MicroRNAs Overcome Cell Fate Barrier by Reducing EZH2-Controlled REST Stability during Neuronal Conversion of Human Adult Fibroblasts

Investigating Müller glia reprogramming in mice: a retrospective of the last decade, and a look to the future

Müller glia, as prominent glial cells within the retina, plays a significant role in maintaining retinal homeostasis in both healthy and diseased states. In lower vertebrates like zebrafish, these cells assume responsibility for spontaneous retinal regeneration, wherein endogenous Müller glia undergo proliferation, transform into Müller glia-derived progenitor cells, and subsequently regenerate the entire retina with restored functionality. Conversely, Müller glia in the mouse and human retina exhibit limited neural reprogramming. Müller glia reprogramming is thus a promising strategy for treating neurodegenerative ocular disorders. Müller glia reprogramming in mice has been accomplished with remarkable success, through various technologies. Advancements in molecular, genetic, epigenetic, morphological, and physiological evaluations have made it easier to document and investigate the Müller glia programming process in mice. Nevertheless, there remain issues that hinder improving reprogramming efficiency and maturity. Thus, understanding the reprogramming mechanism is crucial toward exploring factors that will improve Müller glia reprogramming efficiency, and for developing novel Müller glia reprogramming strategies. This review describes recent progress in relatively successful Müller glia reprogramming strategies. It also provides a basis for developing new Müller glia reprogramming strategies in mice, including epigenetic remodeling, metabolic modulation, immune regulation, chemical small-molecules regulation, extracellular matrix remodeling, and cell-cell fusion, to achieve Müller glia reprogramming in mice.

Post-transcriptional mechanisms controlling neurogenesis and direct neuronal reprogramming

Neurogenesis is a tightly regulated process in time and space both in the developing embryo and in adult neurogenic niches. A drastic change in the transcriptome and proteome of radial glial cells or neural stem cells towards the neuronal state is achieved due to sophisticated mechanisms of epigenetic, transcriptional, and post-transcriptional regulation. Understanding these neurogenic mechanisms is of major importance, not only for shedding light on very complex and crucial developmental processes, but also for the identification of putative reprogramming factors, that harbor hierarchically central regulatory roles in the course of neurogenesis and bare thus the capacity to drive direct reprogramming towards the neuronal fate. The major transcriptional programs that orchestrate the neurogenic process have been the focus of research for many years and key neurogenic transcription factors, as well as repressor complexes, have been identified and employed in direct reprogramming protocols to convert non-neuronal cells, into functional neurons. The post-transcriptional regulation of gene expression during nervous system development has emerged as another important and intricate regulatory layer, strongly contributing to the complexity of the mechanisms controlling neurogenesis and neuronal function. In particular, recent advances are highlighting the importance of specific RNA binding proteins that control major steps of mRNA life cycle during neurogenesis, such as alternative splicing, polyadenylation, stability, and translation. Apart from the RNA binding proteins, microRNAs, a class of small non-coding RNAs that block the translation of their target mRNAs, have also been shown to play crucial roles in all the stages of the neurogenic process, from neural stem/progenitor cell proliferation, neuronal differentiation and migration, to functional maturation. Here, we provide an overview of the most prominent post-transcriptional mechanisms mediated by RNA binding proteins and microRNAs during the neurogenic process, giving particular emphasis on the interplay of specific RNA binding proteins with neurogenic microRNAs. Taking under consideration that the molecular mechanisms of neurogenesis exert high similarity to the ones driving direct neuronal reprogramming, we also discuss the current advances in in vitro and in vivo direct neuronal reprogramming approaches that have employed microRNAs or RNA binding proteins as reprogramming factors, highlighting the so far known mechanisms of their reprogramming action.

Androgen deprivation induces neuroendocrine phenotypes in prostate cancer cells through CREB1/EZH2-mediated downregulation of REST

Although effective initially, prolonged androgen deprivation therapy (ADT) promotes neuroendocrine differentiation (NED) and prostate cancer (PCa) progression. It is incompletely understood how ADT transcriptionally induces NE genes in PCa cells. CREB1 and REST are known to positively and negatively regulate neuronal gene expression in the brain, respectively. No direct link between these two master neuronal regulators has been elucidated in the NED of PCa. We show that REST mRNA is downregulated in NEPC cell and mouse models, as well as in patient samples. Phenotypically, REST overexpression increases ADT sensitivity, represses NE genes, inhibits colony formation in culture, and xenograft tumor growth of PCa cells. As expected, ADT downregulates REST in PCa cells in culture and in mouse xenografts. Interestingly, CREB1 signaling represses REST expression. In studying the largely unclear mechanism underlying transcriptional repression of REST by ADT, we found that REST is a direct target of EZH2 epigenetic repression. Finally, genetic rescue experiments demonstrated that ADT induces NED through EZH2's repression of REST, which is enhanced by ADT-activated CREB1 signaling. In summary, our study has revealed a key pathway underlying NE gene upregulation by ADT, as well as established novel relationships between CREB1 and REST, and between EZH2 and REST, which may also have implications in other cancer types and in neurobiology.

Small molecules reprogram reactive astrocytes into neuronal cells in the injured adult spinal cord

INTRODUCTION

Ectopic expression of transcription factor-mediated in vivo neuronal reprogramming provides promising strategy to compensate for neuronal loss, while its further clinical application may be hindered by delivery and safety concerns. As a novel and attractive alternative, small molecules may offer a non-viral and non-integrative chemical approach for reprogramming cell fates. Recent definitive evidences have shown that small molecules can convert non-neuronal cells into neurons in vitro. However, whether small molecules alone can induce neuronal reprogramming in vivo remains largely unknown.

OBJECTIVES

To identify chemical compounds that can induce in vivo neuronal reprogramming in the adult spinal cord.

METHODS

Immunocytochemistry, immunohistochemistry, qRT-PCR and fate-mapping are performed to analyze the role of small molecules in reprogramming astrocytes into neuronal cells in vitro and in vivo.

RESULTS

By screening, we identify a chemical cocktail with only two chemical compounds that can directly and rapidly reprogram cultured astrocytes into neuronal cells. Importantly, this chemical cocktail can also successfully trigger neuronal reprogramming in the injured adult spinal cord without introducing exogenous genetic factors. These chemically induced cells showed typical neuronal morphologies and neuron-specific marker expression and could become mature and survive for more than 12 months. Lineage tracing indicated that the chemical compound-converted neuronal cells mainly originated from post-injury spinal reactive astrocytes.

CONCLUSION

Our proof-of-principle study demonstrates that in vivo glia-to-neuron conversion can be manipulated in a chemical compound-based manner. Albeit our current chemical cocktail has a lowreprogramming efficiency, it will bring in vivo cell fate reprogramming closer to clinical application in brain and spinal cord repair. Future studies should focus on further refining our chemical cocktail and reprogramming approach to boost the reprogramming efficiency.

Patient-derived neuron model: Capturing age-dependent adult-onset degenerative pathology in Huntington's disease

MicroRNAs play a crucial role in directly reprogramming (converting) human fibroblasts into neurons. Specifically, miR-9/9* and miR-124 (miR-9/9*-124) display neurogenic and cell fate-switching activities when ectopically expressed in human fibroblasts by erasing fibroblast identity and inducing a pan-neuronal state. These converted neurons maintain the biological age of the starting fibroblasts and thus provide a human neuron-based platform to study cellular properties in aged neurons and model adult-onset neurodegenerative disorders using patient-derived cells. Furthermore, the expression of striatal-enriched transcription factors in conjunction with miR-9/9*-124 guides the identity of medium spiny neurons (MSNs), the primary targets in Huntington's disease (HD). Converted MSNs from HD patient-derived fibroblasts (HD-MSNs) can replicate HD-related phenotypes including neurodegeneration associated with age-related declines in critical cellular functions such as autophagy. Here, we review the role of microRNAs in the direct conversion of patient-derived fibroblasts into MSNs and the practical application of converted HD-MSNs as a model for studying adult-onset neuropathology in HD. We provide valuable insights into age-related, cell-intrinsic changes contributing to neurodegeneration in HD-MSNs. Ultimately, we address a comprehensive understanding of the complex molecular landscape underlying HD pathology, offering potential avenues for therapeutic application.

MiR-128-3p - a gray eminence of the human central nervous system

MicroRNA-128-3p (miR-128-3p) is a versatile molecule with multiple functions in the physiopathology of the human central nervous system. Perturbations of miR-128-3p, which is enriched in the brain, contribute to a plethora of neurodegenerative disorders, brain injuries, and malignancies, as this miRNA is a crucial regulator of gene expression in the brain, playing an essential role in the maintenance and function of cells stemming from neuronal lineage. However, the differential expression of miR-128-3p in pathologies underscores the importance of the balance between its high and low levels. Significantly, numerous reports pointed to miR-128-3p as one of the most depleted in glioblastoma, implying it is a critical player in the disease's pathogenesis and thus may serve as a therapeutic agent for this most aggressive form of brain tumor. In this review, we summarize the current knowledge of the diverse roles of miR-128-3p. We focus on its involvement in the neurogenesis and pathophysiology of malignant and neurodegenerative diseases. We also highlight the promising potential of miR-128-3p as an antitumor agent for the future therapy of human cancers, including glioblastoma, and as the linchpin of brain development and function, potentially leading to the development of new therapies for neurological conditions.

Canalizing cell fate by transcriptional repression

Precision in the establishment and maintenance of cellular identities is crucial for the development of multicellular organisms and requires tight regulation of gene expression. While extensive research has focused on understanding cell type-specific gene activation, the complex mechanisms underlying the transcriptional repression of alternative fates are not fully understood. Here, we provide an overview of the repressive mechanisms involved in cell fate regulation. We discuss the molecular machinery responsible for suppressing alternative fates and highlight the crucial role of sequence-specific transcription factors (TFs) in this process. Depletion of these TFs can result in unwanted gene expression and increased cellular plasticity. We suggest that these TFs recruit cell type-specific repressive complexes to their cis-regulatory elements, enabling them to modulate chromatin accessibility in a context-dependent manner. This modulation effectively suppresses master regulators of alternative fate programs and their downstream targets. The modularity and dynamic behavior of these repressive complexes enables a limited number of repressors to canalize and maintain major and minor cell fate decisions at different stages of development.

Transcription Factors in Brain Regeneration: A Potential Novel Therapeutic Target

Transcription factors play a crucial role in providing identity to each cell population. To maintain cell identity, it is essential to balance the expression of activator and inhibitor transcription factors. Cell plasticity and reprogramming offer great potential for future therapeutic applications, as they can regenerate damaged tissue. Specific niche factors can modify gene expression and differentiate or transdifferentiate the target cell to the required fate. Ongoing research is being carried out on the possibilities of transcription factors in regenerating neurons, with neural stem cells (NSCs) being considered the preferred cells for generating new neurons due to their epigenomic and transcriptome memory. NEUROD1/ASCL1, BRN2, MYTL1, and other transcription factors can induce direct reprogramming of somatic cells, such as fibroblasts, into neurons. However, the molecular biology of transcription factors in reprogramming and differentiation still needs to be fully understood.

A cutting-edge strategy for spinal cord injury treatment: resident cellular transdifferentiation

Spinal cord injury causes varying degrees of motor and sensory function loss. However, there are no effective treatments for spinal cord repair following an injury. Moreover, significant preclinical advances in bioengineering and regenerative medicine have not yet been translated into effective clinical therapies. The spinal cord's poor regenerative capacity makes repairing damaged and lost neurons a critical treatment step. Reprogramming-based neuronal transdifferentiation has recently shown great potential in repair and plasticity, as it can convert mature somatic cells into functional neurons for spinal cord injury repair in vitro and in vivo, effectively halting the progression of spinal cord injury and promoting functional improvement. However, the mechanisms of the neuronal transdifferentiation and the induced neuronal subtypes are not yet well understood. This review analyzes the mechanisms of resident cellular transdifferentiation based on a review of the relevant recent literature, describes different molecular approaches to obtain different neuronal subtypes, discusses the current challenges and improvement methods, and provides new ideas for exploring therapeutic approaches for spinal cord injury.

Autonomous action and cooperativity between the ONECUT2 transcription factor and its 3' untranslated region

The transcription factor ONECUT2 (OC2) is a master transcriptional regulator operating in metastatic castration-resistant prostate cancer that suppresses androgen receptor activity and promotes neural differentiation and tumor cell survival. OC2 mRNA possesses an unusually long (14,575 nt), evolutionarily conserved 3' untranslated region (3' UTR) with many microRNA binding sites, including up to 26 miR-9 sites. This is notable because miR-9 targets many of the same genes regulated by the OC2 protein. Paradoxically, OC2 expression is high in tissues with high miR-9 expression. The length and complex secondary structure of OC2 mRNA suggests that it is a potent master competing endogenous RNA (ceRNA) capable of sequestering miRNAs. Here, we describe a novel role for OC2 3' UTR in lethal prostate cancer consistent with a function as a ceRNA. A plausible ceRNA network in OC2-driven tumors was constructed computationally and then confirmed in prostate cancer cell lines. Genes regulated by OC2 3' UTR exhibited high overlap (up to 45%) with genes driven by the overexpression of the OC2 protein in the absence of 3' UTR, indicating a cooperative functional relationship between the OC2 protein and its 3' UTR. These overlapping networks suggest an evolutionarily conserved mechanism to reinforce OC2 transcription by protection of OC2-regulated mRNAs from miRNA suppression. Both the protein and 3' UTR showed increased polycomb-repressive complex activity. The expression of OC2 3' UTR mRNA alone (without protein) dramatically increased the metastatic potential by in vitro assays. Additionally, OC2 3' UTR increased the expression of Aldo-Keto reductase and UDP-glucuronyl transferase family genes responsible for altering the androgen synthesis pathway. ONECUT2 represents the first-described dual-modality transcript that operates as both a key transcription factor driving castration-resistant prostate cancer and a master ceRNA that promotes and protects the same transcriptional network.

Epi-miRNAs: Modern mediators of methylation status in human cancers

Methylation of the fundamental macromolecules, DNA/RNA, and proteins, is remarkably abundant, evolutionarily conserved, and functionally significant in cellular homeostasis and normal tissue/organism development. Disrupted methylation imprinting is strongly linked to loss of the physiological equilibrium and numerous human pathologies, and most importantly to carcinogenesis, tumor heterogeneity, and cancer progression. Mounting recent evidence has documented the active implication of miRNAs in the orchestration of the multicomponent cellular methylation machineries and the deregulation of methylation profile in the epigenetic, epitranscriptomic, and epiproteomic levels during cancer onset and progression. The elucidation of such regulatory networks between the miRNome and the cellular methylation machineries has led to the emergence of a novel subclass of miRNAs, namely "epi-miRNAs" or "epi-miRs." Herein, we have summarized the existing knowledge on the functional role of epi-miRs in the methylation dynamic landscape of human cancers and their clinical utility in modern cancer diagnostics and tailored therapeutics. This article is categorized under: RNA in Disease and Development > RNA in Disease.

Co-culture of iNeurons with primary human skin cells provides a reliable model to examine intercellular communication

OBJECTIVE

The skin is a sensory organ, densely innervated with various types of sensory nerve endings, capable of discriminating touch, environmental sensations, proprioception, and physical affection. Neurons communication with skin cells confer to the tissue the ability to undergo adaptive modifications during response to environmental changes or wound healing after injury. Thought for a long time to be dedicated to the central nervous system, the glutamatergic neuromodulation is increasingly described in peripheral tissues. Glutamate receptors and transporters have been identified in the skin. There is a strong interest in understanding the communication between keratinocytes and neurons, as the close contacts with intra-epidermal nerve fibers is a favorable site for efficient communication. To date, various coculture models have been described. However, these models were based on non-human or immortalized cell line. Even the use of induced pluripotent stem cells (iPSCs) is posing limitations because of epigenetic variations during the reprogramming process.

METHODS

In this study, we performed small molecule-driven direct conversion of human skin primary fibroblasts into induced neurons (iNeurons).

RESULTS

The resulting iNeurons were mature, showed pan-neuronal markers, and exhibited a glutamatergic subtype and C-type fibers characteristics. Autologous coculture of iNeurons with human primary keratinocytes, fibroblasts, and melanocytes was performed and remained healthy for many days, making possible to study the establishment of intercellular interactions.

CONCLUSION

Here, we report that iNeurons and primary skin cells established contacts, with neurite ensheathment by keratinocytes, and demonstrated that iNeurons cocultured with primary skin cells provide a reliable model to examine intercellular communication.

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.

Age-related Huntington's disease progression modeled in directly reprogrammed patient-derived striatal neurons highlights impaired autophagy

Huntington's disease (HD) is an inherited neurodegenerative disorder with adult-onset clinical symptoms, but the mechanism by which aging drives the onset of neurodegeneration in patients with HD remains unclear. In this study we examined striatal medium spiny neurons (MSNs) directly reprogrammed from fibroblasts of patients with HD to model the age-dependent onset of pathology. We found that pronounced neuronal death occurred selectively in reprogrammed MSNs from symptomatic patients with HD (HD-MSNs) compared to MSNs derived from younger, pre-symptomatic patients (pre-HD-MSNs) and control MSNs from age-matched healthy individuals. We observed age-associated alterations in chromatin accessibility between HD-MSNs and pre-HD-MSNs and identified miR-29b-3p, whose age-associated upregulation promotes HD-MSN degeneration by impairing autophagic function through human-specific targeting of the STAT3 3' untranslated region. Reducing miR-29b-3p or chemically promoting autophagy increased the resilience of HD-MSNs against neurodegeneration. Our results demonstrate miRNA upregulation with aging in HD as a detrimental process driving MSN degeneration and potential approaches for enhancing autophagy and resilience of HD-MSNs.

MicroRNA Roles in Cell Reprogramming Mechanisms

Cell reprogramming is a groundbreaking technology that, in few decades, generated a new paradigm in biomedical science. To date we can use cell reprogramming to potentially generate every cell type by converting somatic cells and suitably modulating the expression of key transcription factors. This approach can be used to convert skin fibroblasts into pluripotent stem cells as well as into a variety of differentiated and medically relevant cell types, including cardiomyocytes and neural cells. The molecular mechanisms underlying such striking cell phenotypes are still largely unknown, but in the last decade it has been proven that cell reprogramming approaches are significantly influenced by non-coding RNAs. Specifically, this review will focus on the role of microRNAs in the reprogramming processes that lead to the generation of pluripotent stem cells, neurons, and cardiomyocytes. As highlighted here, non-coding RNA-forced expression can be sufficient to support some cell reprogramming processes, and, therefore, we will also discuss how these molecular determinants could be used in the future for biomedical purposes.

Direct neuronal reprogramming: Fast forward from new concepts toward therapeutic approaches

Differentiated cells have long been considered fixed in their identity. However, about 20 years ago, the first direct conversion of glial cells into neurons in vitro opened the field of "direct neuronal reprogramming." Since then, neuronal reprogramming has achieved the generation of fully functional, mature neurons with remarkable efficiency, even in diseased brain environments. Beyond their clinical implications, these discoveries provided basic insights into crucial mechanisms underlying conversion of specific cell types into neurons and maintenance of neuronal identity. Here we discuss such principles, including the importance of the starter cell for shaping the outcome of neuronal reprogramming. We further highlight technical concerns for in vivo reprogramming and propose a code of conduct to avoid artifacts and pitfalls. We end by pointing out next challenges for development of less invasive cell replacement therapies for humans.

Current Approaches and Molecular Mechanisms for Directly Reprogramming Fibroblasts Into Neurons and Dopamine Neurons

Parkinson's disease is mainly caused by specific degeneration of dopaminergic neurons (DA neurons) in the substantia nigra of the middle brain. Over the past two decades, transplantation of neural stem cells (NSCs) from fetal brain-derived neural stem cells (fNSCs), human embryonic stem cells (hESCs), and induced pluripotent stem cells (iPSCs) has been shown to improve the symptoms of motor dysfunction in Parkinson's disease (PD) animal models and PD patients significantly. However, there are ethical concerns with fNSCs and hESCs and there is an issue of rejection by the immune system, and the iPSCs may involve tumorigenicity caused by the integration of the transgenes. Recent studies have shown that somatic fibroblasts can be directly reprogrammed to NSCs, neurons, and specific dopamine neurons. Directly induced neurons (iN) or induced DA neurons (iDANs) from somatic fibroblasts have several advantages over iPSC cells. The neurons produced by direct transdifferentiation do not pass through a pluripotent state. Therefore, direct reprogramming can generate patient-specific cells, and it can overcome the safety problems of rejection by the immune system and teratoma formation related to hESCs and iPSCs. However, there are some critical issues such as the low efficiency of direct reprogramming, biological functions, and risks from the directly converted neurons, which hinder their clinical applications. Here, the recent progress in methods, mechanisms, and future challenges of directly reprogramming somatic fibroblasts into neurons or dopamine neurons were summarized to speed up the clinical translation of these directly converted neural cells to treat PD and other neurodegenerative diseases.

Multifaceted Regulation of MicroRNA Biogenesis: Essential Roles and Functional Integration in Neuronal and Glial Development

MicroRNAs (miRNAs) are small, non-coding RNAs that function as endogenous gene silencers. Soon after the discovery of miRNAs, a subset of brain-enriched and brain-specific miRNAs were identified and significant advancements were made in delineating miRNA function in brain development. However, understanding the molecular mechanisms that regulate miRNA biogenesis in normal and diseased brains has become a prevailing challenge. Besides transcriptional regulation of miRNA host genes, miRNA processing intermediates are subjected to multifaceted regulation by canonical miRNA processing enzymes, RNA binding proteins (RBPs) and epitranscriptomic modifications. Further still, miRNA activity can be regulated by the sponging activity of other non-coding RNA classes, namely circular RNAs (circRNAs) and long non-coding RNAs (lncRNAs). Differential abundance of these factors in neuronal and glial lineages partly underlies the spatiotemporal expression and function of lineage-specific miRNAs. Here, we review the continuously evolving understanding of the regulation of neuronal and glial miRNA biogenesis at the transcriptional and posttranscriptional levels and the cooperativity of miRNA species in targeting key mRNAs to drive lineage-specific development. In addition, we review dysregulation of neuronal and glial miRNAs and the detrimental impacts which contribute to developmental brain disorders.

Nutraceutical regulation of miRNAs involved in neurodegenerative diseases and brain cancers

The human brain is a well-connected, intricate network of neurons and supporting glial cells. Neurodegenerative diseases arise as a consequence of extensive loss of neuronal cells leading to disruption of their natural structure and function. On the contrary, rapid proliferation and growth of glial as well as neuronal cells account for the occurrence of malignancy in brain. In both cases, the molecular microenvironment holds pivotal importance in the progression of the disease. MicroRNAs (miRNA) are one of the major components of the molecular microenvironment. miRNAs are small, noncoding RNAs that control gene expression post-transcriptionally. As compared to other tissues, the brain expresses a substantially high number of miRNAs. In the early stage of neurodegeneration, miRNA expression upregulates, while in oncogenesis, miRNA expression is gradually lost. Neurodegeneration and brain cancer is presumed to be under the influence of identical pathways of cell proliferation, differentiation and cell death which are tightly regulated by miRNAs. It has been confirmed experimentally that miRNA expression can be regulated by nutraceuticals - macronutrients, micronutrients or natural products derived from food; thereby making dietary supplements immensely significant for targeting miRNAs having altered expression patterns during neurodegeneration or oncogenesis. In this review, we will discuss in detail, about the common miRNAs involved in brain cancers and neurodegenerative diseases along with the comprehensive list of miRNAs involved separately in both pathological conditions. We will also discuss the role of nutraceuticals in the regulation of those miRNAs which are involved in both of these pathological conditions.

Tumor Suppressive Effects of miR-124 and Its Function in Neuronal Development

MicroRNA-124 (miR-124) is strongly expressed in neurons, and its expression increases as neurons mature. Through DNA methylation in the miR-124 promoter region and adsorption of miR-124 by non-coding RNAs, miR-124 expression is known to be reduced in many cancer cells, especially with high malignancy. Recently, numerous studies have focused on miR-124 due to its promising tumor-suppressive effects; however, the overview of their results is unclear. We surveyed the tumor-suppressive effect of miR-124 in glial cell lineage cancers, which are the most frequently reported cancer types involving miR-124, and in lung, colon, liver, stomach, and breast cancers, which are the top five causes of cancer death. Reportedly, miR-124 not only inhibits proliferation and accelerates apoptosis, but also comprehensively suppresses tumor malignant transformation. Moreover, we found that miR-124 exerts its anti-tumor effects by regulating a wide range of target genes, most notably STAT3 and EZH2. In addition, when compared to the original role of miR-124 in neuronal development, we found that the miR-124 target genes that contribute to neuronal maturation share similarities with genes that cause cancer cell metastasis and epithelial-mesenchymal transition. We believe that the two apparently unrelated fields, cancer and neuronal development, can bring new discoveries to each other through the study of miR-124.

MiR-124 synergism with ELAVL3 enhances target gene expression to promote neuronal maturity

Neuron-enriched microRNAs (miRNAs), miR-9/9* and miR-124 (miR-9/9*-124), direct cell fate switching of human fibroblasts to neurons when ectopically expressed by repressing antineurogenic genes. How these miRNAs function after the repression of fibroblast genes for neuronal fate remains unclear. Here, we identified targets of miR-9/9*-124 as reprogramming cells activate the neuronal program and reveal the role of miR-124 that directly promotes the expression of its target genes associated with neuronal development and function. The mode of miR-124 as a positive regulator is determined by the binding of both AGO and a neuron-enriched RNA-binding protein, ELAVL3, to target transcripts. Although existing literature indicates that miRNA-ELAVL family protein interaction can result in either target gene up-regulation or down-regulation in a context-dependent manner, we specifically identified neuronal ELAVL3 as the driver for miR-124 target gene up-regulation in neurons. In primary human neurons, repressing miR-124 and ELAVL3 led to the down-regulation of genes involved in neuronal function and process outgrowth and cellular phenotypes of reduced inward currents and neurite outgrowth. Our results highlight the synergistic role between miR-124 and RNA-binding proteins to promote target gene regulation and neuronal function.

MicroRNAs Instruct and Maintain Cell Type Diversity in the Nervous System

Characterizing the diverse cell types that make up the nervous system is essential for understanding how the nervous system is structured and ultimately how it functions. The astonishing range of cellular diversity found in the nervous system emerges from a small pool of neural progenitor cells. These progenitors and their neuronal progeny proceed through sequential gene expression programs to produce different cell lineages and acquire distinct cell fates. These gene expression programs must be tightly regulated in order for the cells to achieve and maintain the proper differentiated state, remain functional throughout life, and avoid cell death. Disruption of developmental programs is associated with a wide range of abnormalities in brain structure and function, further indicating that elucidating their contribution to cellular diversity will be key to understanding brain health. A growing body of evidence suggests that tight regulation of developmental genes requires post-transcriptional regulation of the transcriptome by microRNAs (miRNAs). miRNAs are small non-coding RNAs that function by binding to mRNA targets containing complementary sequences and repressing their translation into protein, thereby providing a layer of precise spatial and temporal control over gene expression. Moreover, the expression profiles and targets of miRNAs show great specificity for distinct cell types, brain regions and developmental stages, suggesting that they are an important parameter of cell type identity. Here, we provide an overview of miRNAs that are critically involved in establishing neural cell identities, focusing on how miRNA-mediated regulation of gene expression modulates neural progenitor expansion, cell fate determination, cell migration, neuronal and glial subtype specification, and finally cell maintenance and survival.

Transcriptional Silencers: Driving Gene Expression with the Brakes On

Silencers are regulatory DNA elements that reduce transcription from their target promoters; they are the repressive counterparts of enhancers. Although discovered decades ago, and despite evidence of their importance in development and disease, silencers have been much less studied than enhancers. Recently, however, a series of papers have reported systematic studies of silencers in various model systems. Silencers are often bifunctional regulatory elements that can also act as enhancers, depending on cellular context, and are enriched for expression quantitative trait loci (eQTLs) and disease-associated variants. There is not yet evidence of a 'silencer chromatin signature', in the distribution of histone modifications or associated proteins, that is common to all silencers; instead, silencers may fall into various subclasses, acting by distinct (and possibly overlapping) mechanisms.

Direct cell reprogramming: approaches, mechanisms and progress

The reprogramming of somatic cells with defined factors, which converts cells from one lineage into cells of another, has greatly reshaped our traditional views on cell identity and cell fate determination. Direct reprogramming (also known as transdifferentiation) refers to cell fate conversion without transitioning through an intermediary pluripotent state. Given that the number of cell types that can be generated by direct reprogramming is rapidly increasing, it has become a promising strategy to produce functional cells for therapeutic purposes. This Review discusses the evolution of direct reprogramming from a transcription factor-based method to a small-molecule-driven approach, the recent progress in enhancing reprogrammed cell maturation, and the challenges associated with in vivo direct reprogramming for translational applications. It also describes our current understanding of the molecular mechanisms underlying direct reprogramming, including the role of transcription factors, epigenetic modifications, non-coding RNAs, and the function of metabolic reprogramming, and highlights novel insights gained from single-cell omics studies.

Molecular Mechanisms Underlying Ascl1-Mediated Astrocyte-to-Neuron Conversion

Direct neuronal reprogramming potentially provides valuable sources for cell-based therapies. Proneural gene Ascl1 converts astrocytes into induced neuronal (iN) cells efficiently both in vitro and in vivo. However, the underlying mechanisms are largely unknown. By combining RNA sequencing and chromatin immunoprecipitation followed by high-throughput sequencing, we found that the expression of 1,501 genes was markedly changed during the early stages of Ascl1-induced astrocyte-to-neuron conversion and that the regulatory regions of 107 differentially expressed genes were directly bound by ASCL1. Among Ascl1's direct targets, Klf10 regulates the neuritogenesis of iN cells at the early stage, Myt1 and Myt1l are critical for the electrophysiological maturation of iN cells, and Neurod4 and Chd7 are required for the efficient conversion of astrocytes into neurons. Together, this study provides more insights into understanding the molecular mechanisms underlying Ascl1-mediated astrocyte-to-neuron conversion and will be of value for the application of direct neuronal reprogramming.

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RNA-seq and ChIP-seq were used to study Ascl1-induced astrocyte-to-neuron conversion

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Early Klf10 regulates neuritogenesis and electrophysiological properties of iN cells

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Myt1 and Myt1l are critical for the electrophysiological maturation of iN cells

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Neurod4 and Chd7 are required for efficient conversion of astrocytes to neurons

A quantitative model of cellular decision making in direct neuronal reprogramming

The direct reprogramming of adult skin fibroblasts to neurons is thought to be controlled by a small set of interacting gene regulators. Here, we investigate how the interaction dynamics between these regulating factors coordinate cellular decision making in direct neuronal reprogramming. We put forward a quantitative model of the governing gene regulatory system, supported by measurements of mRNA expression. We found that nPTB needs to feed back into the direct neural conversion network most likely via PTB in order to accurately capture quantitative gene interaction dynamics and correctly predict the outcome of various overexpression and knockdown experiments. This was experimentally validated by nPTB knockdown leading to successful neural conversion. We also proposed a novel analytical technique to dissect system behaviour and reveal the influence of individual factors on resulting gene expression. Overall, we demonstrate that computational analysis is a powerful tool for understanding the mechanisms of direct (neuronal) reprogramming, paving the way for future models that can help improve cell conversion strategies.

Müller Glia-Mediated Retinal Regeneration

Müller glia originate from neuroepithelium and are the principal glial cells in the retina. During retinal development, Müller glia are one of the last cell types to be born. In lower vertebrates, such as zebrafish, Müller glia possess a remarkable capacity for retinal regeneration following various forms of injury through a reprogramming process in which endogenous Müller glia proliferate and differentiate into all types of retinal cells. In mammals, Müller glia become reactive in response to damage to protect or to further impair retinal function. Although mammalian Müller glia have regenerative potential, it is limited as far as repairing damaged retina. Lessons learned from zebrafish will help reveal the critical mechanisms involved in Müller glia reprogramming. Progress has been made in triggering Müller glia to reprogram and generate functional neurons to restore vision in mammals indicating that Müller glia reprogramming may be a promising therapeutic strategy for human retinal diseases. This review comprehensively summarizes the mechanisms related to retinal regeneration in model animals and the critical advanced progress made in Müller glia reprogramming in mammals.

Generation of Human Neurons by microRNA-Mediated Direct Conversion of Dermal Fibroblasts

MicroRNAs (miRNAs), miR-9/9^*, and miR-124 (miR-9/9^*-124) display fate-reprogramming activities when ectopically expressed in human fibroblasts by erasing the fibroblast identity and evoking a pan-neuronal state. In contrast to induced pluripotent stem cell-derived neurons, miRNA-induced neurons (miNs) retain the biological age of the starting fibroblasts through direct fate conversion and thus provide a human neuron-based platform to study cellular properties inherent in aged neurons and model adult-onset neurodegenerative disorders using patient-derived cells. Furthermore, expression of neuronal subtype-specific transcription factors in conjunction with miR-9/9^*-124 guides the miNs to distinct neuronal fates, a feature critical for modeling disorders that affect specific neuronal subtypes. Here, we describe the miR-9/9^*-124-based neuronal reprogramming protocols for the generation of several disease-relevant neuronal subtypes: striatal medium spiny neurons, cortical neurons, and spinal cord motor neurons.

Direct Neuronal Reprogramming of Common Marmoset Fibroblasts by ASCL1, microRNA-9/9*, and microRNA-124 Overexpression

The common marmoset (Callithrix jacchus) has attracted considerable attention, especially in the biomedical science and neuroscience research fields, because of its potential to recapitulate the complex and multidimensional phenotypes of human diseases, and several neurodegenerative transgenic models have been reported. However, there remain several issues as (i) it takes years to generate late-onset disease models, and (ii) the onset age and severity of phenotypes can vary among individuals due to differences in genetic background. In the present study, we established an efficient and rapid direct neuronal induction method (induced neurons; iNs) from embryonic and adult marmoset fibroblasts to investigate cellular-level phenotypes in the marmoset brain in vitro. We overexpressed reprogramming effectors, i.e., microRNA-9/9*, microRNA-124, and Achaete-Scute family bHLH transcription factor 1, in fibroblasts with a small molecule cocktail that facilitates neuronal induction. The resultant iNs from embryonic and adult marmoset fibroblasts showed neuronal characteristics within two weeks, including neuron-specific gene expression and spontaneous neuronal activity. As directly reprogrammed neurons have been shown to model neurodegenerative disorders, the neuronal reprogramming of marmoset fibroblasts may offer new tools for investigating neurological phenotypes associated with disease progression in non-human primate neurological disease models.

Engineering cell fate: Applying synthetic biology to cellular reprogramming

Cellular reprogramming drives cells from one stable identity to a new cell fate. By generating a diversity of previously inaccessible cell types from diverse genetic backgrounds, cellular reprogramming is rapidly transforming how we study disease. However, low efficiency and limited maturity have limited the adoption of in vitro-derived cellular models. To overcome these limitations and improve mechanistic understanding of cellular reprogramming, a host of synthetic biology tools have been deployed. Recent synthetic biology approaches have advanced reprogramming by tackling three significant challenges to reprogramming: delivery of reprogramming factors, epigenetic roadblocks, and latent donor identity. In addition, emerging insight from the molecular systems biology of reprogramming reveal how systems-level drivers of reprogramming can be harnessed to further advance reprogramming technologies. Furthermore, recently developed synthetic biology tools offer new modes for engineering cell fate.

Downregulation of the Polycomb-Associated Methyltransferase Ezh2 during Maturation of Hippocampal Neurons Is Mediated by MicroRNAs Let-7 and miR-124

Ezh2 is a catalytic subunit of the polycomb repressive complex 2 (PRC2) which mediates epigenetic gene silencing through depositing the mark histone H3 lysine 27 trimethylation (H3K27me3) at target genomic sequences. Previous studies have demonstrated that Enhancer of Zeste Homolog 2 (Ezh2) was differentially expressed during maturation of hippocampal neurons; in immature neurons, Ezh2 was abundantly expressed, whereas in mature neurons the expression Ezh2 was significantly reduced. Here, we report that Ezh2 is downregulated by microRNAs (miRs) that are expressed during the hippocampal maturation process. We show that, in mature hippocampal neurons, lethal-7 (let-7) and microRNA-124 (miR-124) are robustly expressed and can target cognate motifs at the 3′-UTR of the Ezh2 gene sequence to downregulate Ezh2 expression. Together, these data demonstrate that the PRC2 repressive activity during hippocampal maturation is controlled through a post-transcriptional mechanism that mediates Ezh2 downregulation in mature neurons.

The multifaceted roles of microRNAs in differentiation

MicroRNAs (miRNAs) are major drivers of cell fate specification and differentiation. The post-transcriptional regulation of key molecular factors by microRNAs contributes to the progression of embryonic and postembryonic development in several organisms. Following the discovery of lin-4 and let-7 in Caenorhabditis elegans and bantam microRNAs in Drosophila melanogaster, microRNAs have emerged as orchestrators of cellular differentiation and developmental timing. Spatiotemporal control of microRNAs and associated protein machinery can modulate microRNA activity. Additionally, adaptive modulation of microRNA expression and function in response to changing environmental conditions ensures that robust cell fate specification during development is maintained. Herein, we review the role of microRNAs in the regulation of differentiation during development.

Dedifferentiation and neuronal repression define familial Alzheimer's disease

Identifying the systems-level mechanisms that lead to Alzheimer's disease, an unmet need, is an essential step toward the development of therapeutics. In this work, we report that the key disease-causative mechanisms, including dedifferentiation and repression of neuronal identity, are triggered by changes in chromatin topology. Here, we generated human induced pluripotent stem cell (hiPSC)-derived neurons from donor patients with early-onset familial Alzheimer's disease (EOFAD) and used a multiomics approach to mechanistically characterize the modulation of disease-associated gene regulatory programs. We demonstrate that EOFAD neurons dedifferentiate to a precursor-like state with signatures of ectoderm and nonectoderm lineages. RNA-seq, ATAC-seq, and ChIP-seq analysis reveals that transcriptional alterations in the cellular state are orchestrated by changes in histone methylation and chromatin topology. Furthermore, we demonstrate that these mechanisms are observed in EOFAD-patient brains, validating our hiPSC-derived neuron models. The mechanistic endotypes of Alzheimer's disease uncovered here offer key insights for therapeutic interventions.

Sweat gland regeneration: Current strategies and future opportunities

For patients with extensive skin defects, loss of sweat glands (SwGs) greatly decreases their quality of life. Indeed, difficulties in thermoregulation, ion reabsorption, and maintaining fluid balance might render them susceptible to hyperthermia, heatstroke, or even death. Despite extensive studies on the stem cell biology of the skin in recent years, in-situ regeneration of SwGs with both structural and functional fidelity is still challenging because of the limited regenerative capacity and cell fate control of resident progenitors. To overcome these challenges, one must consider both the intrinsic factors relevant to genetic and epigenetic regulation and cues from the cellular microenvironment. Here, we describe recent progress in molecular biology, developmental pathways, and cellular evolution associated with SwGdevelopment and maturation. This is followed by a summary of the current strategies used for cell-fate modulation, transmembrane drug delivery, and scaffold design associated with SwGregeneration. Finally, we offer perspectives for creating more sophisticated systems to accelerate patients' innate healing capacity and developing engineered skin constructs to treat or replace damaged tissues structurally and functionally.

REST and Neural Gene Network Dysregulation in iPSC Models of Alzheimer's Disease

The molecular basis of the earliest neuronal changes that lead to Alzheimer’s disease (AD) is unclear. Here, we analyze neural cells derived from sporadic AD (SAD), APOE4 gene-edited and control induced pluripotent stem cells (iPSCs). We observe major differences in iPSC-derived neural progenitor (NP) cells and neurons in gene networks related to neuronal differentiation, neurogenesis, and synaptic transmission. The iPSC-derived neural cells from SAD patients exhibit accelerated neural differentiation and reduced progenitor cell renewal. Moreover, a similar phenotype appears in NP cells and cerebral organoids derived from APOE4 iPSCs. Impaired function of the transcriptional repressor REST is strongly implicated in the altered transcriptome and differentiation state. SAD and APOE4 expression result in reduced REST nuclear translocation and chromatin binding, and disruption of the nuclear lamina. Thus, dysregulation of neural gene networks may set in motion the pathologic cascade that leads to AD.

Meyer et al. derive neural progenitors, neurons, and cerebral organoids from sporadic Alzheimer’s disease (SAD) and APOE4 gene-edited iPSCs. SAD and APOE4 expression alter the neural transcriptome and differentiation in part through loss of function of the transcriptional repressor REST. Thus, neural gene network dysregulation may lead to Alzheimer’s disease.

miRNA-Based Rapid Differentiation of Purified Neurons from hPSCs Advancestowards Quick Screening for Neuronal Disease Phenotypes In Vitro

Obtaining differentiated cells with high physiological functions by an efficient, but simple and rapid differentiation method is crucial for modeling neuronal diseases in vitro using human pluripotent stem cells (hPSCs). Currently, methods involving the transient expression of one or a couple of transcription factors have been established as techniques for inducing neuronal differentiation in a rapid, single step. It has also been reported that microRNAs can function as reprogramming effectors for directly reprogramming human dermal fibroblasts to neurons. In this study, we tested the effect of adding neuronal microRNAs, miRNA-9/9*, and miR-124 (miR-9/9*-124), for the neuronal induction method of hPSCs using Tet-On-driven expression of the Neurogenin2 gene (Ngn2), a proneural factor. While it has been established that Ngn2 can facilitate differentiation from pluripotent stem cells into neurons with high purity due to its neurogenic effect, a long or indefinite time is required for neuronal maturation with Ngn2 misexpression alone. With the present method, the cells maintained a high neuronal differentiation rate while exhibiting increased gene expression of neuronal maturation markers, spontaneous calcium oscillation, and high electrical activity with network bursts as assessed by a multipoint electrode system. Moreover, when applying this method to iPSCs from Alzheimer's disease (AD) patients with presenilin-1 (PS1) or presenilin-2 (PS2) mutations, cellular phenotypes such as increased amount of extracellular secretion of amyloid β42, abnormal oxygen consumption, and increased reactive oxygen species in the cells were observed in a shorter culture period than those previously reported. Therefore, it is strongly anticipated that the induction method combining Ngn2 and miR-9/9*-124 will enable more rapid and simple screening for various types of neuronal disease phenotypes and promote drug discovery.

The Proteasome and Its Network: Engineering for Adaptability

The proteasome, the most complex protease known, degrades proteins that have been conjugated to ubiquitin. It faces the unique challenge of acting enzymatically on hundreds and perhaps thousands of structurally diverse substrates, mechanically unfolding them from their native state and translocating them vectorially from one specialized compartment of the enzyme to another. Moreover, substrates are modified by ubiquitin in myriad configurations of chains. The many unusual design features of the proteasome may have evolved in part to endow this enzyme with a robust ability to process substrates regardless of their identity. The proteasome plays a major role in preserving protein homeostasis in the cell, which requires adaptation to a wide variety of stress conditions. Modulation of proteasome function is achieved through a large network of proteins that interact with it dynamically, modify it enzymatically, or fine-tune its levels. The resulting adaptability of the proteasome, which is unique among proteases, enables cells to control the output of the ubiquitin-proteasome pathway on a global scale.

News about the Role of the Transcription Factor REST in Neurons: From Physiology to Pathology

RE-1 silencing transcription factor (REST) (known also as NRSF) is a well-known transcription repressor whose strong decrease induces the distinction of neurons with respect to the other cells. Such distinction depends on the marked increased/decreased expression of specific genes, accompanied by parallel changes of the corresponding proteins. Many properties of REST had been identified in the past. Here we report those identified during the last 5 years. Among physiological discoveries are hundreds of genes governed directly/indirectly by REST, the mechanisms of its neuron/fibroblast conversions, and the cooperations with numerous distinct factors induced at the epigenetic level and essential for REST specific functions. New effects induced in neurons during brain diseases depend on the localization of REST, in the nucleus, where functions and toxicity occur, and in the cytoplasm. The effects of REST, including cell aggression or protection, are variable in neurodegenerative diseases in view of the distinct mechanisms of their pathology. Moreover, cooperations are among the mechanisms that govern the severity of brain cancers, glioblastomas, and medulloblastomas. Interestingly, the role in cancers is relevant also for therapeutic perspectives affecting the REST cooperations. In conclusion, part of the new REST knowledge in physiology and pathology appears promising for future developments in research and brain diseases.

Next-generation disease modeling with direct conversion: a new path to old neurons

Within just over a decade, human reprogramming-based disease modeling has developed from a rather outlandish idea into an essential part of disease research. While iPSCs are a valuable tool for modeling developmental and monogenetic disorders, their rejuvenated identity poses limitations for modeling age-associated diseases. Direct cell-type conversion of fibroblasts into induced neurons (iNs) circumvents rejuvenation and preserves hallmarks of cellular aging. iNs are thus advantageous for modeling diseases that possess strong age-related and epigenetic contributions and can complement iPSC-based strategies for disease modeling. In this review, we provide an overview of the state of the art of direct iN conversion and describe the key epigenetic, transcriptomic, and metabolic changes that occur in converting fibroblasts. Furthermore, we summarize new insights into this fascinating process, particularly focusing on the rapidly changing criteria used to define and characterize in vitro-born human neurons. Finally, we discuss the unique features that distinguish iNs from other reprogramming-based neuronal cell models and how iNs are relevant to disease modeling.

ATRX In-Frame Fusion Neuroblastoma Is Sensitive to EZH2 Inhibition via Modulation of Neuronal Gene Signatures

ATRX alterations occur at high frequency in neuroblastoma of adolescents and young adults. Particularly intriguing are the large N-terminal deletions of ATRX (Alpha Thalassemia/Mental Retardation, X-linked) that generate in-frame fusion (IFF) proteins devoid of key chromatin interaction domains, while retaining the SWI/SNF-like helicase region. We demonstrate that ATRX IFF proteins are redistributed from H3K9me3-enriched chromatin to promoters of active genes and identify REST as an ATRX IFF target whose activation promotes silencing of neuronal differentiation genes. We further show that ATRX IFF cells display sensitivity to EZH2 inhibitors, due to derepression of neurogenesis genes, including a subset of REST targets. Taken together, we demonstrate that ATRX structural alterations are not loss-of-function and put forward EZH2 inhibitors as a potential therapy for ATRX IFF neuroblastoma.

The Role of Neuronal Factors in the Epigenetic Reprogramming of Microglia in the Normal and Diseased Central Nervous System

Twenty years ago, the scientific community exhibited relatively little interest in the study of microglial cells. However, recent technical and conceptual advances in this field have greatly increased interest in the basic biology of these cells within various neurodegenerative diseases, including multiple sclerosis, Alzheimer’s disease, and traumatic brain/spinal cord injuries. The main functions of these cells in the normal central nervous system (CNS) remain poorly understood, despite considerable elucidation of their roles in pathological conditions. Microglia populate the brain before birth and remain in close lifelong contact with CNS-resident cells under the influence of the local microenvironment. Within the CNS parenchyma, microglia actively interact with two main cell types, astrocytes and neurons, which produce many factors that affect microglia phenotypes in the normal CNS and during neuroinflammation. These factors include interleukin (IL)-34, macrophage colony-stimulating factor, transforming growth factor-β, and IL-4, which promote microglial expansion, survival, and differentiation to an anti-inflammatory phenotype in the normal CNS. Under inflammatory conditions, however, astrocytes produce several pro-inflammatory factors that contribute to microglial activation. The interactions of microglia with neurons in the normal and diseased CNS are especially intriguing. Microglia are known to interact actively with neurons by facilitating axonal pruning during development, while neurons provide specific factors that alter microglial phenotypes and functions. This review focuses mainly on the roles of soluble neuronal factors that affect microglial phenotypes and functions and the possible involvement of these factors in the pathology of neurodegenerative diseases.

Mitigating Antagonism between Transcription and Proliferation Allows Near-Deterministic Cellular Reprogramming

Although cellular reprogramming enables the generation of new cell types for disease modeling and regenerative therapies, reprogramming remains a rare cellular event. By examining reprogramming of fibroblasts into motor neurons and multiple other somatic lineages, we find that epigenetic barriers to conversion can be overcome by endowing cells with the ability to mitigate an inherent antagonism between transcription and DNA replication. We show that transcription factor overexpression induces unusually high rates of transcription and that sustaining hypertranscription and transgene expression in hyperproliferative cells early in reprogramming is critical for successful lineage conversion. However, hypertranscription impedes DNA replication and cell proliferation, processes that facilitate reprogramming. We identify a chemical and genetic cocktail that dramatically increases the number of cells capable of simultaneous hypertranscription and hyperproliferation by activating topoisomerases. Further, we show that hypertranscribing, hyperproliferating cells reprogram at 100-fold higher, near-deterministic rates. Therefore, relaxing biophysical constraints overcomes molecular barriers to cellular reprogramming.

MicroRNAs miR-25, let-7 and miR-124 regulate the neurogenic potential of Müller glia in mice

Müller glial cells (MG) generate retinal progenitor (RPC)-like cells after injury in non-mammalian species, although this does not occur in the mammalian retina. Studies have profiled gene expression in these cells to define genes that may be relevant to their differences in neurogenic potential. However, less is known about differences in micro-RNA (miRNA) expression. In this study, we compared miRNAs from RPCs and MG to identify miRNAs more highly expressed in RPCs, and others more highly expressed in MG. To determine whether these miRNAs are relevant to the difference in neurogenic potential between these two cell types, we tested them in dissociated cultures of MG using either mimics or antagomiRs to increase or reduce expression, respectively. Among the miRNAs tested, miR-25 and miR-124 overexpression, or let-7 antagonism, induced Ascl1 expression and conversion of ∼40% of mature MG into a neuronal/RPC phenotype. Our results suggest that the differences in miRNA expression between MG and RPCs contribute to their difference in neurogenic potential, and that manipulations in miRNAs provide a new tool with which to reprogram MG for retinal regeneration.

EZH2 as a novel therapeutic target for atrial fibrosis and atrial fibrillation

Angiotensin II (Ang-II)-induced fibroblast differentiation plays an important role in the development of atrial fibrosis and atrial fibrillation (AF). Here, we show that the expression of the histone methyltransferase enhancer of zeste homolog 2 (EZH2) is increased in atrial muscle and atrial fibroblasts in patients with AF, accompanied by significant atrial fibrosis and atrial fibroblast differentiation. In addition, EZH2 is induced in murine models of atrial fibrosis. Furthermore, either pharmacological GSK126 inhibition or molecular silencing of EZH2 can inhibit the differentiation of atrial fibroblasts and the ability to produce ECM induced by Ang-II. Simultaneously, inhibition of EZH2 can block the Ang-II-induced migration of atrial fibroblasts. We found that EZH2 promotes fibroblast differentiation mainly through the Smad signaling pathway and can form a transcription complex with Smad2 to bind to the promoter region of the ACTA2 gene. Finally, our in vivo experiments demonstrated that the EZH2 inhibitor GSK126 significantly inhibited Ang-II-induced atrial enlargement and fibrosis and reduced AF vulnerability. Our results demonstrate that targeting EZH2 or EZH2-regulated genes might present therapeutic potential in AF.

MicroRNA in Brain pathology: Neurodegeneration the Other Side of the Brain Cancer

The mammalian brain is made up of billions of neurons and supporting cells (glial cells), intricately connected. Molecular perturbations often lead to neurodegeneration by progressive loss of structure and malfunction of neurons, including their death. On the other side, a combination of genetic and cellular factors in glial cells, and less frequently in neurons, drive oncogenic transformation. In both situations, microenvironmental niches influence the progression of diseases and therapeutic responses. Dynamic changes that occur in cellular transcriptomes during the progression of developmental lineages and pathogenesis are controlled through a variety of regulatory networks. These include epigenetic modifications, signaling pathways, and transcriptional and post-transcriptional mechanisms. One prominent component of the latter is small non-coding RNAs, including microRNAs, that control the vast majority of these networks including genes regulating neural stemness, differentiation, apoptosis, projection fates, migration and many others. These cellular processes are also profoundly dependent on the microenvironment, stemness niche, hypoxic microenvironment, and interactions with associated cells including endothelial and immune cells. Significantly, the brain of all other mammalian organs expresses the highest number of microRNAs, with an additional gain in expression in the early stage of neurodegeneration and loss in expression in oncogenesis. However, a mechanistic explanation of the concept of an apparent inverse correlation between the odds of cancer and neurodegenerative diseases is only weakly developed. In this review, we thus will discuss widespread de-regulation of microRNAome observed in these two major groups of brain pathologies. The deciphering of these intricacies is of importance, as therapeutic restoration of pre-pathological microRNA landscape in neurodegeneration must not lead to oncogenesis and vice versa. We thus focus on microRNAs engaged in cellular processes that are inversely regulated in these diseases. We also aim to define the difference in microRNA networks between pro-survival and pro-apoptotic signaling in the brain.

Potentials of Cellular Reprogramming as a Novel Strategy for Neuroregeneration

Cellular reprogramming technology holds great potential for tissue repair and regeneration to replace cells that are lost due to diseases or injuries. In addition to the landmark discovery of induced pluripotent stem cells, advances in cellular reprogramming allow the direct lineage conversion of one somatic cell type to another using defined transcription factors. This direct reprogramming technology represents a rapid way to generate target cells in the laboratory, which can be used for transplantation and studies of biology and diseases. More importantly, recent work has demonstrated the exciting application of direct reprogramming to stimulate regeneration in vivo, providing an alternative approach to transplantation of donor cells. Here, we provide an overview of the underlying concept of using cellular reprogramming to convert cell fates and discuss the current advances in cellular reprogramming both in vitro and in vivo, with particular focuses on the neural and retinal systems. We also discuss the potential of in vivo reprogramming in regenerative medicine, the challenges and potential solutions to translate this technology to the clinic.

CRISPR Activation Screens Systematically Identify Factors that Drive Neuronal Fate and Reprogramming

Comprehensive identification of factors that can specify neuronal fate could provide valuable insights into lineage specification and reprogramming, but systematic interrogation of transcription factors, and their interactions with each other, has proven technically challenging. We developed a CRISPR activation (CRISPRa) approach to systematically identify regulators of neuronal-fate specification. We activated expression of all endogenous transcription factors and other regulators via a pooled CRISPRa screen in embryonic stem cells, revealing genes including epigenetic regulators such as Ezh2 that can induce neuronal fate. Systematic CRISPR-based activation of factor pairs allowed us to generate a genetic interaction map for neuronal differentiation, with confirmation of top individual and combinatorial hits as bona fide inducers of neuronal fate. Several factor pairs could directly reprogram fibroblasts into neurons, which shared similar transcriptional programs with endogenous neurons. This study provides an unbiased discovery approach for systematic identification of genes that drive cell-fate acquisition.

Down-regulated lncRNA MEG3 promotes osteogenic differentiation of human dental follicle stem cells by epigenetically regulating Wnt pathway

Our previous long noncoding RNA (lncRNA) microarray results showed that lncRNA MEG3 (maternally expressed 3) was significantly downregulated in human dental follicle cells than human periodontal ligament cells. Latest studies show that MEG3 contributes to polycomb repressive complex 2 (PRC2) recruitment to silence gene expression. The enhancer of zeste homolog 2 (EZH2), a crucial catalytic subunit of PRC2, mediates gene silencing and participates in cell lineage determination via methyltransferase activity. In this study, we found that the expression of EZH2 and H3K27me3 (trimethylation on lysine 27 in histone H3) decreased during osteogenesis of human dental follicle stem cells (hDFSCs). Knockdown studies of MEG3 and EZH2 by siRNA showed that MEG3/EZH2 negatively regulated osteogenesis of hDFSCs. We investigated the role of Wnt signaling pathway during the osteogenesis of hDFSCs and its relationship with EZH2. Besides, we studied the key genes of the canonical/noncanonical Wnt signaling pathway which might be related to EZH2. ChIP (chromatin immunoprecipitation) analysis showed that these effects were due to the EZH2 regulation of H3K27me3 level on the Wnt genes promotors. We first demonstrated that the decrease of MEG3 or EZH2 activated the Wnt/β-catenin signaling pathway via epigenetically regulating the H3K27me3 level on the Wnt genes promotors. Our research offers a new target for periodontal tissue engineering and osteogenic tissue regeneration.

Mechanistic Insights Into MicroRNA-Induced Neuronal Reprogramming of Human Adult Fibroblasts

The use of transcriptional factors as cell fate regulators are often the primary focus in the direct reprogramming of somatic cells into neurons. However, in human adult fibroblasts, deriving functionally mature neurons with high efficiency requires additional neurogenic factors such as microRNAs (miRNAs) to evoke a neuronal state permissive to transcription factors to exert their reprogramming activities. As such, increasing evidence suggests brain-enriched miRNAs, miR-9/9^∗ and miR-124, as potent neurogenic molecules through simultaneously targeting of anti-neurogenic effectors while allowing additional transcription factors to generate specific subtypes of human neurons. In this review, we will focus on methods that utilize neuronal miRNAs and provide mechanistic insights by which neuronal miRNAs, in synergism with brain-region specific transcription factors, drive the conversion of human fibroblasts into clinically relevant subtypes of neurons. Furthermore, we will provide insights into the age signature of directly converted neurons and how the converted human neurons can be utilized to model late-onset neurodegenerative disorders.