MicroRNA targeting of CoREST controls polarization of migrating cortical neurons

Neuronal miR-17-5p contributes to interhemispheric cortical connectivity defects induced by prenatal alcohol exposure

Structural and functional deficits in brain connectivity are reported in patients with fetal alcohol spectrum disorders (FASDs), but whether and how prenatal alcohol exposure (PAE) affects axonal development of neurons and disrupts wiring between brain regions is unknown. Here, we develop a mouse model of moderate alcohol exposure during prenatal brain wiring to study the effects of PAE on corpus callosum (CC) development. PAE induces aberrant navigation of interhemispheric CC axons that persists even after exposure ends, leading to ectopic termination in the contralateral cortex. The neuronal miR-17-5p and its target ephrin type A receptor 4 (EphA4) mediate the effect of alcohol on the contralateral targeting of CC axons. Thus, altered microRNA-mediated regulation of axonal guidance may have implications for interhemispheric cortical connectivity and associated behaviors in FASD.

Comprehensive analysis of REST corepressors (RCORs) in pan-cancer

REST corepressors (RCORs) are the core component of the LSD1/CoREST/HDACs transcriptional repressor complex, which have been revealed differently expressed in various cancers, but the therapeutic and prognostic mechanisms in cancer are still poorly understood. In this study, we analyzed expression, prognostic value, molecular subtypes, genetic alteration, immunotherapy response and drug sensitivity of RCORs in pan-cancer. Clinical correlation, stemness index, immune infiltration and regulatory networks of RCORs in hepatocellular carcinoma (HCC) were detected through TCGA and GSCA database. In-vitro experiments were conducted to explore the role of RCOR1 in HCC cells. The expression of RCORs varied among different cancers, and have prognostic values in several cancers. Cancer subtypes were categorized according to the expression of RCORs with clinical information. RCORs were significantly correlated with immunotherapy response, MSI, drug sensitivity and genetic alteration in pan-cancer. In HCC, RCORs were considered as potential predictor of stemness and also had association with immune infiltration. The ceRNA-TF-kinase regulatory networks of RCORs were constructed. Besides, RCOR1 acts as an oncogene in HCC and promotes the proliferation of HCC cells by inhibiting cell cycle arrest and cell apoptosis. Taken together, our study revealed the potential molecular mechanisms of RCORs in pan-cancer, offering a benchmark for disease-related research.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

MicroRNA‑124: an emerging therapeutic target in central nervous system disorders

The central nervous system (CNS) consists of neuron and non-neuron cells including neural stem/precursor cells (NSPCs), neuroblasts, glia cells (mainly astrocyte, oligodendroglia and microglia), which thereby form a precise and complicated network and exert diverse functions through interactions of numerous bioactive ingredients. MicroRNAs (miRNAs), with small size approximately  ~ 21nt and as well-documented post-transcriptional key regulators of gene expression, are a cluster of evolutionarily conserved endogenous non-coding RNAs. More than 2000 different miRNAs has been discovered till now. MicroRNA-124(miR-124), the most brain-rich microRNA, has been validated to possess important functions in the central nervous system, including neural stem cell proliferation and differentiation, cell fate determination, neuron migration, synapse plasticity and cognition, cell apoptosis etc. According to recent studies, herein, we provide a review of this conversant miR-124 to further understand the potential functions and therapeutic and clinical value in brain diseases.

A miR-124-mediated post-transcriptional mechanism controlling the cell fate switch of astrocytes to induced neurons

The microRNA (miRNA) miR-124 has been employed supplementary to neurogenic transcription factors (TFs) and other miRNAs to enhance direct neurogenic conversion. The aim of this study was to investigate whether miR-124 is sufficient to drive direct reprogramming of astrocytes to induced neurons (iNs) on its own and elucidate its independent mechanism of reprogramming action. Our data show that miR-124 is a potent driver of the reprogramming switch of astrocytes toward an immature neuronal fate by directly targeting the RNA-binding protein Zfp36L1 implicated in ARE-mediated mRNA decay and subsequently derepressing Zfp36L1 neurogenic interactome. To this end, miR-124 contribution in iNs' production largely recapitulates endogenous neurogenesis pathways, being further enhanced upon addition of the neurogenic compound ISX9, which greatly improves iNs' differentiation and functional maturation. Importantly, miR-124 is potent in guiding direct conversion of reactive astrocytes to immature iNs in vivo following cortical trauma, while ISX9 supplementation confers a survival advantage to newly produced iNs.

Molecular Mechanisms Involved in the Regulation of Neurodevelopment by miR-124

miR-124 is a miRNA predominantly expressed in the nervous system and accounts for more than a quarter of the total miRNAs in the brain. It regulates neurogenesis, neuronal differentiation, neuronal maturation, and synapse formation and is the most important miRNA in the brain. Furthermore, emerging evidence has suggested miR-124 may be associated with the pathogenesis of various neurodevelopmental and neuropsychiatric disorders. Here, we provide an overview of the role of miR-124 in neurodevelopment and the underling mechanisms, and finally, we prospect the significance of miR-124 research to the field of neuroscience.

Human cytomegalovirus infection perturbs neural progenitor cell fate via the expression of viral microRNAs

Human cytomegalovirus (HCMV) preferentially targets neural progenitor cells (NPCs) in congenitally infected fetal brains, inducing neurodevelopmental disorders. While HCMV expresses several microRNAs (miRNAs) during infection, their roles in NPC infection are unclear. Here, we characterized expression of cellular and viral miRNAs in HCMV-infected NPCs during early infection by microarray and identified seven differentially expressed cellular miRNAs and six significantly upregulated HCMV miRNAs. Deep learning approaches were used to identify potential targets of significantly upregulated HCMV miRNAs against differentially expressed cellular messenger RNA (mRNAs), and the associations with miRNA-mRNA expression changes were observed. Gene ontology enrichment analysis indicated cellular gene targets were significantly enriched in pathways involved in neurodevelopment and cell-cycle processes. Viral modulation of selected miRNAs and cellular gene targets involved in neurodevelopmental processes were further validated by real-time quantitative reverse transcription polymerase chain reaction. Finally, a predicted 3' untranslated region target site of hcmv-miR-US25-1 in Jag1, a factor important for neurogenesis, was confirmed by mutagenesis. Reduction of Jag1 RNA and protein levels in NPCs was observed in response to transient expression of hcmv-miR-US25-1. A hcmv-miR-US25-1 mutant virus (ΔmiR-US25) displayed limited ability to downregulate Jag1 mRNA levels and protein levels during the early infection stage compared with the wild type virus. Our collective experimental and computational investigation of miRNAs and cellular mRNAs expression in HCMV-infected NPCs yields new insights into the roles of viral miRNAs in regulating NPC fate and their contributions to HCMV neuropathogenesis.

Optical Control of MicroRNA Function in Zebrafish Embryos

MicroRNAs play crucial and dynamic roles in vertebrate development and diseases. Some, like miR-430, are highly expressed during early embryo development and regulate hundreds of transcripts, which can make it difficult to study their role in the timing and location of specific developmental processes using conventional morpholino oligonucleotide (MO) knockdown or genetic deletion approaches. We demonstrate that light-activated circular morpholino oligonucleotides (cMOs) can be applied to the conditional control of microRNA function. We targeted miR-430 in zebrafish embryos to study its role in the development of the embryo body and the heart. Using 405 nm irradiation, precise spatial and temporal control over miR-430 function was demonstrated, offering insight into the cell populations and developmental timepoints involved in each process.

Engineering extracellular vesicles by three-dimensional dynamic culture of human mesenchymal stem cells

Human mesenchymal stem cell (hMSC) derived extracellular vesicles (EVs) have shown therapeutic potential in recent studies. However, the corresponding therapeutic components are largely unknown, and scale-up production of hMSC EVs is a major challenge for translational applications. In the current study, hMSCs were grown as 3D aggregates under wave motion to promote EV secretion. Results demonstrate that 3D hMSC aggregates promote activation of the endosomal sorting complexes required for transport (ESCRT)-dependent and -independent pathways. mRNA sequencing revealed global transcriptome alterations for 3D hMSC aggregates. Compared to 2D-hMSC-EVs, the quantity of 3D-hMSC-EVs was enhanced significantly (by 2-fold), with smaller sizes, higher miR-21 and miR-22 expression, and an altered protein cargo (e.g., upregulation of cytokines and anti-inflammatory factors) uncovered by proteomics analysis, possibly due to altered EV biogenesis. Functionally, 3D-hMSC-EVs rejuvenated senescent stem cells and exhibited enhanced immunomodulatory potentials. In summary, this study provides a promising strategy for scalable production of high-quality EVs from hMSCs with enhanced therapeutic potential.

miR-137 and miR-122, two outer subventricular zone non-coding RNAs, regulate basal progenitor expansion and neuronal differentiation

Cortical expansion in primate brains relies on enlargement of germinal zones during a prolonged developmental period. Although most mammals have two cortical germinal zones, the ventricular zone (VZ) and subventricular zone (SVZ), gyrencephalic species display an additional germinal zone, the outer subventricular zone (oSVZ), which increases the number and diversity of neurons generated during corticogenesis. How the oSVZ emerged during evolution is poorly understood, but recent studies suggest a role for non-coding RNAs, which allow tight genetic program regulation during development. Here, using in vivo functional genetics, single-cell RNA sequencing, live imaging, and electrophysiology to assess progenitor and neuronal properties in mice, we identify two oSVZ-expressed microRNAs (miRNAs), miR-137 and miR-122, which regulate key cellular features of cortical expansion. miR-137 promotes basal progenitor self-replication and superficial layer neuron fate, whereas miR-122 decreases the pace of neuronal differentiation. These findings support a cell-type-specific role of miRNA-mediated gene expression in cortical expansion.

•

oSVZ-expressed microRNAs 137 and 122 promote superficial layer identity of neurons

•

miR-137 promotes basal progenitor proliferation and layer 2/3 neuron generation

•

miR-122 slows down neuronal differentiation pace

Translational control of gene function through optically regulated nucleic acids

Translation of mRNA into protein is one of the most fundamental processes within biological systems. Gene expression is tightly regulated both in space and time, often involving complex signaling or gene regulatory networks, as most prominently observed in embryo development. Thus, studies of gene function require tools with a matching level of external control. Light is an excellent conditional trigger as it is minimally invasive, can be easily tuned in wavelength and amplitude, and can be applied with excellent spatial and temporal resolution. To this end, modification of established oligonucleotide-based technologies with optical control elements, in the form of photocaging groups and photoswitches, has rendered these tools capable of navigating the dynamic regulatory pathways of mRNA translation in cellular and in vivo models. In this review, we discuss the different optochemical approaches used to generate photoresponsive nucleic acids that activate and deactivate gene expression and function at the translational level.

Nuclear and Mitochondrial Genome, Epigenome and Gut Microbiome: Emerging Molecular Biomarkers for Parkinson's Disease

BACKGROUND

Parkinson's disease (PD) is currently the second most common neurodegenerative disorder, burdening about 10 million elderly individuals worldwide. The multifactorial nature of PD poses a difficult obstacle for understanding the mechanisms involved in its onset and progression. Currently, diagnosis depends on the appearance of clinical signs, some of which are shared among various neurologic disorders, hindering early diagnosis. There are no effective tools to prevent PD onset, detect the disease in early stages or accurately report the risk of disease progression. Hence, there is an increasing demand for biomarkers that may identify disease onset and progression, as treatment-based medicine may not be the best approach for PD. Over the last few decades, the search for molecular markers to predict susceptibility, aid in accurate diagnosis and evaluate the progress of PD have intensified, but strategies aimed to improve individualized patient care have not yet been established.

CONCLUSIONS

Genomic variation, regulation by epigenomic mechanisms, as well as the influence of the host gut microbiome seem to have a crucial role in the onset and progress of PD, thus are considered potential biomarkers. As such, the human nuclear and mitochondrial genome, epigenome, and the host gut microbiome might be the key elements to the rise of personalized medicine for PD patients.

MiRNAs in early brain development and pediatric cancer: At the intersection between healthy and diseased embryonic development

The size and organization of the brain are determined by the activity of progenitor cells early in development. Key mechanisms regulating progenitor cell biology involve miRNAs. These small noncoding RNA molecules bind mRNAs with high specificity, controlling their abundance and expression. The role of miRNAs in brain development has been studied extensively, but their involvement at early stages remained unknown until recently. Here, recent findings showing the important role of miRNAs in the earliest phases of brain development are reviewed, and it is discussed how loss of specific miRNAs leads to pathological conditions, particularly adult and pediatric brain tumors. Let-7 miRNA downregulation and the initiation of embryonal tumors with multilayered rosettes (ETMR), a novel link recently discovered by the laboratory, are focused upon. Finally, it is discussed how miRNAs may be used for the diagnosis and therapeutic treatment of pediatric brain tumors, with the hope of improving the prognosis of these devastating diseases.

Long Noncoding RNA Maternally Expressed Gene 3 Targets miR-30b and Regulates the AKT Serine/Threonine Kinase 1/Phosphoinositide 3-Kinase Signaling Pathway of H2O2-Induced Proliferation, Apoptosis, and Oxidative Stress in Retinal Ganglion Cells

Oxidative stress is an important factor affecting retinal ganglion cell (RGC) apoptosis. RGC apoptosis is the main pathophysiological feature of visual impairment as a result of glaucoma. Recently, it has been found that long noncoding RNA (lncRNA) and microRNAs are involved in RGC apoptosis. Here, the function of lncRNA maternally expressed gene 3 (MEG3) and miR-30b in H2 O2-induced RGC proliferation, apoptosis, and oxidative stress was investigated. The expression levels of MEG3 and miR-30b were detected by RT-PCR; the effects of MEG3 and miR-30b on the proliferation and apoptosis of RGCs were observed by flow cytometry; the levels of apoptosis-related proteins and AKT/PI3K signal pathway proteins were detected by protein immunoassay; and the regulation of miR-34a by pvt1 was verified by in vivo and in vitro experiments. The expression of MEG3 and miR-30b increased and decreased significantly in RGCs treated by H2O2. MEG3 expression decreased, apoptosis level-related proteins decreased, the apoptosis rate reduced, and the activity of MDA and SOD decreased. When the expression of miR-34a was inhibited, the proliferation rate of RGCs increased, the apoptosis rate decreased, and the level of apoptosis-related proteins decreased, which reversed MEG3’s effect on RGC apoptosis and proliferation. Furthermore, pvt1 could bind the miR-30b promoter and regulate it with in vitro expression and in vivo expression. Besides, we found that miR-30b can regulate the AKT/PI3K signaling pathway and participate in cell apoptosis and hyperplasia in stress response. LncRNA MEG3 targets miR-30b and regulates the AKT/PI3K signaling pathway on H2 O2-induced cell apoptosis, hyperplasia, and oxidative stress of RGCs.

Neuroprotective effects of human amniotic fluid stem cells-derived secretome in an ischemia/reperfusion model

Stem cells offer the basis for the promotion of robust new therapeutic approaches for a variety of human disorders. There are still many limitations to be overcome before clinical therapeutic application, including a better understanding of the mechanism by which stem cell therapies may lead to enhanced recovery. In vitro investigations are necessary to dissect the mechanisms involved and to support the potential development in stem cell-based therapies. In spite of growing interest in human amniotic fluid stem cells, not much is known about the characteristics of their secretome and regarding the potential neuroprotective mechanism in different pathologies, including stroke. To get more insight on amniotic fluid cells therapeutic potential, signal transduction pathways activated by human amniotic fluid stem cells (hAFSCs)-derived secretome in a stroke in vitro model (ischemia/reperfusion [I/R] model) were investigated by Western blot. Moreover, miRNA expression in the exosomal fraction of the conditioned medium was analyzed. hAFSCs-derived secretome was able to activate pro-survival and anti-apoptotic pathways. MicroRNA analysis in the exosomal component revealed a panel of 16 overexpressed miRNAs involved in the regulation of coherent signaling pathways. In particular, the pathways of relevance in ischemia/reperfusion, such as neurotrophin signaling, and those related to neuroprotection and neuronal cell death, were analyzed. The results obtained strongly point toward the neuroprotective effects of the hAFSCs-conditioned medium in the in vitro stroke model here analyzed. This can be achieved by the modulation and activation of pro-survival processes, at least in part, due to the activity of secreted miRNAs.

MicroRNAs at the Crossroad of the Dichotomic Pathway Cell Death vs. Stemness in Neural Somatic and Cancer Stem Cells: Implications and Therapeutic Strategies

Stemness and apoptosis may highlight the dichotomy between regeneration and demise in the complex pathway proceeding from ontogenesis to the end of life. In the last few years, the concept has emerged that the same microRNAs (miRNAs) can be concurrently implicated in both apoptosis-related mechanisms and cell differentiation. Whether the differentiation process gives rise to the architecture of brain areas, any long-lasting perturbation of miRNA expression can be related to the occurrence of neurodevelopmental/neuropathological conditions. Moreover, as a consequence of neural stem cell (NSC) transformation to cancer stem cells (CSCs), the fine modulation of distinct miRNAs becomes necessary. This event implies controlling the expression of pro/anti-apoptotic target genes, which is crucial for the management of neural/neural crest-derived CSCs in brain tumors, neuroblastoma, and melanoma. From a translational point of view, the current progress on the emerging miRNA-based neuropathology therapeutic applications and antitumor strategies will be disclosed and their advantages and shortcomings discussed.

An evolutionarily acquired microRNA shapes development of mammalian cortical projections

The mammalian central nervous system contains unique projections from the cerebral cortex thought to underpin complex motor and cognitive skills, including the corticospinal tract and corpus callosum. The neurons giving rise to these projections—corticospinal and callosal projection neurons—develop from the same progenitors, but acquire strikingly different fates. The broad evolutionary conservation of known genes controlling cortical projection neuron fates raises the question of how the more narrowly conserved corticospinal and callosal projections evolved. We identify a microRNA cluster selectively expressed by corticospinal projection neurons and exclusive to placental mammals. One of these microRNAs promotes corticospinal fate via regulation of the callosal gene LMO4, suggesting a mechanism whereby microRNA regulation during development promotes evolution of neuronal diversity.

The corticospinal tract is unique to mammals and the corpus callosum is unique to placental mammals (eutherians). The emergence of these structures is thought to underpin the evolutionary acquisition of complex motor and cognitive skills. Corticospinal motor neurons (CSMN) and callosal projection neurons (CPN) are the archetypal projection neurons of the corticospinal tract and corpus callosum, respectively. Although a number of conserved transcriptional regulators of CSMN and CPN development have been identified in vertebrates, none are unique to mammals and most are coexpressed across multiple projection neuron subtypes. Here, we discover 17 CSMN-enriched microRNAs (miRNAs), 15 of which map to a single genomic cluster that is exclusive to eutherians. One of these, miR-409-3p, promotes CSMN subtype identity in part via repression of LMO4, a key transcriptional regulator of CPN development. In vivo, miR-409-3p is sufficient to convert deep-layer CPN into CSMN. This is a demonstration of an evolutionarily acquired miRNA in eutherians that refines cortical projection neuron subtype development. Our findings implicate miRNAs in the eutherians’ increase in neuronal subtype and projection diversity, the anatomic underpinnings of their complex behavior.

Translational Control during Mammalian Neocortex Development and Postembryonic Neuronal Function

The control of mRNA translation has key roles in the regulation of gene expression and biological processes such as mammalian cellular differentiation and identity. Methodological advances in the last decade have resulted in considerable progress towards understanding how translational control contributes to the regulation of diverse biological phenomena. In this review, we discuss recent findings in the involvement of translational control in the mammalian neocortex development and neuronal biology. We focus on regulatory mechanisms that modulate translational efficiency during neural stem cells self-renewal and differentiation, as well as in neuronal-related processes such as synapse, plasticity, and memory.

MicroRNA Alterations in Induced Pluripotent Stem Cell-Derived Neurons from Bipolar Disorder Patients: Pathways Involved in Neuronal Differentiation, Axon Guidance, and Plasticity

Bipolar disorder (BP) is a complex psychiatric condition characterized by severe fluctuations in mood for which underlying pathological mechanisms remain unclear. Family and twin studies have identified a hereditary component to the disorder, but a single causative gene (or set of genes) has not been identified. MicroRNAs (miRNAs) are small, noncoding RNAs ∼20 nucleotides in length, that are responsible for the posttranslational regulation of multiple genes. They have been shown to play important roles in neural development as well as in the adult brain, and several miRNAs have been reported to be dysregulated in postmortem brain tissue isolated from bipolar patients. Because there are no viable cellular models to study BP, we have taken advantage of the recent discovery that somatic cells can be reprogrammed to pluripotency then directed to form the full complement of neural cells. Analysis of RNAs extracted from Control and BP patient-derived neurons identified 58 miRNAs that were differentially expressed between the two groups. Using quantitative polymerase chain reaction we validated six miRNAs that were elevated and two miRNAs that were expressed at lower levels in BP-derived neurons. Analysis of the targets of the miRNAs indicate that they may regulate a number of cellular pathways, including axon guidance, Mapk, Ras, Hippo, Neurotrophin, and Wnt signaling. Many are involved in processes previously implicated in BP, such as cell migration, axon guidance, dendrite and synapse development, and function. We have validated targets of several different miRNAs, including AXIN2, BDNF, RELN, and ANK3 as direct targets of differentially expressed miRNAs using luciferase assays. Identification of pathways altered in patient-derived neurons suggests that disruption of these regulatory networks that may contribute to the complex phenotypes in BP.

Validation of differentially expressed brain-enriched microRNAs in the plasma of PD patients

Objective

There is a pressing need to identify and validate, minimally invasive, molecular biomarkers that will complement current practices and increase the diagnostic accuracy in Parkinson’s disease (PD). Brain‐enriched miRNAs regulate all aspects of neuron development and function; importantly, they are secreted by neurons in amounts that can be readily detected in the plasma. Τhe aim of the present study was to validate a set of previously identified brain‐enriched miRNAs with diagnostic potential for idiopathic PD and recognize the molecular pathways affected by these deregulated miRNAs.

Methods

RT‐qPCR was performed in the plasma of 92 healthy controls and 108 idiopathic PD subjects. Statistical and in silico analyses were used to validate deregulated miRNAs and pathways in PD, respectively.

Results

miR‐22‐3p, miR‐124‐3p, miR‐136‐3p, miR‐154‐5p, and miR‐323a‐3p levels were found to be differentially expressed between healthy controls and PD patients. miR‐330‐5p, miR‐433‐3p, and miR‐495‐3p levels were overall higher in male subjects. Most of these miRNAs are clustered at Chr14q32 displaying CREB1, CEBPB, and MAZ transcription factor binding sites. Gene Ontology annotation analysis of deregulated miRNA targets revealed that “Protein modification,” “Transcription factor activity,” and “Cell death” terms were over‐represented. Kyoto Encyclopedia of Genes and Genome analysis revealed that “Long‐term depression,” “TGF‐beta signaling,” and “FoxO signaling” pathways were significantly affected.

Interpretation

We validated a panel of brain‐enriched miRNAs that can be used along with other measures for the detection of PD, revealed molecular pathways targeted by these deregulated miRNAs, and identified upstream transcription factors that may be directly implicated in PD pathogenesis.

ETV5 is Essential for Neuronal Differentiation of Human Neural Progenitor Cells by Repressing NEUROG2 Expression

Neural progenitor cells (NPCs) are multipotent cells that have the potential to produce neurons and glial cells in the neural system. NPCs undergo identity maintenance or differentiation regulated by different kinds of transcription factors. Here we present evidence that ETV5, which is an ETS transcription factor, promotes the generation of glial cells and drives the neuronal subtype-specific genes in newly differentiated neurons from the human embryonic stem cells-derived NPCs. Next, we find a new role for ETV5 in the repression of NEUROG2 expression in NPCs. ETV5 represses NEUROG2 transcription via NEUROG2 promoter and requires the ETS domain. We identify ETV5 has the binding sites and is implicated in silent chromatin in NEUROG2 promoter by chromatin immunoprecipitation (ChIP) assays. Further, NEUROG2 transcription repression by ETV5 was shown to be dependent on a transcriptional corepressor (CoREST). During NPC differentiation toward neurons, ETV5 represses NEUROG2 expression and blocks the appearance of glutamatergic neurons. This finding suggests that ETV5 negatively regulates NEUROG2 expression and increases the number of GABAergic subtype neurons derived from NPCs. Thus, ETV5 represents a potent new candidate protein with benefits for the generation of GABAergic neurons.

Neurogenesis, neuronal migration, and axon guidance

Development of the central nervous system (CNS) is a complex, dynamic process that involves a precisely orchestrated sequence of genetic, environmental, biochemical, and physical factors from early embryonic stages to postnatal life. Duringthe past decade, great strides have been made to unravel mechanisms underlying human CNS development through the employment of modern genetic techniques and experimental approaches. In this chapter, we review the current knowledge regarding the main developmental processes and signaling mechanisms of (i) neurogenesis, (ii) neuronal migration, and (iii) axon guidance. We discuss mechanisms related to neural stem cells proliferation, migration, terminal translocation of neuronal progenitors, and axon guidance and pathfinding. For each section, we also provide a comprehensive overview of the underlying regulatory processes, including transcriptional, posttranscriptional, and epigenetic factors, and a myriad of signaling pathways that are pivotal to determine the fate of neuronal progenitors and newly formed migrating neurons. We further highlight how impairment of this complex regulating system, such as mutations in its core components, may cause cortical malformation, epilepsy, intellectual disability, and autism in humans. A thorough understanding of normal human CNS development is thus crucial to decipher mechanisms responsible for neurodevelopmental disorders and in turn guide the development of effective and targeted therapeutic strategies.

Challenges in microRNAs' targetome prediction and validation

MicroRNAs (miRNAs) are small RNA molecules with important roles in post-transcriptional regulation of gene expression. In recent years, the predicted number of miRNAs has skyrocketed, largely as a consequence of high-throughput sequencing technologies becoming ubiquitous. This dramatic increase in miRNA candidates poses multiple challenges in terms of data deposition, curation, and validation. Although multiple databases containing miRNA annotations and targets have been developed, ensuring data quality by validating miRNA-target interactions requires the efforts of the research community. In order to generate databases containing biologically active miRNAs, it is imperative to overcome a multitude of hurdles, including restricted miRNA expression patterns, distinct miRNA biogenesis machineries, and divergent miRNA-mRNA interaction dynamics. In the present review, we discuss recent advances and limitations in miRNA prediction, identification, and validation. Lastly, we focus on the most enriched neuronal miRNA, miR-124, and its gene regulatory network in human neurons, which has been revealed using a combined computational and experimental approach.

Translating neural stem cells to neurons in the mammalian brain

The mammalian neocortex underlies our perception of sensory information, performance of motor activities, and higher-order cognition. During mammalian embryogenesis, radial glial precursor cells sequentially give rise to diverse populations of excitatory cortical neurons, followed by astrocytes and oligodendrocytes. A subpopulation of these embryonic neural precursors persists into adulthood as neural stem cells, which give rise to inhibitory interneurons and glia. Although the intrinsic mechanisms instructing the genesis of these distinct progeny have been well-studied, most work to date has focused on transcriptional, epigenetic, and cell-cycle control. Recent studies, however, have shown that posttranscriptional mechanisms also regulate the cell fate choices of transcriptionally primed neural precursors during cortical development. These mechanisms are mediated primarily by RNA-binding proteins and microRNAs that coordinately regulate mRNA translation, stability, splicing, and localization. Together, these findings point to an extensive network of posttranscriptional control and provide insight into both normal cortical development and disease. They also add another layer of complexity to brain development and raise important biological questions for future investigation.

The RNA helicase DDX17 controls the transcriptional activity of REST and the expression of proneural microRNAs in neuronal differentiation

The Repressor Element 1-silencing transcription factor (REST) represses a number of neuronal genes in non-neuronal cells or in undifferentiated neural progenitors. Here, we report that the DEAD box RNA helicase DDX17 controls important REST-related processes that are critical during the early phases of neuronal differentiation. First, DDX17 associates with REST, promotes its binding to the promoter of a subset of REST-targeted genes and co-regulates REST transcriptional repression activity. During neuronal differentiation, we observed a downregulation of DDX17 along with that of the REST complex that contributes to the activation of neuronal genes. Second, DDX17 and its paralog DDX5 regulate the expression of several proneural microRNAs that are known to target the REST complex during neurogenesis, including miR-26a/b that are also direct regulators of DDX17 expression. In this context, we propose a new mechanism by which RNA helicases can control the biogenesis of intronic miRNAs. We show that the processing of the miR-26a2 precursor is dependent on RNA helicases, owing to an intronic regulatory region that negatively impacts on both miRNA processing and splicing of its host intron. Our work places DDX17 in the heart of a pathway involving REST and miRNAs that allows neuronal gene repression.

Neuronal Polarization Packs a One-Two Punch

In this issue of Neuron, Ambrozkiewicz et al. (2018) define a new molecular circuit controlling neuronal polarization and migration through the transcription factor SOX9 to coordinate the production of regulators of both protein synthesis and degradation.

MicroRNA-129 modulates neuronal migration by targeting Fmr1 in the developing mouse cortex

During cortical development, neuronal migration is one of the most important steps for normal cortical formation and function, and defects in this process cause many brain diseases. However, the molecular mechanisms underlying this process remain largely unknown. In this study, we found that miR-129-5p and miR-129-3p were expressed in both neural progenitor cells and cortical neurons in the developing murine cortex. Moreover, abnormal miR-129 expression could block radial migration of both the deeper layer and upper layer neurons, and impair the multipolar to bipolar transition. However, antagomir-mediated inhibition resulted in overmigration of neurons. In addition, we showed that Fragile X Mental Retardation gene 1 (Fmr1), which is mutated in the autism spectrum disorder fragile X syndrome, is an important regulatory target for miR-129-5p. Furthermore, Fmr1 loss-of-function and gain-of-function experiments showed opposite effects on miR-129 regulation of neuronal migration, and restoring Fmr1 expression could counteract the deleterious effect of miR-129 on neuronal migration. Taken together, our results suggest that miR-129-5p could modulate the expression of fragile X mental retardation 1 protein (FMRP) to ensure normal neuron positioning in the developing cerebral cortex.

MicroRNA-22 Controls Aberrant Neurogenesis and Changes in Neuronal Morphology After Status Epilepticus

Prolonged seizures (status epilepticus, SE) may drive hippocampal dysfunction and epileptogenesis, at least partly, through an elevation in neurogenesis, dysregulation of migration and aberrant dendritic arborization of newly-formed neurons. MicroRNA-22 was recently found to protect against the development of epileptic foci, but the mechanisms remain incompletely understood. Here, we investigated the contribution of microRNA-22 to SE-induced aberrant adult neurogenesis. SE was induced by intraamygdala microinjection of kainic acid (KA) to model unilateral hippocampal neuropathology in mice. MicroRNA-22 expression was suppressed using specific oligonucleotide inhibitors (antagomir-22) and newly-formed neurons were visualized using the thymidine analog iodo-deoxyuridine (IdU) and a green fluorescent protein (GFP)-expressing retrovirus to visualize the dendritic tree and synaptic spines. Using this approach, we quantified differences in the rate of neurogenesis and migration, the structure of the apical dendritic tree and density and morphology of dendritic spines in newly-formed neurons.SE resulted in an increased rate of hippocampal neurogenesis, including within the undamaged contralateral dentate gyrus (DG). Newly-formed neurons underwent aberrant migration, both within the granule cell layer and into ectopic sites. Inhibition of microRNA-22 exacerbated these changes. The dendritic diameter and the density and average volume of dendritic spines were unaffected by SE, but these parameters were all elevated in mice in which microRNA-22 was suppressed. MicroRNA-22 inhibition also reduced the length and complexity of the dendritic tree, independently of SE. These data indicate that microRNA-22 is an important regulator of morphogenesis of newly-formed neurons in adults and plays a role in supressing aberrant neurogenesis associated with SE.

Evolution of New miRNAs and Cerebro-Cortical Development

The noncoding portion of the genome, including microRNAs, has been fertile evolutionary soil for cortical development in primates. A major contribution to cortical expansion in primates is the generation of novel precursor cell populations. Because miRNA expression profiles track closely with cell identity, it is likely that numerous novel microRNAs have contributed to cellular diversity in the brain. The tools to determine the genomic context within which novel microRNAs emerge and how they become integrated into molecular circuitry are now in hand.

Optochemical Control of Biological Processes in Cells and Animals

Biological processes are naturally regulated with high spatial and temporal control, as is perhaps most evident in metazoan embryogenesis. Chemical tools have been extensively utilized in cell and developmental biology to investigate cellular processes, and conditional control methods have expanded applications of these technologies toward resolving complex biological questions. Light represents an excellent external trigger since it can be controlled with very high spatial and temporal precision. To this end, several optically regulated tools have been developed and applied to living systems. In this review we discuss recent developments of optochemical tools, including small molecules, peptides, proteins, and nucleic acids that can be irreversibly or reversibly controlled through light irradiation, with a focus on applications in cells and animals.

The dynamic recruitment of TRBP to neuronal membranes mediates dendritogenesis during development

MicroRNAs are important regulators of local protein synthesis during neuronal development. We investigated the dynamic regulation of microRNA production and found that the majority of the microRNA‐generating complex, consisting of Dicer, TRBP, and PACT, specifically associates with intracellular membranes in developing neurons. Stimulation with brain‐derived neurotrophic factor (BDNF), which promotes dendritogenesis, caused the redistribution of TRBP from the endoplasmic reticulum into the cytoplasm, and its dissociation from Dicer, in a Ca²⁺‐dependent manner. As a result, the processing of a subset of neuronal precursor microRNAs, among them the dendritically localized pre‐miR16, was impaired. Decreased production of miR‐16‐5p, which targeted the BDNF mRNA itself, was rescued by expression of a membrane‐targeted TRBP. Moreover, miR‐16‐5p or membrane‐targeted TRBP expression blocked BDNF‐induced dendritogenesis, demonstrating the importance of neuronal TRBP dynamics for activity‐dependent neuronal development. We propose that neurons employ specialized mechanisms to modulate local gene expression in dendrites, via the dynamic regulation of microRNA biogenesis factors at intracellular membranes of the endoplasmic reticulum, which in turn is crucial for neuronal dendrite complexity and therefore neuronal circuit formation and function.

RNA on the brain: emerging layers of post-transcriptional regulation in cerebral cortex development

Embryonic development is a critical period during which neurons of the brain are generated and organized. In the developing cerebral cortex, this requires complex processes of neural progenitor proliferation, neuronal differentiation, and migration. Each step relies upon highly regulated control of gene expression. In particular, RNA splicing, stability, localization, and translation have emerged as key post-transcriptional regulatory nodes of mouse corticogenesis. Trans-regulators of RNA metabolism, including microRNAs (miRs) and RNA-binding proteins (RBPs), orchestrate diverse steps of cortical development. These trans-factors function either individually or cooperatively to influence RNAs, often of similar classes, termed RNA regulons. New technological advances raise the potential for an increasingly sophisticated understanding of post-transcriptional control in the developing neocortex. Many RNA-binding factors are also implicated in neurodevelopmental diseases of the cortex. Therefore, elucidating how RBPs and miRs converge to influence mRNA expression in progenitors and neurons will give valuable insights into mechanisms of cortical development and disease. WIREs Dev Biol 2018, 7:e290. doi: 10.1002/wdev.290 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory RNA Nervous System Development > Vertebrates: Regional Development Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Disease.

The miR-183/ItgA3 axis is a key regulator of prosensory area during early inner ear development

MicroRNAs are important regulators of gene expression and are involved in cellular processes such as proliferation or differentiation, particularly during development of numerous organs including the inner ear. However, it remains unknown if miRNAs are required during the earliest stages of otocyst and cochlear duct development. Here, we report that a conditional loss of Dicer expression in the otocyst impairs the early development of the inner ear as a result of the accumulation of DNA damage that trigger p53-mediated apoptosis. Moreover, cochlear progenitors in the prosensory domain do not exit the cell cycle. Our unbiased approach identified ItgA3 as a target of miR-183, which are both enriched in the otic vesicle. We observed that the repression of integrin alpha 3 by miR-183 controls cell proliferation in the developing cochlea. Collectively, our results reveal that Dicer and miRNAs play essential roles in the regulation of early inner ear development.

MicroRNAs in neural development: from master regulators to fine-tuners

The proper formation and function of neuronal networks is required for cognition and behavior. Indeed, pathophysiological states that disrupt neuronal networks can lead to neurodevelopmental disorders such as autism, schizophrenia or intellectual disability. It is well-established that transcriptional programs play major roles in neural circuit development. However, in recent years, post-transcriptional control of gene expression has emerged as an additional, and probably equally important, regulatory layer. In particular, it has been shown that microRNAs (miRNAs), an abundant class of small regulatory RNAs, can regulate neuronal circuit development, maturation and function by controlling, for example, local mRNA translation. It is also becoming clear that miRNAs are frequently dysregulated in neurodevelopmental disorders, suggesting a role for miRNAs in the etiology and/or maintenance of neurological disease states. Here, we provide an overview of the most prominent regulatory miRNAs that control neural development, highlighting how they act as ‘master regulators’ or ‘fine-tuners’ of gene expression, depending on context, to influence processes such as cell fate determination, cell migration, neuronal polarization and synapse formation.

MicroRNA-338 modulates cortical neuronal placement and polarity

The precise spatial and temporal regulation of gene expression orchestrates the many intricate processes during brain development. In the present study we examined the role of the brain-enriched microRNA-338 (miR-338) during mouse cortical development. Reduction of miR-338 levels in the developing mouse cortex, using a sequence-specific miR-sponge, resulted in a loss of neuronal polarity in the cortical plate and significantly reduced the number of neurons within this cortical layer. Conversely, miR-338 overexpression in developing mouse cortex increased the number of neurons, which exhibited a multipolar morphology. All together, our results raise the possibility for a direct role for this non-coding RNA, which was recently associated with schizophrenia, in the regulation of cortical neuronal polarity and layer placement.

Cortical Morphogenesis during Embryonic Development Is Regulated by miR-34c and miR-204

The porcine brain closely resembles the human brain in aspects such as development and morphology. Temporal miRNA profiling in the developing embryonic porcine cortex revealed a distinct set of miRNAs, including miR-34c and miR-204, which exhibited a highly specific expression profile across the time of cortical folding. These miRNAs were found to target Doublecortin (DCX), known to be involved in neuron migration during cortical folding of gyrencephalic brains. In vivo modulation of miRNA expression in mouse embryos confirmed that miR-34c and miR-204 can control neuronal migration and cortical morphogenesis, presumably by posttranscriptional regulation of DCX.

Hippocampal adult neurogenesis: Does the immune system matter?

Adult hippocampal neurogenesis involves proliferation, survival, differentiation and integration of newborn neurons into pre-existing neuronal networks. Although its functional significance in the central nervous system (CNS) has not comprehensively elucidated, adult neurogenesis has been attributed a role in cognition, learning and memory. There is a growing body of evidence that CNS resident as well as peripheral immune cells participate in regulating hippocampal adult neurogenesis. Microglial cells are closely associated with neural stem/progenitor cell (NSPC) in the neurogenic niche engaged in a bidirectional communication with neurons, which may be important for adult neurogenesis. Microglial and neuronal crosstalk is mediated in part by CX3CL1/CX3CR1 signaling and a disruption in this pathway has been associated with impaired neurogenesis. It has been also reported that microglial neuroprotective or neurotoxic effects in adult neurogenesis occur in a context-dependent manner. Apart from microglia other brain resident and peripheral immune cells including pericytes, perivascular macrophages, mast cells and T-cells also modulate this phenomenon. It is worth mentioning that under some physiological circumstances such as normal aging there is a significant decrease in hippocampal neurogenesis. A role for innate and adaptive immune system in adult neurogenesis has been also reported during aging. Here, we review the current evidence regarding neuro-immune interactions in the regulation of neurogenesis under distinct conditions, including aging.

Elongator controls cortical interneuron migration by regulating actomyosin dynamics

The migration of cortical interneurons is a fundamental process for the establishment of cortical connectivity and its impairment underlies several neurological disorders. During development, these neurons are born in the ganglionic eminences and they migrate tangentially to populate the cortical layers. This process relies on various morphological changes that are driven by dynamic cytoskeleton remodelings. By coupling time lapse imaging with molecular analyses, we show that the Elongator complex controls cortical interneuron migration in mouse embryos by regulating nucleokinesis and branching dynamics. At the molecular level, Elongator fine-tunes actomyosin forces by regulating the distribution and turnover of actin microfilaments during cell migration. Thus, we demonstrate that Elongator cell-autonomously promotes cortical interneuron migration by controlling actin cytoskeletal dynamics.

MicroRNAs and psychiatric disorders: From aetiology to treatment

The emergence of psychiatric disorders relies on the interaction between genetic vulnerability and environmental adversities. Several studies have demonstrated a crucial role for epigenetics (e.g. DNA methylation, post-translational histone modifications and microRNA-mediated post-transcriptional regulation) in the translation of environmental cues into adult behavioural outcome, which can prove to be harmful thus increasing the risk to develop psychopathology. Within this frame, non-coding RNAs, especially microRNAs, came to light as pivotal regulators of many biological processes occurring in the Central Nervous System, both during the neuronal development as well as in the regulation of adult function, including learning, memory and neuronal plasticity. On these basis, in recent years it has been hypothesised a central role for microRNA modulation and expression regulation in many brain disorders, including neurodegenerative disorders and mental illnesses. Indeed, the aim of the present review is to present the most recent state of the art regarding microRNA involvement in psychiatric disorders. We will first describe the mechanisms that regulate microRNA biogenesis and we will report evidences of microRNA dysregulation in peripheral body fluids, in postmortem brain tissues from patients suffering from psychopathology as well as in animal models. Last, we will discuss the potential to consider microRNAs as putative target for pharmacological intervention, using common psychotropic drugs or more specific tools, with the aim to normalize functions that are disrupted in different psychiatric conditions.

miR-9 and miR-124 synergistically affect regulation of dendritic branching via the AKT/GSK3β pathway by targeting Rap2a

A single microRNA (miRNA) can regulate expression of multiple proteins, and expression of an individual protein may be controlled by numerous miRNAs. This regulatory pattern strongly suggests that synergistic effects of miRNAs play critical roles in regulating biological processes. miR-9 and miR-124, two of the most abundant miRNAs in the mammalian nervous system, have important functions in neuronal development. In this study, we identified the small GTP-binding protein Rap2a as a common target of both miR-9 and miR-124. miR-9 and miR-124 together, but neither miRNA alone, strongly suppressed Rap2a, thereby promoting neuronal differentiation of neural stem cells (NSCs) and dendritic branching of differentiated neurons. Rap2a also diminished the dendritic complexity of mature neurons by decreasing the levels of pAKT and pGSK3β. Our results reveal a novel pathway in which miR-9 and miR-124 synergistically repress expression of Rap2a to sustain homeostatic dendritic complexity during neuronal development and maturation.

MiR-980 Is a Memory Suppressor MicroRNA that Regulates the Autism-Susceptibility Gene A2bp1

MicroRNAs have been associated with many different biological functions, but little is known about their roles in conditioned behavior. We demonstrate that Drosophila miR-980 is a memory suppressor gene functioning in multiple regions of the adult brain. Memory acquisition and stability were both increased by miR-980 inhibition. Whole cell recordings and functional imaging experiments indicated that miR-980 regulates neuronal excitability. We identified the autism susceptibility gene, A2bp1, as an mRNA target for miR-980. A2bp1 levels varied inversely with miR-980 expression; memory performance was directly related to A2bp1 levels. In addition, A2bp1 knockdown reversed the memory gains produced by miR-980 inhibition, consistent with A2bp1 being a downstream target of miR-980 responsible for the memory phenotypes. Our results indicate that miR-980 represses A2bp1 expression to tune the excitable state of neurons, and the overall state of excitability translates to memory impairment or improvement.

miR-124-9-9* potentiates Ascl1-induced reprogramming of cultured Müller glia

The Müller glia of fish provide a source for neuronal regeneration after injury, but they do not do so in mammals. We previously showed that lentiviral gene transfer of the transcription factor Achaete-scute homolog 1 (Ascl1/Mash1) in murine Müller glia cultures resulted in partial reprogramming of the cells to retinal progenitors. The microRNAs (miRNAs) miR-124-9-9* facilitate neuronal reprogramming of fibroblasts, but their role in glia reprogramming has not been reported. The aim of this study was to test whether (1) lentiviral gene transfer of miR-124-9-9* can reprogram Müller glia into retinal neurons and (2) miR-124-9-9* can improve Ascl1-induced reprogramming. Primary Müller glia cultures were generated from postnatal day (P) 11/12 mice, transduced with lentiviral particles, i.e., miR-124-9-9*-RFP, nonsense-RFP, Ascl1-GFP, or GFP-control. Gene expression and immunofluorescence analyses were performed within 3 weeks after infection. 1. Overexpression of miR-124-9-9* induced the expression of the proneural factor Ascl1 and additional markers of neurons, including TUJ1 and MAP2. 2. When Ascl1 and miR-124-9-9* were combined, 50 to 60% of Müller glia underwent neuronal reprogramming, whereas Ascl1 alone results in a 30 to 35% reprogramming rate. 3. Analysis of the miR-124-9-9* treated glial cells showed a reduction in the level of Ctdsp1 and Ptbp1, indicating a critical role for the REST pathway in the repression of neuronal genes in Müller glia. Our data further suggest that miR-124-9-9* and the REST complex may play a role in regulating the reprogramming of Müller glia to progenitors that underlies retinal regeneration in zebrafish.

Real-time Recordings of Migrating Cortical Neurons from GFP and Cre Recombinase Expressing Mice

The cerebral cortex is one of the most intricate regions of the brain that requires elaborate cell migration patterns for its development. Experimental observations show that projection neurons migrate radially within the cortical wall, whereas interneurons migrate along multiple tangential paths to reach the developing cortex. Tight regulation of the cell migration processes ensures proper positioning and functional integration of neurons to specific cerebral cortical circuits. Disruption of neuronal migration often leads to cortical dysfunction and/or malformation associated with neurological disorders. Unveiling the molecular control of neuron migration is thus fundamental to understanding the physiological or pathological development of the cerebral cortex. In this unit, protocols allowing detailed analysis of patterns of migration of both interneurons and projection neurons under different experimental conditions (i.e., loss or gain of function) are presented.

Essential Function of Dicer in Resolving DNA Damage in the Rapidly Dividing Cells of the Developing and Malignant Cerebellum

Maintenance of genomic integrity is critical during neurodevelopment, particularly in rapidly dividing cerebellar granule neuronal precursors that experience constitutive replication-associated DNA damage. As Dicer was recently recognized to have an unexpected function in the DNA damage response, we examined whether Dicer was important for preserving genomic integrity in the developing brain. We report that deletion of Dicer in the developing mouse cerebellum resulted in the accumulation of DNA damage leading to cerebellar progenitor degeneration, which was rescued with p53 deficiency; deletion of DGCR8 also resulted in similar DNA damage and cerebellar degeneration. Dicer deficiency also resulted in DNA damage and death in other rapidly dividing cells including embryonic stem cells and the malignant cerebellar progenitors in a mouse model of medulloblastoma. Together, these results identify an essential function of Dicer in resolving the spontaneous DNA damage that occurs during the rapid proliferation of developmental progenitors and malignant cells.

Molecular Pathways Underlying Projection Neuron Production and Migration during Cerebral Cortical Development

Glutamatergic neurons of the mammalian cerebral cortex originate from radial glia (RG) progenitors in the ventricular zone (VZ). During corticogenesis, neuroblasts migrate toward the pial surface using two different migration modes. One is multipolar (MP) migration with random directional movement, and the other is locomotion, which is a unidirectional movement guided by the RG fiber. After reaching their final destination, the neurons finalize their migration by terminal translocation, which is followed by maturation via dendrite extension to initiate synaptogenesis and thereby complete neural circuit formation. This switching of migration modes during cortical development is unique in mammals, which suggests that the RG-guided locomotion mode may contribute to the evolution of the mammalian neocortical 6-layer structure. Many factors have been reported to be involved in the regulation of this radial neuronal migration process. In general, the radial migration can be largely divided into four steps; (1) maintenance and departure from the VZ of neural progenitor cells, (2) MP migration and transition to bipolar cells, (3) RG-guided locomotion, and (4) terminal translocation and dendrite maturation. Among these, many different gene mutations or knockdown effects have resulted in failure of the MP to bipolar transition (step 2), suggesting that it is a critical step, particularly in radial migration. Moreover, this transition occurs at the subplate layer. In this review, we summarize recent advances in our understanding of the molecular mechanisms underlying each of these steps. Finally, we discuss the evolutionary aspects of neuronal migration in corticogenesis.

Peri-Implantation Hormonal Milieu: Elucidating Mechanisms of Adverse Neurodevelopmental Outcomes

While live births resulting from assisted reproductive technology (ART) exceed 1% of total births annually, the effect of ART on fetal development is not well understood. Data have demonstrated that IVF leads to alterations in DNA methylation and gene expression in the placenta that may have long-term effects on health and disease. Studies have linked adverse neurodevelopmental outcomes to ART, although human studies are inconclusive. In order to isolate the peri-implantation environment and its effects on brain development, we utilized a mouse model with and without superovulation and examined the effect of adult behavior as well as adult cortical neuronal density. Adult offspring of superovulated dams showed increased anxiety-like behavior compared to offspring of naturally mated dams (P < .05). There was no difference in memory and learning tests between the 2 groups. The adult brains from offspring of superovulated recipients had fewer neurons per field compared to naturally mated control offspring (P < .05). In order to examine potential pathways leading to these changes, we measured messenger RNA and microRNA (miRNA) expression in fetal brains at E18.5. Microarray analysis found that miRNAs miR-122, miR-144, and miR-211, involved in regulation of neuronal migration and differentiation, were downregulated in brains of offspring exposed to a superovulated environment(P < .05). There was also altered expression of genes involved in neuronal development. These results suggest that the peri-implantation environment can affect neurodevelopment and can lead to behavioral changes in adulthood. Human studies with long-term follow-up of children from ART are necessary to further investigate the influence of ART on the offspring.

MicroRNAs: Not "Fine-Tuners" but Key Regulators of Neuronal Development and Function

MicroRNAs (miRNAs) are a class of short non-coding RNAs that operate as prominent post-transcriptional regulators of eukaryotic gene expression. miRNAs are abundantly expressed in the brain of most animals and exert diverse roles. The anatomical and functional complexity of the brain requires the precise coordination of multilayered gene regulatory networks. The flexibility, speed, and reversibility of miRNA function provide precise temporal and spatial gene regulatory capabilities that are crucial for the correct functioning of the brain. Studies have shown that the underlying molecular mechanisms controlled by miRNAs in the nervous systems of invertebrate and vertebrate models are remarkably conserved in humans. We endeavor to provide insight into the roles of miRNAs in the nervous systems of these model organisms and discuss how such information may be used to inform regarding diseases of the human brain.

MicroRNA-124 Regulates Cell Specification in the Cochlea through Modulation of Sfrp4/5

The organ of Corti, the auditory organ of the mammalian inner ear, contains sensory hair cells and supporting cells that arise from a common sensory progenitor. The molecular bases allowing the specification of these progenitors remain elusive. In the present study, by combining microarray analyses with conditional deletion of Dicer in the developing inner ear, we identified that miR-124 controls cell fate in the developing organ of Corti. By targeting secreted frizzled-related protein 4 (Sfrp4) and Sfrp5, two inhibitors of the Wnt pathway, we showed that miR-124 controls the β-catenin-dependent and also the PCP-related non-canonical Wnt pathways that contribute to HC differentiation and polarization in the organ of Corti. Thus, our work emphasizes the importance of miR-124 as an epigenetic safeguard that fine-tunes the expression of genes critical for cell patterning during cochlear differentiation.