dendrite-specific regulation of the mitochondrial surface area, whereas axon initial segment (AIS) mitochondria show changes in polarity; both responses are lost in the Fmr1 KO. Taken together, our results demonstrate impairments in mitochondrial plasticity in FXS, which has not previously been reported. These results suggest that mitochondrial dysregulation in FXS could contribute to abnormal neuronal plasticity, with broader implications to other neurodevelopmental disorders and therapeutic strategies.