miRNA-Mediated Regulation of Adult Hippocampal Neurogenesis; Implications for Epilepsy

The emerging role of miRNAs in epilepsy: From molecular signatures to diagnostic potential

Epilepsy is a medical condition characterized by intermittent seizures accompanied by changes in consciousness. Epilepsy significantly impairs the daily functioning and overall well-being of affected individuals. Epilepsy is a chronic neurological disorder characterized by recurrent seizures resulting from various dysfunctions in brain activity. The molecular processes underlying changes in neuronal structure, impaired apoptotic responses in neurons, and disruption of regenerative pathways in glial cells in epilepsy remain unknown. MicroRNAs (miRNAs) play a crucial role in regulating apoptosis, autophagy, oxidative stress, neuroinflammation, and the body's regenerative and immune responses. miRNAs have been shown to influence many pathogenic processes in epilepsy including inflammatory responses, neuronal necrosis and apoptosis, dendritic growth, synaptic remodeling, and other processes related to the development of epilepsy. Therefore, the purpose of our current analysis was to determine the role of miRNAs in the etiology and progression of epilepsy. Furthermore, they have been examined for their potential application as biomarkers and therapeutic targets.

Extracellular vesicles in the glioblastoma microenvironment: A diagnostic and therapeutic perspective

Glioblastoma (GBM), is the most malignant form of gliomas and the most common and lethal primary brain tumor in adults. Conventional cancer treatments have limited to no efficacy on GBM. GBM cells respond and adapt to the surrounding brain parenchyma known as tumor microenvironment (TME) to promote tumor preservation. Among specific TME, there are 3 of particular interest for GBM biology: the perivascular niche, the subventricular zone neurogenic niche, and the immune microenvironment. GBM cells and TME cells present a reciprocal feedback which results in tumor maintenance. One way that these cells can communicate is through extracellular vesicles. These vesicles include exosomes and microvesicles that have the ability to carry both cancerous and non-cancerous cargo, such as miRNA, RNA, proteins, lipids, and DNA. In this review we will discuss the booming topic that is extracellular vesicles, and how they have the novelty to be a diagnostic and targetable vehicle for GBM.

microRNA-dependent regulation of gene expression in GABAergic interneurons

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

Extracellular vesicles throughout development: A potential roadmap for emerging glioblastoma therapies

Extracellular vesicles (EVs) are membrane-delimited vesicular bodies carrying different molecules, classified according to their size, density, cargo, and origin. Research on this topic has been actively growing through the years, as EVs are associated with critical pathological processes such as neurodegenerative diseases and cancer. Despite that, studies exploring the physiological functions of EVs are sparse, with particular emphasis on their role in organismal development, initial cell differentiation, and morphogenesis. In this review, we explore the topic of EVs from a developmental perspective, discussing their role in the earliest cell-fate decisions and neural tissue morphogenesis. We focus on the function of EVs through development to highlight possible conserved or novel processes that can impact disease progression. Specifically, we take advantage of what was learned about their role in development so far to discuss EVs impact on glioblastoma, a particular brain tumor of stem-cell origin and poor prognosis, and how their function can be hijacked to improve current therapies.

H19 silencing decreases kainic acid-induced hippocampus neuron injury via activating the PI3K/AKT pathway via the H19/miR-206 axis

Temporal lobe epilepsy (TLE) is the most common type of intractable epilepsy and is refractory to medications. However, the role and mechanism of H19 in regulating TLE remains largely undefined. Expression of H19 and miR-206 was detected using real-time quantitative PCR (RT-qPCR). Cell apoptosis, autophagy and inflammatory response were determined by flow cytometry, western blotting and enzyme-linked immunosorbent assay (ELISA). The interaction between H19 and miR-206 was predicted on the miRcode database and confirmed by luciferase reporter assay, RNA immunoprecipitation (RIP) and RNA pull-down. H19 was upregulated and miR-206 was downregulated in the rat hippocampus neurons after kainic acid (KA) treatment. Functionally, both H19 knockdown and miR-206 overexpression weakened KA-induced apoptosis, autophagy, inflammatory response, and oxidative stress in hippocampus neurons. Mechanically, the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway was activated by H19 knockdown and miR-206 was confirmed to be targeted and negatively regulated by H19. Moreover, downregulation of miR-206 could counteract the effects of H19 knockdown in KA-induced hippocampus neurons. Knockdown of H19 suppressed hippocampus neuronal apoptosis, autophagy and inflammatory response presumably through directly upregulating miR-206 and activating the PI3K/AKT signaling pathway.

Effects of exosomes on adult hippocampal neurogenesis and neuropsychiatric disorders

Exosomes are extracellular vesicles originating from the endosomal system, which are involved in intercellular substance transfer and cell waste elimination. Recent studies implicate the roles of exosomes in adult hippocampal neurogenesis, a process through which new granule cells are generated in the dentate gyrus, and which is closely related to mood and cognition, as well as psychiatric disorders. As such, exosomes are recognized as potential biomarkers of neurologic and psychiatric disorders. This review briefly introduces the synthesis and secretion mechanism of exosomes, and discuss the relationship between exosomes and hippocampal neurogenesis, and their roles in regulating depression, epilepsy and schizophrenia. Finally, we discuss the prospects of their application in diagnosing disorders of the central nervous system (CNS).

Adult Neural Stem Cell Regulation by Small Non-coding RNAs: Physiological Significance and Pathological Implications

The adult neurogenic niches are complex multicellular systems, receiving regulatory input from a multitude of intracellular, juxtacrine, and paracrine signals and biological pathways. Within the niches, adult neural stem cells (aNSCs) generate astrocytic and neuronal progeny, with the latter predominating in physiological conditions. The new neurons generated from this neurogenic process are functionally linked to memory, cognition, and mood regulation, while much less is known about the functional contribution of aNSC-derived newborn astrocytes and adult-born oligodendrocytes. Accumulating evidence suggests that the deregulation of aNSCs and their progeny can impact, or can be impacted by, aging and several brain pathologies, including neurodevelopmental and mood disorders, neurodegenerative diseases, and also by insults, such as epileptic seizures, stroke, or traumatic brain injury. Hence, understanding the regulatory underpinnings of aNSC activation, differentiation, and fate commitment could help identify novel therapeutic avenues for a series of pathological conditions. Over the last two decades, small non-coding RNAs (sncRNAs) have emerged as key regulators of NSC fate determination in the adult neurogenic niches. In this review, we synthesize prior knowledge on how sncRNAs, such as microRNAs (miRNAs) and piwi-interacting RNAs (piRNAs), may impact NSC fate determination in the adult brain and we critically assess the functional significance of these events. We discuss the concepts that emerge from these examples and how they could be used to provide a framework for considering aNSC (de)regulation in the pathogenesis and treatment of neurological diseases.

Exosomal microRNAs from mesenchymal stem/stromal cells: Biology and applications in neuroprotection

Mesenchymal stem/stromal cells (MSCs) are extensively studied as cell-therapy agents for neurological diseases. Recent studies consider exosomes secreted by MSCs as important mediators for MSCs’ neuroprotective functions. Exosomes transfer functional molecules including proteins, lipids, metabolites, DNAs, and coding and non-coding RNAs from MSCs to their target cells. Emerging evidence shows that exosomal microRNAs (miRNAs) play a key role in the neuroprotective properties of these exosomes by targeting several genes and regulating various biological processes. Multiple exosomal miRNAs have been identified to have neuroprotective effects by promoting neurogenesis, neurite remodeling and survival, and neuroplasticity. Thus, exosomal miRNAs have significant therapeutic potential for neurological disorders such as stroke, traumatic brain injury, and neuroinflammatory or neurodegenerative diseases and disorders. This review discusses the neuroprotective effects of selected miRNAs (miR-21, miR-17-92, miR-133, miR-138, miR-124, miR-30, miR146a, and miR-29b) and explores their mechanisms of action and applications for the treatment of various neurological disease and disorders. It also provides an overview of state-of-the-art bioengineering approaches for isolating exosomes, optimizing their yield and manipulating the miRNA content of their cargo to improve their therapeutic potential.

Oxidative-Signaling in Neural Stem Cell-Mediated Plasticity: Implications for Neurodegenerative Diseases

The adult mammalian brain is capable of generating new neurons from existing neural stem cells (NSCs) in a process called adult neurogenesis. This process, which is critical for sustaining cognition and mental health in the mature brain, can be severely hampered with ageing and different neurological disorders. Recently, it is believed that the beneficial effects of NSCs in the injured brain relies not only on their potential to differentiate and integrate into the preexisting network, but also on their secreted molecules. In fact, further insight into adult NSC function is being gained, pointing to these cells as powerful endogenous "factories" that produce and secrete a large range of bioactive molecules with therapeutic properties. Beyond anti-inflammatory, neurogenic and neurotrophic effects, NSC-derived secretome has antioxidant proprieties that prevent mitochondrial dysfunction and rescue recipient cells from oxidative damage. This is particularly important in neurodegenerative contexts, where oxidative stress and mitochondrial dysfunction play a significant role. In this review, we discuss the current knowledge and the therapeutic opportunities of NSC secretome for neurodegenerative diseases with a particular focus on mitochondria and its oxidative state.

Differentially expressed circRNA and functional pathways in the hippocampus of epileptic mice based on next-generation sequencing

Epilepsy is a clinical syndrome caused by the highly synchronized abnormal discharge of brain neurons. It has the characteristics of paroxysmal, transient, repetitive, and stereotyped. Circular RNAs (circRNAs) are a recently discovered type of noncoding RNA with diverse cellular functions related to their excellent stability; additionally, some circRNAs can bind and regulate microRNAs (miRNAs). The present study was designed to screen the differentially expressed circRNA in an acute seizure model of epilepsy in mice, analyze the related miRNA and mRNA, and study their participating functions and enrichment pathways. In order to obtain the differential expression of circRNA in epilepsy and infer their function, we used next-generation sequencing and found significantly different transcripts. CIRI (circRNA identifier) software was used to predict circRNA from the hippocampus cDNA, EdgeR was applied for the differential circRNA analysis between samples, and Cytoscape 3.7.2 software was used to draw the network diagram. A total of 10,388 differentially expressed circRNAs were identified, of which 34 were upregulated and 66 were downregulated. Among them, mm9_circ_008777 and mm9_circ_004424 were the key upregulated genes, and their expression in the epilepsy group was verified using Quantitative real-time PCR (QPCR). The analysis indicated that the extracted gene ontology terms and Kyoto Encyclopedia of Genes and Genomes pathways were closely related to several epilepsy-associated processes. This study determined that mm9_circ_008777 and mm9_circ_004424 are potential biomarkers of epilepsy, which play important roles in epilepsy-related pathways. These results could help improve the understanding of the biological mechanisms of circRNAs and epilepsy treatments.

Long-term chronic caloric restriction alters miRNA profiles in the brain of ageing mice

Calorie restriction (CR) has been shown to be one of the most effective methods in alleviating the effects of ageing and age-related diseases. Although the protective effects of CR have been reported, the exact molecular mechanism still needs to be clarified. This study aims to determine differentially expressed (DE) miRNAs and altered gene pathways due to long-term chronic (CCR) and intermittent (ICR) CR in the brain of mice to understand the preventive roles of miRNAs resulting from long-term CR. Ten weeks old mice were enrolled into three different dietary groups; ad libitum, CCR or ICR, and fed until 82 weeks of age. miRNAs were analysed using GeneChip 4.1 microarray and the target of DE miRNAs was determined using miRNA target databases. Out of a total 3,163 analysed miRNAs, 55 of them were differentially expressed either by different CR protocols or by ageing. Brain samples from the CCR group had increased expression levels of mmu-miR-713 while decreasing expression levels of mmu-miR-184-3p and mmu-miR-351-5p compared to the other dietary groups. Also, current results indicated that CCR showed better preventive effects than that of ICR. Thus, CCR may perform its protective effects by modulating these specific miRNAs since they are shown to play roles in neurogenesis, chromatin and histone regulation. In conclusion, these three miRNAs could be potential targets for neurodegenerative and ageing-related diseases and may play important roles in the protective effects of CR in the brain.

An Insight into Molecular Mechanisms and Novel Therapeutic Approaches in Epileptogenesis

Epilepsy is the second most common neurological disease with abnormal neural activity involving the activation of various intracellular signalling transduction mechanisms. The molecular and system biology mechanisms responsible for epileptogenesis are not well defined or understood. Neuroinflammation, neurodegeneration and Epigenetic modification elicit epileptogenesis. The excessive neuronal activities in the brain are associated with neurochemical changes underlying the deleterious consequences of excitotoxicity. The prolonged repetitive excessive neuronal activities extended to brain tissue injury by the activation of microglia regulating abnormal neuroglia remodelling and monocyte infiltration in response to brain lesions inducing axonal sprouting contributing to neurodegeneration. The alteration of various downstream transduction pathways resulted in intracellular stress responses associating endoplasmic reticulum, mitochondrial and lysosomal dysfunction, activation of nucleases, proteases mediated neuronal death. The recently novel pharmacological agents modulate various receptors like mTOR, COX-2, TRK, JAK-STAT, epigenetic modulators and neurosteroids are used for attenuation of epileptogenesis. Whereas the various molecular changes like the mutation of the cell surface, nuclear receptor and ion channels focusing on repetitive episodic seizures have been explored by preclinical and clinical studies. Despite effective pharmacotherapy for epilepsy, the inadequate understanding of precise mechanisms, drug resistance and therapeutic failure are the current fundamental problems in epilepsy. Therefore, the novel pharmacological approaches evaluated for efficacy on experimental models of epilepsy need to be identified and validated. In addition, we need to understand the downstream signalling pathways of new targets for the treatment of epilepsy. This review emphasizes on the current state of novel molecular targets as therapeutic approaches and future directions for the management of epileptogenesis. Novel pharmacological approaches and clinical exploration are essential to make new frontiers in curing epilepsy.

Epigenetic Regulation of the Hippocampus, with Special Reference to Radiation Exposure

The hippocampus is crucial in learning, memory and emotion processing, and is involved in the development of different neurological and neuropsychological disorders. Several epigenetic factors, including DNA methylation, histone modifications and non-coding RNAs, have been shown to regulate the development and function of the hippocampus, and the alteration of epigenetic regulation may play important roles in the development of neurocognitive and neurodegenerative diseases. This review summarizes the epigenetic modifications of various cell types and processes within the hippocampus and their resulting effects on cognition, memory and overall hippocampal function. In addition, the effects of exposure to radiation that may induce a myriad of epigenetic changes in the hippocampus are reviewed. By assessing and evaluating the current literature, we hope to prompt a more thorough understanding of the molecular mechanisms that underlie radiation-induced epigenetic changes, an area which can be further explored.

Extracellular Vesicles, Influential Players of Intercellular Communication within Adult Neurogenic Niches

Adult neurogenesis, involving the generation of functional neurons from adult neural stem cells (NSCs), occurs constitutively in discrete brain regions such as hippocampus, sub-ventricular zone (SVZ) and hypothalamus. The intrinsic structural plasticity of the neurogenic process allows the adult brain to face the continuously changing external and internal environment and requires coordinated interplay between all cell types within the specialized microenvironment of the neurogenic niche. NSC-, neuronal- and glia-derived factors, originating locally, regulate the balance between quiescence and self-renewal of NSC, their differentiation programs and the survival and integration of newborn cells. Extracellular Vesicles (EVs) are emerging as important mediators of cell-to-cell communication, representing an efficient way to transfer the biologically active cargos (nucleic acids, proteins, lipids) by which they modulate the function of the recipient cells. Current knowledge of the physiological role of EVs within adult neurogenic niches is rather limited. In this review, we will summarize and discuss EV-based cross-talk within adult neurogenic niches and postulate how EVs might play a critical role in the regulation of the neurogenic process.

Lysophosphatidic Acid Receptor 1 Specifically Labels Seizure-Induced Hippocampal Reactive Neural Stem Cells and Regulates Their Division

A population of neural stem cells (NSCs) dwelling in the dentate gyrus (DG) is able to generate neurons throughout adult life in the hippocampus of most mammals. These NSCs generate also astrocytes naturally and are capable of generating oligodendrocytes after gene manipulation. It has been more recently shown that adult hippocampal NSCs after epileptic seizures as well as subventricular zone NSCs after stroke can give rise to reactive astrocytes (RAs). In the hippocampus, the induction of seizures triggers the conversion of NSCs into reactive NSCs (React-NSCs) characterized by a drastic morphological transformation, abnormal migration, and massive activation or entry into the cell cycle to generate more React-NSCs that ultimately differentiate into RAs. In the search for tools to investigate the properties of React-NSCs, we have explored the LPA₁–green fluorescent protein (GFP) transgenic line of mice in which hippocampal NSCs are specifically labeled due to the expression of lysophosphatidic acid receptor 1 (LPA₁). We first addressed the validity of the transgene expression as true marker of LPA₁ expression and then demonstrated how, after seizures, LPA₁-GFP labeled exclusively React-NSCs for several weeks. Then React-NSCs lost LPA₁-GFP expression as neurons of the granule cell layer started to express it. Finally, we used knockout for LPA₁ transgenic mice to show that LPA₁ plays a functional role in the activation of React-NSCs. Thus, we confirmed that LPA₁-GFP expression is a valid tool to study both NSCs and React-NSCs and that the LPA₁ pathway could be a target in the intent to preserve NSCs after seizures.

Untangling biological factors influencing trajectory inference from single cell data

Advances in single-cell RNA sequencing over the past decade has shifted the discussion of cell identity toward the transcriptional state of the cell. While the incredible resolution provided by single-cell RNA sequencing has led to great advances in unraveling tissue heterogeneity and inferring cell differentiation dynamics, it raises the question of which sources of variation are important for determining cellular identity. Here we show that confounding biological sources of variation, most notably the cell cycle, can distort the inference of differentiation trajectories. We show that by factorizing single cell data into distinct sources of variation, we can select a relevant set of factors that constitute the core regulators for trajectory inference, while filtering out confounding sources of variation (e.g. cell cycle) which can perturb the inferred trajectory. Script are available publicly on https://github.com/mochar/cell_variation.

Circulating MicroRNAs from Serum Exosomes May Serve as a Putative Biomarker in the Diagnosis and Treatment of Patients with Focal Cortical Dysplasia

Focal cortical dysplasia (FCD) is a congenital malformation of cortical development where the cortical neurons located in the brain area fail to migrate in the proper formation. Epilepsy, particularly medically refractory epilepsy, is the most common clinical presentation for all types of FCD. This study aimed to explore the expression change of circulating miRNAs in patients with FCD from serum exosomes. A total of nine patients with FCD and four healthy volunteers were enrolled in this study. The serum exosomes were isolated from the peripheral blood of the subjects. Transmission electron microscopy (TEM) was used to identify the exosomes. Both exosomal markers and neuronal markers were detected by Western blotting analysis to prove that we could obtain central nervous system-derived exosomes from the circulation. The expression profiles of circulating exosomal miRNAs were assessed using next-generation sequencing analysis (NGS). We obtained a total of 107 miRNAs with dominant fold change (>2-fold) from both the annotated 5p-arm and 3p-arm of 2780 mature miRNAs. Based on the integrated platform of HMDD v3.2, miRway DB and DIANA-miRPath v3.0 online tools, and confirmed by MiRBase analysis, four potentially predicted miRNAs from serum exosomes in patients with FCD were identified, including miR194-2-5p, miR15a-5p, miR-132-3p, and miR-145-5p. All four miRNAs presented upregulated expression in patients with FCD compared with controls. Through Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis and pathway category of four target miRNAs, we found eight possible signaling pathways that may be related to FCD. Among them, we suggest that the mTOR signaling pathway, PI3K-Akt signaling pathway, p53 signaling pathway, and cell cycle regulation and TGF-beta signaling pathway are high-risk pathways that play a crucial role in the pathogenesis of FCD and refractory epilepsy. Our results suggest that the circulating miRNAs from exosomes may provide a potential biomarker for diagnostic, prognostic, and therapeutic adjuncts in patients with FCD and refractory epilepsy.

Modulation of MicroRNAs as a Potential Molecular Mechanism Involved in the Beneficial Actions of Physical Exercise in Alzheimer Disease

Alzheimer disease (AD) is one of the most common neurodegenerative diseases, affecting middle-aged and elderly individuals worldwide. AD pathophysiology involves the accumulation of beta-amyloid plaques and neurofibrillary tangles in the brain, along with chronic neuroinflammation and neurodegeneration. Physical exercise (PE) is a beneficial non-pharmacological strategy and has been described as an ally to combat cognitive decline in individuals with AD. However, the molecular mechanisms that govern the beneficial adaptations induced by PE in AD are not fully elucidated. MicroRNAs are small non-coding RNAs involved in the post-transcriptional regulation of gene expression, inhibiting or degrading their target mRNAs. MicroRNAs are involved in physiological processes that govern normal brain function and deregulated microRNA profiles are associated with the development and progression of AD. It is also known that PE changes microRNA expression profile in the circulation and in target tissues and organs. Thus, this review aimed to identify the role of deregulated microRNAs in the pathophysiology of AD and explore the possible role of the modulation of microRNAs as a molecular mechanism involved in the beneficial actions of PE in AD.

Genetic and molecular basis of epilepsy-related cognitive dysfunction

Epilepsy is a common neurological disease characterized by recurrent seizures. About 70 million people were affected by epilepsy or epileptic seizures. Epilepsy is a complicated complex or symptomatic syndromes induced by structural, functional, and genetic causes. Meanwhile, several comorbidities are accompanied by epileptic seizures. Cognitive dysfunction is a long-standing complication associated with epileptic seizures, which severely impairs quality of life. Although the definitive pathogenic mechanisms underlying epilepsy-related cognitive dysfunction remain unclear, accumulating evidence indicates that multiple risk factors are probably involved in the development and progression of cognitive dysfunction in patients with epilepsy. These factors include the underlying etiology, recurrent seizures or status epilepticus, structural damage that induced secondary epilepsy, genetic variants, and molecular alterations. In this review, we summarize several theories that may explain the genetic and molecular basis of epilepsy-related cognitive dysfunction.

Omega-3 Docosahexaenoic Acid Is a Mediator of Fate-Decision of Adult Neural Stem Cells

The mammalian brain is enriched with lipids that serve as energy catalyzers or secondary messengers of essential signaling pathways. Docosahexaenoic acid (DHA) is an omega-3 fatty acid synthesized de novo at low levels in humans, an endogenous supply from its precursors, and is mainly incorporated from nutrition, an exogeneous supply. Decreased levels of DHA have been reported in the brains of patients with neurodegenerative diseases. Preventing this decrease or supplementing the brain with DHA has been considered as a therapy for the DHA brain deficiency that could be linked with neuronal death or neurodegeneration. The mammalian brain has, however, a mechanism of compensation for loss of neurons in the brain: neurogenesis, the birth of neurons from neural stem cells. In adulthood, neurogenesis is still present, although at a slower rate and with low efficiency, where most of the newly born neurons die. Neural stem/progenitor cells (NSPCs) have been shown to require lipids for proper metabolism for proliferation maintenance and neurogenesis induction. Recent studies have focused on the effects of these essential lipids on the neurobiology of NSPCs. This review aimed to introduce the possible use of DHA to impact NSPC fate-decision as a therapy for neurodegenerative diseases.

The orphan nuclear receptor TLX: an emerging master regulator of cross-talk between microglia and neural precursor cells

Neuroinflammation and neurogenesis have both been the subject of intensive investigation over the past 20 years. The sheer complexity of their regulation and their ubiquity in various states of health and disease have sometimes obscured the progress that has been made in unraveling their mechanisms and regulation. A recent study by Kozareva et al. (Neuronal Signaling (2019) 3), provides evidence that the orphan nuclear receptor TLX is central to communication between microglia and neural precursor cells and could help us understand how inflammation, mediated by microglia, influences the development of new neurons in the adult hippocampus. Here, we put recent studies on TLX into the context of what is known about adult neurogenesis and microglial activation in the brain, along with the many hints that these processes must be inter-related.

MicroRNAs and Regeneration in Animal Models of CNS Disorders

microRNAs (miRNAs) are recently identified small RNA molecules that regulate gene expression and significantly influence the essential cellular processes associated with CNS repair after trauma and neuropathological conditions including stroke and neurodegenerative disorders. A number of specific miRNAs are implicated in regulating the development and propagation of CNS injury, as well as its subsequent regeneration. The review focuses on the functions of the miRNAs and their role in brain recovery following CNS damage. The article introduces a brief description of miRNA biogenesis and mechanisms of miRNA-induced gene suppression, followed by an overview of miRNAs involved in the processes associated with CNS repair, including neuroprotection, neuronal plasticity and axonal regeneration, vascular reorganization, neuroinflammation, and endogenous stem cell activation. Specific emphasis is placed on the role of multifunctional miRNA miR-155, as it appears to be involved in multiple neurorestorative processes during different CNS pathologies. In association with our own studies on miR-155, I introduce a new and unexplored approach to cerebral regeneration: regulation of brain tissue repair through a direct modulation of specific miRNA activity. The review concludes with discussion on the challenges and the future potential of miRNA-based therapeutic approaches to CNS repair.

Co-administration of Anti microRNA-124 and -137 Oligonucleotides Prevents Hippocampal Neural Stem Cell Loss Upon Non-convulsive Seizures

Convulsive seizures promote adult hippocampal neurogenesis (AHN) through a transient activation of neural stem/progenitor cells (NSPCs) in the subgranular zone (SGZ) of the dentate gyrus (DG). However, in a significant population of epilepsy patients, non-convulsive seizures (ncSZ) are observed. The response of NSPCs to non-convulsive seizure induction has not been characterized before. We here studied first the short-term effects of controlled seizure induction on NSPCs fate and identity. We induced seizures of controlled intensity by intrahippocampally injecting increasing doses of the chemoconvulsant kainic acid (KA) and analyzed their effect on subdural EEG recordings, hippocampal structure, NSPC proliferation and the number and location of immature neurons shortly after seizure onset. After establishing a KA dose that elicits ncSZ, we then analyzed the effects of ncSZ on NSPC proliferation and NSC identity in the hippocampus. ncSZ specifically triggered neuroblast proliferation, but did not induce proliferation of NSPCs in the SGZ, 3 days post seizure onset. However, ncSZ induced significant changes in NSPC composition in the hippocampus, including the generation of reactive NSCs. Interestingly, intrahippocampal injection of a combination of two anti microRNA oligonucleotides targeting microRNA-124 and -137 normalized neuroblast proliferation and prevented NSC loss in the DG upon ncSZ. Our results show for the first time that ncSZ induce significant changes in neuroblast proliferation and NSC composition. Simultaneous antagonism of both microRNA-124 and -137 rescued seizure-induced alterations in NSPC, supporting their coordinated action in the regulation of NSC fate and proliferation and their potential for future seizure therapies.

Possible epigenetic regulatory effect of dysregulated circular RNAs in epilepsy

Circular RNAs (circRNAs) involve in the epigenetic regulation and its major mechanism is the sequestration of the target micro RNAs (miRNAs). We hypothesized that circRNAs might be related with the pathophysiology of chronic epilepsy and evaluated the altered circRNA expressions and their possible regulatory effects on their target miRNAs and mRNAs in a mouse epilepsy model. The circRNA expression profile in the hippocampus of the pilocarpine mice was analyzed and compared with control. The correlation between the expression of miRNA binding sites (miRNA response elements, MRE) in the dysregulated circRNAs and the expression of their target miRNAs was evaluated. As miRNAs also inhibit their target mRNAs, circRNA–miRNA-mRNA regulatory network, comprised of dysregulated RNAs that targets one another were searched. For the identified networks, bioinformatics analyses were performed. As the result, Forty-three circRNAs were dysregulated in the hippocampus (up-regulated, 26; down-regulated, 17). The change in the expression of MRE in those circRNAs negatively correlated with the change in the relevant target miRNA expression (r = -0.461, P<0.001), supporting that circRNAs inhibit their target miRNA. 333 dysregulated circRNA–miRNA-mRNA networks were identified. Gene ontology and pathway analyses demonstrated that the up-regulated mRNAs in those networks were closely related to the major processes in epilepsy. Among them, STRING analysis identified 37 key mRNAs with abundant (≥4) interactions with other dysregulated target mRNAs. The dysregulation of the circRNAs which had multiple interactions with key mRNAs were validated by PCR. We concluded that dysregulated circRNAs might have a pathophysiologic role in chronic epilepsy by regulating multiple disease relevant mRNAs via circRNA−miRNA−mRNA interactions.

MicroRNAs Engage in Complex Circuits Regulating Adult Neurogenesis

The finding that the adult mammalian brain is still capable of producing neurons has ignited a new field of research aiming to identify the molecular mechanisms regulating adult neurogenesis. An improved understanding of these mechanisms could lead to the development of novel approaches to delay cognitive decline and facilitate neuroregeneration in the adult human brain. Accumulating evidence suggest microRNAs (miRNAs), which represent a class of post-transcriptional gene expression regulators, as crucial part of the gene regulatory networks governing adult neurogenesis. This review attempts to illustrate how miRNAs modulate key processes in the adult neurogenic niche by interacting with each other and with transcriptional regulators. We discuss the function of miRNAs in adult neurogenesis following the life-journey of an adult-born neuron from the adult neural stem cell (NSCs) compartment to its final target site. We first survey how miRNAs control the initial step of adult neurogenesis, that is the transition of quiescent to activated proliferative adult NSCs, and then go on to discuss the role of miRNAs to regulate neuronal differentiation, survival, and functional integration of the newborn neurons. In this context, we highlight miRNAs that converge on functionally related targets or act within cross talking gene regulatory networks. The cooperative manner of miRNA action and the broad target repertoire of each individual miRNA could make the miRNA system a promising tool to gain control on adult NSCs in the context of therapeutic approaches.