Dual Requirement of CHD8 for Chromatin Landscape Establishment and Histone Methyltransferase Recruitment to Promote CNS Myelination and Repair

Regulators of Oligodendrocyte Differentiation

Myelination has evolved as a mechanism to ensure fast and efficient propagation of nerve impulses along axons. Within the central nervous system (CNS), myelination is carried out by highly specialized glial cells, oligodendrocytes. The formation of myelin is a prolonged aspect of CNS development that occurs well into adulthood in humans, continuing throughout life in response to injury or as a component of neuroplasticity. The timing of myelination is tightly tied to the generation of oligodendrocytes through the differentiation of their committed progenitors, oligodendrocyte precursor cells (OPCs), which reside throughout the developing and adult CNS. In this article, we summarize our current understanding of some of the signals and pathways that regulate the differentiation of OPCs, and thus the myelination of CNS axons.

Small-molecule-induced epigenetic rejuvenation promotes SREBP condensation and overcomes barriers to CNS myelin regeneration

Remyelination failure in diseases like multiple sclerosis (MS) was thought to involve suppressed maturation of oligodendrocyte precursors; however, oligodendrocytes are present in MS lesions yet lack myelin production. We found that oligodendrocytes in the lesions are epigenetically silenced. Developing a transgenic reporter labeling differentiated oligodendrocytes for phenotypic screening, we identified a small-molecule epigenetic-silencing-inhibitor (ESI1) that enhances myelin production and ensheathment. ESI1 promotes remyelination in animal models of demyelination and enables de novo myelinogenesis on regenerated CNS axons. ESI1 treatment lengthened myelin sheaths in human iPSC-derived organoids and augmented (re)myelination in aged mice while reversing age-related cognitive decline. Multi-omics revealed that ESI1 induces an active chromatin landscape that activates myelinogenic pathways and reprograms metabolism. Notably, ESI1 triggered nuclear condensate formation of master lipid-metabolic regulators SREBP1/2, concentrating transcriptional co-activators to drive lipid/cholesterol biosynthesis. Our study highlights the potential of targeting epigenetic silencing to enable CNS myelin regeneration in demyelinating diseases and aging.

Chromatin remodeler CHD8 is required for spermatogonial proliferation and early meiotic progression

Meiosis is a key step during germ cell differentiation, accompanied by the activation of thousands of genes through germline-specific chromatin reorganization. The chromatin remodeling mechanisms underpinning early meiotic stages remain poorly understood. Here we focus on the function of one of the major autism genes, CHD8, in spermatogenesis, based on the epidemiological association between autism and low fertility rates. Specific ablation of Chd8 in germ cells results in gradual depletion of undifferentiated spermatogonia and the failure of meiotic double-strand break (DSB) formation, leading to meiotic prophase I arrest and cell death. Transcriptional analyses demonstrate that CHD8 is required for extensive activation of spermatogenic genes in spermatogonia, necessary for spermatogonial proliferation and meiosis. CHD8 directly binds and regulates genes crucial for meiosis, including H3K4me3 histone methyltransferase genes, meiotic cohesin genes, HORMA domain-containing genes, synaptonemal complex genes, and DNA damage response genes. We infer that CHD8 contributes to meiotic DSB formation and subsequent meiotic progression through combined regulation of these meiosis-related genes. Our study uncovers an essential role of CHD8 in the proliferation of undifferentiated spermatogonia and the successful progression of meiotic prophase I.

The CHD family chromatin remodeling enzyme, Kismet, promotes both clathrin-mediated and activity-dependent bulk endocytosis

Chromodomain helicase DNA binding domain (CHD) proteins, including CHD7 and CHD8, remodel chromatin to enable transcriptional programs. Both proteins are important for proper neural development as heterozygous mutations in Chd7 and Chd8 are causative for CHARGE syndrome and correlated with autism spectrum disorders, respectively. Their roles in mature neurons are poorly understood despite influencing the expression of genes required for cell adhesion, neurotransmission, and synaptic plasticity. The Drosophila homolog of CHD7 and CHD8, Kismet (Kis), promotes neurotransmission, endocytosis, and larval locomotion. Endocytosis is essential in neurons for replenishing synaptic vesicles, maintaining protein localization, and preserving the size and composition of the presynaptic membrane. Several forms of endocytosis have been identified including clathrin-mediated endocytosis, which is coupled with neural activity and is the most prevalent form of synaptic endocytosis, and activity-dependent bulk endocytosis, which occurs during periods of intense stimulation. Kis modulates the expression of gene products involved in endocytosis including promoting shaggy/GSK3β expression while restricting PI3K92E. kis mutants electrophysiologically phenocopy a liquid facets mutant in response to paradigms that induce clathrin-mediated endocytosis and activity-dependent bulk endocytosis. Further, kis mutants do not show further reductions in endocytosis when activity-dependent bulk endocytosis or clathrin-mediated endocytosis are pharmacologically inhibited. We find that Kis is important in postsynaptic muscle for proper endocytosis but the ATPase domain of Kis is dispensable for endocytosis. Collectively, our data indicate that Kis promotes both clathrin-mediated endocytosis and activity-dependent bulk endocytosis possibly by promoting transcription of several endocytic genes and maintaining the size of the synaptic vesicle pool.

The CHD Protein Kismet Restricts the Synaptic Localization of Cell Adhesion Molecules at the Drosophila Neuromuscular Junction

The appropriate expression and localization of cell surface cell adhesion molecules must be tightly regulated for optimal synaptic growth and function. How neuronal plasma membrane proteins, including cell adhesion molecules, cycle between early endosomes and the plasma membrane is poorly understood. Here we show that the Drosophila homolog of the chromatin remodeling enzymes CHD7 and CHD8, Kismet, represses the synaptic levels of several cell adhesion molecules. Neuroligins 1 and 3 and the integrins αPS2 and βPS are increased at kismet mutant synapses but Kismet only directly regulates transcription of neuroligin 2. Kismet may therefore regulate synaptic CAMs indirectly by activating transcription of gene products that promote intracellular vesicle trafficking including endophilin B (endoB) and/or rab11. Knock down of EndoB in all tissues or neurons increases synaptic FasII while knock down of EndoB in kis mutants does not produce an additive increase in FasII. In contrast, neuronal expression of Rab11, which is deficient in kis mutants, leads to a further increase in synaptic FasII in kis mutants. These data support the hypothesis that Kis influences the synaptic localization of FasII by promoting intracellular vesicle trafficking through the early endosome.

Neurodevelopmental functions of CHD8: new insights and questions

Heterozygous, de novo, loss-of-function variants of the CHD8 gene are associated with a high penetrance of autism and other neurodevelopmental phenotypes. Identifying the neurodevelopmental functions of high-confidence autism risk genes like CHD8 may improve our understanding of the neurodevelopmental mechanisms that underlie autism spectrum disorders. Over the last decade, a complex picture of pleiotropic CHD8 functions and mechanisms of action has emerged. Multiple brain and non-brain cell types and progenitors appear to be affected by CHD8 haploinsufficiency. Behavioural, cellular and synaptic phenotypes are dependent on the nature of the gene mutation and are modified by sex and genetic background. Here, I review some of the CHD8-interacting proteins and molecular mechanisms identified to date, as well as the impacts of CHD8 deficiency on cellular processes relevant to neurodevelopment. I endeavour to highlight some of the critical questions that still require careful and concerted attention over the next decade to bring us closer to the goal of understanding the salient mechanisms whereby CHD8 deficiency causes neurodevelopmental disorders.

Macromolecular Complex Including MLL3, Carabin and Calcineurin Regulates Cardiac Remodeling

BACKGROUND

Cardiac hypertrophy is an intermediate stage in the development of heart failure. The structural and functional processes occurring in cardiac hypertrophy include extensive gene reprogramming, which is dependent on epigenetic regulation and chromatin remodeling. However, the chromatin remodelers and their regulatory functions involved in the pathogenesis of cardiac hypertrophy are not well characterized.

METHODS

Protein interaction was determined by immunoprecipitation assay in primary cardiomyocytes and mouse cardiac samples subjected or not to transverse aortic constriction for 1 week. Chromatin immunoprecipitation and DNA sequencing (ChIP-seq) experiments were performed on the chromatin of adult mouse cardiomyocytes.

RESULTS

We report that the calcium-activated protein phosphatase CaN (calcineurin), its endogenous inhibitory protein carabin, the STK24 (STE20-like protein kinase 3), and the histone monomethyltransferase, MLL3 (mixed lineage leukemia 3) form altogether a macromolecular complex at the chromatin of cardiomyocytes. Under basal conditions, carabin prevents CaN activation while the serine/threonine kinase STK24 maintains MLL3 inactive via phosphorylation. After 1 week of transverse aortic constriction, both carabin and STK24 are released from the CaN-MLL3 complex leading to the activation of CaN, dephosphorylation of MLL3, and in turn, histone H3 lysine 4 monomethylation. Selective cardiac MLL3 knockdown mitigates hypertrophy, and chromatin immunoprecipitation and DNA sequencing analysis demonstrates that MLL3 is de novo recruited at the transcriptional start site of genes implicated in cardiomyopathy in stress conditions. We also show that CaN and MLL3 colocalize at chromatin and that CaN activates MLL3 histone methyl transferase activity at distal intergenic regions under hypertrophic conditions.

CONCLUSIONS

Our study reveals an unsuspected epigenetic mechanism of CaN that directly regulates MLL3 histone methyl transferase activity to promote cardiac remodeling.

Uncovering oligodendrocyte enhancers that control Cnp expression

Oligodendrocytes (OLs) produce myelin sheaths around axons in the central nervous system (CNS). Myelin accelerates the propagation of action potentials along axons and supports the integrity of axons. Impaired myelination has been linked to neurological and neuropsychiatric disorders. As a major component of CNS myelin, 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) plays an indispensable role in the axon-supportive function of myelin. Notably, this function requires a high-level expression of CNP in OLs, as evidenced by downregulated expression of CNP in mental disorders and animal models. Little is known about how CNP expression is regulated in OLs. Especially, OL enhancers that govern CNP remain elusive. We have recently developed a powerful method that links OL enhancers to target genes in a principled manner. Here, we applied it to Cnp, uncovering two OL enhancers for it (termed Cnp-E1 and Cnp-E2). Epigenome editing analysis revealed that Cnp-E1 and Cnp-E2 are dedicated to Cnp. ATAC-seq and ChIP-seq data show that Cnp-E1 and Cnp-E2 are conserved OL-specific enhancers. Single cell multi-omics data that jointly profile gene expression and chromatin accessibility suggest that Cnp-E2 plays an important role in Cnp expression in the early stage of OL differentiation while Cnp-E1 sustains it in mature OLs.

Systemic and intrinsic functions of ATRX in glial cell fate and CNS myelination in male mice

Myelin, an extension of the oligodendrocyte plasma membrane, wraps around axons to facilitate nerve conduction. Myelination is compromised in ATR-X intellectual disability syndrome patients, but the causes are unknown. We show that loss of ATRX leads to myelination deficits in male mice that are partially rectified upon systemic thyroxine administration. Targeted ATRX inactivation in either neurons or oligodendrocyte progenitor cells (OPCs) reveals OPC-intrinsic effects on myelination. OPCs lacking ATRX fail to differentiate along the oligodendrocyte lineage and acquire a more plastic state that favors astrocytic differentiation in vitro and in vivo. ATRX chromatin occupancy in OPCs greatly overlaps with that of the chromatin remodelers CHD7 and CHD8 as well as H3K27Ac, a mark of active enhancers. Overall, our data indicate that ATRX regulates the onset of myelination systemically via thyroxine, and by promoting OPC differentiation and suppressing astrogliogenesis. These functions of ATRX identified in mice could explain white matter pathogenesis observed in ATR-X syndrome patients.

The stability of the myelinating oligodendrocyte transcriptome is regulated by the nuclear lamina

Oligodendrocytes are specialized cells that insulate and support axons with their myelin membrane, allowing proper brain function. Here, we identify lamin A/C (LMNA/C) as essential for transcriptional and functional stability of myelinating oligodendrocytes. We show that LMNA/C levels increase with differentiation of progenitors and that loss of Lmna in differentiated oligodendrocytes profoundly alters their chromatin accessibility and transcriptional signature. Lmna deletion in myelinating glia is compatible with normal developmental myelination. However, altered chromatin accessibility is detected in fully differentiated oligodendrocytes together with increased expression of progenitor genes and decreased levels of lipid-related transcription factors and inner mitochondrial membrane transcripts. These changes are accompanied by altered brain metabolism, lower levels of myelin-related lipids, and altered mitochondrial structure in oligodendrocytes, thereby resulting in myelin thinning and the development of a progressively worsening motor phenotype. Overall, our data identify LMNA/C as essential for maintaining the transcriptional and functional stability of myelinating oligodendrocytes.

Promyelinating drugs promote functional recovery in an autism spectrum disorder mouse model of Pitt-Hopkins syndrome

Pitt-Hopkins syndrome is an autism spectrum disorder caused by autosomal dominant mutations in the human transcription factor 4 gene (TCF4). One pathobiological process caused by murine Tcf4 mutation is a cell autonomous reduction in oligodendrocytes and myelination. In this study, we show that the promyelinating compounds, clemastine, sobetirome and Sob-AM2 are effective at restoring myelination defects in a Pitt-Hopkins syndrome mouse model. In vitro, clemastine treatment reduced excess oligodendrocyte precursor cells and normalized oligodendrocyte density. In vivo, 2-week intraperitoneal administration of clemastine also normalized oligodendrocyte precursor cell and oligodendrocyte density in the cortex of Tcf4 mutant mice and appeared to increase the number of axons undergoing myelination, as EM imaging of the corpus callosum showed a significant increase in the proportion of uncompacted myelin and an overall reduction in the g-ratio. Importantly, this treatment paradigm resulted in functional rescue by improving electrophysiology and behaviour. To confirm behavioural rescue was achieved via enhancing myelination, we show that treatment with the thyroid hormone receptor agonist sobetirome or its brain penetrating prodrug Sob-AM2, was also effective at normalizing oligodendrocyte precursor cell and oligodendrocyte densities and behaviour in the Pitt-Hopkins syndrome mouse model. Together, these results provide preclinical evidence that promyelinating therapies may be beneficial in Pitt-Hopkins syndrome and potentially other neurodevelopmental disorders characterized by dysmyelination.

PBAF Subunit Pbrm1 Selectively Influences the Transition from Progenitors to Pre-Myelinating Cells during Oligodendrocyte Development

Oligodendrocyte development is accompanied by defined changes in the state of chromatin that are brought about by chromatin remodeling complexes. Many such remodeling complexes exist, but only a few have been studied for their impact on oligodendrocytes as the myelin-forming cells of the central nervous system. To define the role of the PBAF remodeling complex, we focused on Pbrm1 as an essential subunit of the PBAF complex and specifically deleted it in the oligodendrocyte lineage at different times of development in the mouse. Deletion in late oligodendrocyte progenitor cells did not lead to substantial changes in the ensuing differentiation and myelination processes. However, when Pbrm1 loss had already occurred in oligodendrocyte progenitor cells shortly after their specification, fewer cells entered the pre-myelinating state. The reduction in pre-myelinating cells later translated into a comparable reduction in myelinating oligodendrocytes. We conclude that Pbrm1 and, by inference, the activity of the PBAF complex is specifically required at the transition from oligodendrocyte progenitor to pre-myelinating oligodendrocyte and ensures the generation of normal numbers of myelinating oligodendrocytes.

Deletion of the autism-related gene Chd8 alters activity-dependent transcriptional responses in mouse postmitotic neurons

CHD8 encodes chromodomain helicase DNA-binding protein 8 and its mutation is a highly penetrant risk factor for autism spectrum disorder (ASD). CHD8 serves as a key transcriptional regulator on the basis of its chromatin-remodeling activity and thereby controls the proliferation and differentiation of neural progenitor cells. However, the function of CHD8 in postmitotic neurons and the adult brain has remained unclear. Here we show that Chd8 homozygous deletion in mouse postmitotic neurons results in downregulation of the expression of neuronal genes as well as alters the expression of activity-dependent genes induced by KCl-mediated neuronal depolarization. Furthermore, homozygous ablation of CHD8 in adult mice was associated with attenuation of activity-dependent transcriptional responses in the hippocampus to kainic acid-induced seizures. Our findings implicate CHD8 in transcriptional regulation in postmitotic neurons and the adult brain, and they suggest that disruption of this function might contribute to ASD pathogenesis associated with CHD8 haploinsufficiency.

The NuRD Complex in Neurodevelopment and Disease: A Case of Sliding Doors

The Nucleosome Remodelling and Deacetylase (NuRD) complex represents one of the major chromatin remodelling complexes in mammalian cells, uniquely coupling the ability to "open" the chromatin by inducing nucleosome sliding with histone deacetylase activity. At the core of the NuRD complex are a family of ATPases named CHDs that utilise the energy produced by the hydrolysis of the ATP to induce chromatin structural changes. Recent studies have highlighted the prominent role played by the NuRD in regulating gene expression during brain development and in maintaining neuronal circuitry in the adult cerebellum. Importantly, components of the NuRD complex have been found to carry mutations that profoundly affect neurological and cognitive development in humans. Here, we discuss recent literature concerning the molecular structure of NuRD complexes and how the subunit composition and numerous permutations greatly determine their functions in the nervous system. We will also discuss the role of the CHD family members in an array of neurodevelopmental disorders. Special emphasis will be given to the mechanisms that regulate the NuRD complex composition and assembly in the cortex and how subtle mutations may result in profound defects of brain development and the adult nervous system.

Olig1/2-Expressing Intermediate Lineage Progenitors Are Predisposed to PTEN/p53-Loss-Induced Gliomagenesis and Harbor Specific Therapeutic Vulnerabilities

Malignant gliomas such as glioblastoma are highly heterogeneous with distinct cells of origin and varied genetic alterations. It remains elusive whether the specific states of neural cell lineages are differentially susceptible to distinct genetic alterations during malignant transformation. Here, an analysis of The Cancer Genome Atlas databases revealed that comutations of PTEN and TP53 are most significantly enriched in human high-grade gliomas. Therefore, we selectively ablated Pten and Trp53 in different progenitors to determine which cell lineage states are susceptible to malignant transformation. Mice with PTEN/p53 ablation mediated by multilineage-expressing human GFAP (hGFAP) promoter-driven Cre developed glioma but with incomplete penetrance and long latency. Unexpectedly, ablation of Pten and Trp53 in Nestin+ neural stem cells (NSC) or Pdgfra+/NG2+ committed oligodendrocyte precursor cells (OPC), two major cells of origin in glioma, did not induce glioma formation in mice. Strikingly, mice lacking Pten and Trp53 in Olig1+/Olig2+ intermediate precursors (pri-OPC) prior to the committed OPCs developed high-grade gliomas with 100% penetrance and short latency. The resulting tumors exhibited distinct tumor phenotypes and drug sensitivities from NSC- or OPC-derived glioma subtypes. Integrated transcriptomic and epigenomic analyses revealed that PTEN/p53-loss induced activation of oncogenic pathways, including HIPPO-YAP and PI3K signaling, to promote malignant transformation. Targeting the core regulatory circuitries YAP and PI3K signaling effectively inhibited tumor cell growth. Thus, our multicell state in vivo mutagenesis analyses suggests that transit-amplifying states of Olig1/2 intermediate lineage precursors are predisposed to PTEN/p53-loss-induced transformation and gliomagenesis, pointing to subtype-specific treatment strategies for gliomas with distinct genetic alterations.

SIGNIFICANCE

Multiple progenitor-state mutagenesis reveal that Olig1/2-expressing intermediate precursors are highly susceptible to PTEN/p53-loss-mediated transformation and impart differential drug sensitivity, indicating tumor-initiating cell states and genetic drivers dictate glioma phenotypes and drug responses. See related commentary by Zamler and Hu, p. 807.

CHD8 mutations increase gliogenesis to enlarge brain size in the nonhuman primate

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition that affects social interaction and behavior. Mutations in the gene encoding chromodomain helicase DNA-binding protein 8 (CHD8) lead to autism symptoms and macrocephaly by a haploinsufficiency mechanism. However, studies of small animal models showed inconsistent findings about the mechanisms for CHD8 deficiency-mediated autism symptoms and macrocephaly. Using the nonhuman primate as a model system, we found that CRISPR/Cas9-mediated CHD8 mutations in the embryos of cynomolgus monkeys led to increased gliogenesis to cause macrocephaly in cynomolgus monkeys. Disrupting CHD8 in the fetal monkey brain prior to gliogenesis increased the number of glial cells in newborn monkeys. Moreover, knocking down CHD8 via CRISPR/Cas9 in organotypic monkey brain slices from newborn monkeys also enhanced the proliferation of glial cells. Our findings suggest that gliogenesis is critical for brain size in primates and that abnormal gliogenesis may contribute to ASD.

KMT2E Haploinsufficiency Manifests Autism-Like Behaviors and Amygdala Neuronal Development Dysfunction in Mice

Autism spectrum disorders (ASD) are highly heterogeneous neurodevelopmental disorders characterized by impaired social interaction skills. Whole exome sequencing has identified loss-of-function mutations in lysine methyltransferase 2E (KMT2E, also named MLL5) in ASD patients and it is listed as an ASD high-risk gene in humans. However, experimental evidence of KMT2E in association with ASD-like manifestations or neuronal function is still missing. Relying on KMT2E^+/- mice, through animal behavior analyses, positron emission tomography (PET) imaging, and neuronal morphological analyses, we explored the role of KMT2E haploinsufficiency in ASD-like symptoms. Behavioral results revealed that KMT2E haploinsufficiency was sufficient to produce social deficit, accompanied by anxiety in mice. Whole-brain ¹⁸F-FDG-PET analysis identified that relative amygdala glycometabolism was selectively decreased in KMT2E^+/- mice compared to wild-type mice. The numbers and soma sizes of amygdala neurons in KMT2E^+/- mice were prominently increased. Additionally, KMT2E mRNA levels in human amygdala were significantly decreased after birth during brain development. Our findings support a causative role of KMT2E in ASD development and suggest that amygdala neuronal development abnormality is likely a major underlying mechanism.

Identifying an oligodendrocyte enhancer that regulates Olig2 expression

Olig2 is a basic helix-loop-helix transcription factor that plays a critical role in the central nervous system. It directs the specification of motor neurons and oligodendrocyte precursor cells (OPCs) from neural progenitors and the subsequent maturation of OPCs into myelin-forming oligodendrocytes (OLs). It is also required for the development of astrocytes. Despite a decade-long search, enhancers that regulate the expression of Olig2 remain elusive. We have recently developed an innovative method that maps promoter-distal enhancers to genes in a principled manner. Here, we applied it to Olig2 in the context of OL lineage cells, uncovering an OL enhancer for it (termed Olig2-E1). Silencing Olig2-E1 by CRISPRi epigenome editing significantly downregulated Olig2 expression. Luciferase assay and ATAC-seq and ChIP-seq data show that Olig2-E1 is an OL-specific enhancer that is conserved across human, mouse and rat. Hi-C data reveal that Olig2-E1 physically interacts with OLIG2 and suggest that this interaction is specific to OL lineage cells. In sum, Olig2-E1 is an evolutionarily conserved OL-specific enhancer that drives the expression of Olig2.

Oligodendrocyte-lineage cell exocytosis and L-type prostaglandin D synthase promote oligodendrocyte development and myelination

In the developing central nervous system, oligodendrocyte precursor cells (OPCs) differentiate into oligodendrocytes, which form myelin around axons. Oligodendrocytes and myelin are essential for the function of the central nervous system, as evidenced by the severe neurological symptoms that arise in demyelinating diseases such as multiple sclerosis and leukodystrophy. Although many cell-intrinsic mechanisms that regulate oligodendrocyte development and myelination have been reported, it remains unclear whether interactions among oligodendrocyte-lineage cells (OPCs and oligodendrocytes) affect oligodendrocyte development and myelination. Here, we show that blocking vesicle-associated membrane protein (VAMP) 1/2/3-dependent exocytosis from oligodendrocyte-lineage cells impairs oligodendrocyte development, myelination, and motor behavior in mice. Adding oligodendrocyte-lineage cell-secreted molecules to secretion-deficient OPC cultures partially restores the morphological maturation of oligodendrocytes. Moreover, we identified L-type prostaglandin D synthase as an oligodendrocyte-lineage cell-secreted protein that promotes oligodendrocyte development and myelination in vivo. These findings reveal a novel autocrine/paracrine loop model for the regulation of oligodendrocyte and myelin development.

The epigenetic landscape of oligodendrocyte lineage cells

The epigenetic landscape of oligodendrocyte lineage cells refers to the cell-specific modifications of DNA, chromatin, and RNA that define a unique gene expression pattern of functionally specialized cells. Here, we focus on the epigenetic changes occurring as progenitors differentiate into myelin-forming cells and respond to the local environment. First, modifications of DNA, RNA, nucleosomal histones, key principles of chromatin organization, topologically associating domains, and local remodeling will be reviewed. Then, the relationship between epigenetic modulators and RNA processing will be explored. Finally, the reciprocal relationship between the epigenome as a determinant of the mechanical properties of cell nuclei and the target of mechanotransduction will be discussed. The overall goal is to provide an interpretative key on how epigenetic changes may account for the heterogeneity of the transcriptional profiles identified in this lineage.

Small molecule modulators of chromatin remodeling: from neurodevelopment to neurodegeneration

The dynamic changes in chromatin conformation alter the organization and structure of the genome and further regulate gene transcription. Basically, the chromatin structure is controlled by reversible, enzyme-catalyzed covalent modifications to chromatin components and by noncovalent ATP-dependent modifications via chromatin remodeling complexes, including switch/sucrose nonfermentable (SWI/SNF), inositol-requiring 80 (INO80), imitation switch (ISWI) and chromodomain-helicase DNA-binding protein (CHD) complexes. Recent studies have shown that chromatin remodeling is essential in different stages of postnatal and adult neurogenesis. Chromatin deregulation, which leads to defects in epigenetic gene regulation and further pathological gene expression programs, often causes a wide range of pathologies. This review first gives an overview of the regulatory mechanisms of chromatin remodeling. We then focus mainly on discussing the physiological functions of chromatin remodeling, particularly histone and DNA modifications and the four classes of ATP-dependent chromatin-remodeling enzymes, in the central and peripheral nervous systems under healthy and pathological conditions, that is, in neurodegenerative disorders. Finally, we provide an update on the development of potent and selective small molecule modulators targeting various chromatin-modifying proteins commonly associated with neurodegenerative diseases and their potential clinical applications.

Epilepsy and Autism Spectrum Disorder (ASD): The Underlying Mechanisms and Therapy Targets Related to Adenosine

Epilepsy and autism spectrum disorder (ASD) are highly mutually comorbid, suggesting potential overlaps in genetic etiology, pathophysiology, and neurodevelopmental abnormalities. Adenosine, an endogenous anticonvulsant and neuroprotective neuromodulator of the brain, has been proved to affect the process of epilepsy and ASD. On the one hand, adenosine plays a crucial role in preventing the progression and development of epilepsy through adenosine receptordependent and -independent ways. On the other hand, adenosine signaling can not only regulate core symptoms but also improve comorbid disorders in ASD. Given the important role of adenosine in epilepsy and ASD, therapeutic strategies related to adenosine, including the ketogenic diet, neuromodulation therapy, and adenosine augmentation therapy, have been suggested for the arrangement of epilepsy and ASD. There are several proposals in this review. Firstly, it is necessary to further discuss the relationship between both diseases based on the comorbid symptoms and mechanisms of epilepsy and ASD. Secondly, it is important to explore the role of adenosine involved in epilepsy and ASD. Lastly, potential therapeutic value and clinical approaches of adenosine-related therapies in treating epilepsy and ASD need to be emphasized.

Age-differential sexual dimorphisms in CHD8-S62X-mutant mouse synapses and transcriptomes

Chd8^+/N2373K mice with a human C-terminal-truncating mutation (N2373K) display autistic-like behaviors in juvenile and adult males but not in females. In contrast, Chd8^+/S62X mice with a human N-terminal-truncating mutation (S62X) display behavioral deficits in juvenile males (not females) and adult males and females, indicative of age-differential sexually dimorphic behaviors. Excitatory synaptic transmission is suppressed and enhanced in male and female Chd8^+/S62X juveniles, respectively, but similarly enhanced in adult male and female mutants. ASD-like transcriptomic changes are stronger in newborn and juvenile (but not adult) Chd8^+/S62X males but in newborn and adult (not juvenile) Chd8^+/S62X females. These results point to age-differential sexual dimorphisms in Chd8^+/S62X mice at synaptic and transcriptomic levels, in addition to the behavioral level.

The autism risk factor CHD8 is a chromatin activator in human neurons and functionally dependent on the ERK-MAPK pathway effector ELK1

The chromodomain helicase DNA-binding protein CHD8 is the most frequently mutated gene in autism spectrum disorder. Despite its prominent disease involvement, little is known about its molecular function in the human brain. CHD8 is a chromatin regulator which binds to the promoters of actively transcribed genes through genomic targeting mechanisms which have yet to be fully defined. By generating a conditional loss-of-function and an endogenously tagged allele in human pluripotent stem cells, we investigated the molecular function and the interaction of CHD8 with chromatin in human neurons. Chromatin accessibility analysis and transcriptional profiling revealed that CHD8 functions as a transcriptional activator at its target genes in human neurons. Furthermore, we found that CHD8 chromatin targeting is cell context-dependent. In human neurons, CHD8 preferentially binds at ETS motif-enriched promoters. This enrichment is particularly prominent on the promoters of genes whose expression significantly changes upon the loss of CHD8. Indeed, among the ETS transcription factors, we identified ELK1 as being most highly correlated with CHD8 expression in primary human fetal and adult cortical neurons and most highly expressed in our stem cell-derived neurons. Remarkably, ELK1 was necessary to recruit CHD8 specifically to ETS motif-containing sites. These findings imply that ELK1 and CHD8 functionally cooperate to regulate gene expression and chromatin states at MAPK/ERK target genes in human neurons. Our results suggest that the MAPK/ERK/ELK1 axis potentially contributes to the pathogenesis caused by CHD8 mutations in human neurodevelopmental disorders.

CHD8 suppression impacts on histone H3 lysine 36 trimethylation and alters RNA alternative splicing

Disruptive mutations in the chromodomain helicase DNA-binding protein 8 gene (CHD8) have been recurrently associated with autism spectrum disorders (ASDs). Here we investigated how chromatin reacts to CHD8 suppression by analyzing a panel of histone modifications in induced pluripotent stem cell-derived neural progenitors. CHD8 suppression led to significant reduction (47.82%) in histone H3K36me3 peaks at gene bodies, particularly impacting on transcriptional elongation chromatin states. H3K36me3 reduction specifically affects highly expressed, CHD8-bound genes and correlates with altered alternative splicing patterns of 462 genes implicated in 'regulation of RNA splicing' and 'mRNA catabolic process'. Mass spectrometry analysis uncovered a novel interaction between CHD8 and the splicing regulator heterogeneous nuclear ribonucleoprotein L (hnRNPL), providing the first mechanistic insights to explain the CHD8 suppression-derived splicing phenotype, partly implicating SETD2, a H3K36me3 methyltransferase. In summary, our results point toward broad molecular consequences of CHD8 suppression, entailing altered histone deposition/maintenance and RNA processing regulation as important regulatory processes in ASD.

Impaired macroglial development and axonal conductivity contributes to the neuropathology of DYRK1A-related intellectual disability syndrome

The correct development and activity of neurons and glial cells is necessary to establish proper brain connectivity. DYRK1A encodes a protein kinase involved in the neuropathology associated with Down syndrome that influences neurogenesis and the morphological differentiation of neurons. DYRK1A loss-of-function mutations in heterozygosity cause a well-recognizable syndrome of intellectual disability and autism spectrum disorder. In this study, we analysed the developmental trajectories of macroglial cells and the properties of the corpus callosum, the major white matter tract of the brain, in Dyrk1a^+/- mice, a mouse model that recapitulates the main neurological features of DYRK1A syndrome. We found that Dyrk1a^+/- haploinsufficient mutants present an increase in astrogliogenesis in the neocortex and a delay in the production of cortical oligodendrocyte progenitor cells and their progression along the oligodendroglial lineage. There were fewer myelinated axons in the corpus callosum of Dyrk1a^+/- mice, axons that are thinner and with abnormal nodes of Ranvier. Moreover, action potential propagation along myelinated and unmyelinated callosal axons was slower in Dyrk1a^+/- mutants. All these alterations are likely to affect neuronal circuit development and alter network synchronicity, influencing higher brain functions. These alterations highlight the relevance of glial cell abnormalities in neurodevelopmental disorders.

Conserved and Distinct Functions of the Autism-Related Chromatin Remodeler CHD8 in Embryonic and Adult Forebrain Neurogenesis

The chromatin remodeler CHD8 represents a high-confidence risk factor in autism, a multistage progressive neurologic disorder, however the underlying stage-specific functions remain elusive. In this study, by analyzing Chd8 conditional knock-out mice (male and female), we find that CHD8 controls cortical neural stem/progenitor cell (NSC) proliferation and survival in a stage-dependent manner. Strikingly, inducible genetic deletion reveals that CHD8 is required for the production and fitness of transit-amplifying intermediate progenitors (IPCs) essential for upper-layer neuron expansion in the embryonic cortex. p53 loss of function partially rescues apoptosis and neurogenesis defects in the Chd8-deficient brain. Further, transcriptomic and epigenomic profiling indicates that CHD8 regulates the chromatin accessibility landscape to activate neurogenesis-promoting factors including TBR2, a key regulator of IPC neurogenesis, while repressing DNA damage- and p53-induced apoptotic programs. In the adult brain, CHD8 depletion impairs forebrain neurogenesis by impeding IPC differentiation from NSCs in both subventricular and subgranular zones; however, unlike in embryos, it does not affect NSC proliferation and survival. Treatment with an antidepressant approved by the Federal Drug Administration (FDA), fluoxetine, partially restores adult hippocampal neurogenesis in Chd8-ablated mice. Together, our multistage functional studies identify temporally specific roles for CHD8 in developmental and adult neurogenesis, pointing to a potential strategy to enhance neurogenesis in the CHD8-deficient brain.SIGNIFICANCE STATEMENT The role of the high-confidence autism gene CHD8 in neurogenesis remains incompletely understood. Here, we identify a stage-specific function of CHD8 in development of NSCs in developing and adult brains by conserved, yet spatiotemporally distinct, mechanisms. In embryonic cortex, CHD8 is critical for the proliferation, survival, and differentiation of both NSC and IPCs during cortical neurogenesis. In adult brain, CHD8 is required for IPC generation but not the proliferation and survival of adult NSCs. Treatment with FDA-approved antidepressant fluoxetine partially rescues the adult neurogenesis defects in CHD8 mutants. Thus, our findings help resolve CHD8 functions throughout life during embryonic and adult neurogenesis and point to a potential avenue to promote neurogenesis in CHD8 deficiency.

Age-differential sexual dimorphism in CHD8-S62X-mutant mouse behaviors

Autism spectrum disorders (ASD) are ~4-times more common in males than females, and CHD8 (a chromatin remodeler)-related ASD shows a strong male bias (~4:1), although the underlying mechanism remains unclear. Chd8-mutant mice with a C-terminal protein-truncating mutation (N2373K) display male-preponderant behavioral deficits as juveniles and adults, although whether this also applies to other Chd8 mutations remains unknown. In addition, it remains unclear whether sexually dimorphic phenotypes in Chd8-mutant mice are differentially observed in males and females across different ages. We here generated new Chd8-mutant (knock-in) mice carrying a patient-derived mutation causing an N-terminal and stronger protein truncation (Chd8+/S62X mice) and characterized the mice by behavioral analyses. Juvenile Chd8+/S62X mice displayed male-preponderant autistic-like behaviors; hypoactivity and enhanced mother-seeking/attachment behavior in males but not in females. Adult male and female Chd8+/S62X mice showed largely similar deficits in repetitive and anxiety-like behavioral domains. Therefore, the CHD8-S62X mutation induces ASD-like behaviors in juvenile male mice and adult male and female mice, pointing to an age-differential sexual dimorphism and also distinct sexual dimorphisms in different Chd8 mutations (N2373K and S62X).

Developmental Cues and Molecular Drivers in Myelinogenesis: Revisiting Early Life to Re-Evaluate the Integrity of CNS Myelin

The mammalian central nervous system (CNS) coordinates its communication through saltatory conduction, facilitated by myelin-forming oligodendrocytes (OLs). Despite the fact that neurogenesis from stem cell niches has caught the majority of attention in recent years, oligodendrogenesis and, more specifically, the molecular underpinnings behind OL-dependent myelinogenesis, remain largely unknown. In this comprehensive review, we determine the developmental cues and molecular drivers which regulate normal myelination both at the prenatal and postnatal periods. We have indexed the individual stages of myelinogenesis sequentially; from the initiation of oligodendrocyte precursor cells, including migration and proliferation, to first contact with the axon that enlists positive and negative regulators for myelination, until the ultimate maintenance of the axon ensheathment and myelin growth. Here, we highlight multiple developmental pathways that are key to successful myelin formation and define the molecular pathways that can potentially be targets for pharmacological interventions in a variety of neurological disorders that exhibit demyelination.

Autism-associated chromatin remodeler CHD8 regulates erythroblast cytokinesis and fine-tunes the balance of Rho GTPase signaling

CHD8 is an ATP-dependent chromatin-remodeling factor whose monoallelic mutation defines a subtype of autism spectrum disorders (ASDs). Previous work found that CHD8 is required for the maintenance of hematopoiesis by integrating ATM-P53-mediated survival of hematopoietic stem/progenitor cells (HSPCs). Here, by using Chd8^F/FMx1-Cre combined with a Trp53^F/F mouse model that suppresses apoptosis of Chd8^−/− HSPCs, we identify CHD8 as an essential regulator of erythroid differentiation. Chd8^−/−P53^−/− mice exhibited severe anemia conforming to congenital dyserythropoietic anemia (CDA) phenotypes. Loss of CHD8 leads to drastically decreased numbers of orthochromatic erythroblasts and increased binucleated and multinucleated basophilic erythroblasts with a cytokinesis failure in erythroblasts. CHD8 binds directly to the gene bodies of multiple Rho GTPase signaling genes in erythroblasts, and loss of CHD8 results in their dysregulated expression, leading to decreased RhoA and increased Rac1 and Cdc42 activities. Our study shows that autism-associated CHD8 is essential for erythroblast cytokinesis.

Tu et al. report that CHD8, an autism-related chromatin remodeler, is essential for erythroid differentiation. Loss of CHD8 leads to unbalanced Rho GTPase signaling and defective erythroblast cytokinesis, mimicking that of congenital dyserythropoietic anemia.

Long Noncoding RNAs in CNS Myelination and Disease

Myelination by oligodendrocytes is crucial for neuronal survival and function, and defects in myelination or failure in myelin repair can lead to axonal degeneration and various neurological diseases. At present, the factors that promote myelination and overcome the remyelination block in demyelinating diseases are poorly defined. Although the roles of protein-coding genes in oligodendrocyte differentiation have been extensively studied, the majority of the mammalian genome is transcribed into noncoding RNAs, and the functions of these molecules in myelination are poorly characterized. Long noncoding RNAs (lncRNAs) regulate transcription at multiple levels, providing spatiotemporal control and robustness for cell type-specific gene expression and physiological functions. lncRNAs have been shown to regulate neural cell-type specification, differentiation, and maintenance of cell identity, and dysregulation of lncRNA function has been shown to contribute to neurological diseases. In this review, we discuss recent advances in our understanding of the functions of lncRNAs in oligodendrocyte development and myelination as well their roles in neurological diseases and brain tumorigenesis. A more systematic characterization of lncRNA functional networks will be instrumental for a better understanding of CNS myelination, myelin disorders, and myelin repair.

Developmental pyrethroid exposure and age influence phenotypes in a Chd8 haploinsufficient autism mouse model

Hundreds of genes have been associated with autism spectrum disorder (ASD), including loss-of-function mutations in chromodomain helicase DNA binding protein 8 (Chd8). Environmental factors also are implicated in autism risk and have the potential to exacerbate phenotypes in genetically sensitized backgrounds. Here we investigate transcriptional and behavioral phenotypes in a Chd8 haploinsufficient (Chd8^V986*/+) mouse line exposed to the pesticide deltamethrin (DM) from conception to postnatal day 22. Vehicle-exposed Chd8^V986*/+ mice displayed ASD-associated phenotypes, including anxiety-like behavior and altered sociability, replicating a previous study with this mouse line. A core set of genes was altered in Chd8^V986*/+ mice at multiple ages, including Usp11, Wars2, Crlf2, and Eglf6, and proximity ligation data indicated direct binding of CHD8 to the 5’ region of these genes. Moreover, oligodendrocyte and neurodegenerative transcriptional phenotypes were apparent in 12 and 18 month old Chd8^V986*/+ mice. Following DM exposure, the mutant mice displayed an exacerbated phenotype in the elevated plus maze, and genes associated with vascular endothelial cells were downregulated in the cerebral cortex of older Chd8^V986*/+ animals. Our study reveals a gene x environment interaction with a Chd8 haploinsufficient mouse line and points to the importance of investigating phenotypes in ASD animal models across the lifespan.

mTOR pathway repressing expression of FoxO3 is a potential mechanism involved in neonatal white matter dysplasia

Neonatal white matter dysplasia (NWMD) is characterized by developmental abnormity of CNS white matter, including abnormal myelination. Besides environmental factors such as suffocation at birth, genetic factors are also main causes. Signaling pathway is an important part of gene function and several signaling pathways play important roles in myelination. Here, we performed genetic analysis on a corhort of 138 patients with NWMD and found that 20% (5/25) cause genes which refered to 28.57% (8/28) patients enriched in mTOR signaling pathway. Depletion of mTOR reduced genesis and proliferation of oligodendrocyte progenitor cells (OPC) during embryonic stage and reduced myelination in corpus callosum besides cerebellum and spinal cord during early postnatal stages which is related to not only differentiation but also proliferation of oligodendrocyte (OL). Transcriptomic analyses indicated that depletion of mTOR in OLs upregulated expression of FoxO3, which is a repressor of expression of myelin basic protein (MBP), and downregulating expresion of FoxO3 by siRNA promoted OPCs develop into MBP+ OLs. Thus, our findings suggested that mTOR signaling pathway is NWMD-related pathway and mTOR is important for myelination of the entire CNS during early developmental stages through regulating expression of FoxO3 at least partially.

Dysmyelination by Oligodendrocyte-Specific Ablation of Ninj2 Contributes to Depressive-Like Behaviors

Depression is a mental disorder affecting more than 300 million people in the world. Abnormalities in white matter are associated with the development of depression. Here, the authors show that mice with oligodendrocyte-specific deletion of Nerve injury-induced protein 2 （Ninj2) exhibit depressive-like behaviors. Loss of Ninj2 in oligodendrocytes inhibits oligodendrocyte development and myelination, and impairs neuronal structure and activities. Ninj2 competitively inhibits TNFα/TNFR1 signaling pathway by directly binding to TNFR1 in oligodendrocytes. Loss of Ninj2 activates TNFα-induced necroptosis, and increases C-C Motif Chemokine Ligand 2 (Ccl2) production, which might mediate the signal transduction from oligodendrocyte to neurons. Inhibition of necroptosis by Nec-1s administration synchronously restores oligodendrocyte development, improves neuronal excitability, and alleviates depressive-like behaviors. This study thus illustrates the role of Ninj2 in the development of depression and myelination, reveals the relationship between oligodendrocytes and neurons, and provides a potential therapeutic target for depression.

Identifying oligodendrocyte enhancers governing Plp1 expression

Oligodendrocytes (OLs) produce myelin in the central nervous system (CNS), which accelerates the propagation of action potentials and supports axonal integrity. As a major component of CNS myelin, proteolipid protein 1 (Plp1) is indispensable for the axon-supportive function of myelin. Notably, this function requires the continuous high-level expression of Plp1 in OLs. Equally important is the controlled expression of Plp1, as illustrated by Pelizaeus-Merzbacher disease for which the most common cause is PLP1 overexpression. Despite a decade-long search, promoter-distal OL enhancers that govern Plp1 remain elusive. We have recently developed an innovative method that maps promoter-distal enhancers to genes in a principled manner. Here, we applied it to Plp1, uncovering two OL enhancers for it (termed Plp1-E1 and Plp1-E2). Remarkably, clustered regularly interspaced short palindromic repeats (CRISPR) interference epigenome editing showed that Plp1-E1 and Plp1-E2 do not regulate two genes in their vicinity, highlighting their exquisite specificity to Plp1. Assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and chromatin immunoprecipitation with high-throughput sequencing (ChIP-seq) data show that Plp1-E1 and Plp1-E2 are OL-specific enhancers that are conserved among human, mouse and rat. Hi-C data reveal that the physical interactions between Plp1-E1/2 and PLP1 are among the strongest in OLs and specific to OLs. We also show that Myrf, a master regulator of OL development, acts on Plp1-E1 and Plp1-E2 to promote Plp1 expression.

DHHC5 facilitates oligodendrocyte development by palmitoylating and activating STAT3

Myelin sheath is an important structure to maintain functions of the nerves in central nervous system. Protein palmitoylation has been established as a sorting determinant for the transport of myelin-forming proteins to the myelin membrane, however, its function in the regulation of oligodendrocyte development remains unknown. Here, we show that an Asp-His-His-Cys (DHHC) motif-containing palmitoyl acyltransferases, DHHC5, is involved in the control of oligodendrocyte development. Loss of Zdhhc5 in oligodendrocytes inhibits myelination and remyelination by reducing total myelinating oligodendrocyte population. STAT3 is the primary substrate for DHHC5 palmitoylation in oligodendrocytes. Zdhhc5 ablation reduces STAT3 palmitoylation and suppresses STAT3 phosphorylation and activation. As a result, the transcription of the myelin-related and anti-apoptosis genes is inhibited, leading to suppressed oligodendrocyte development and myelination. Our findings demonstrate a key role DHHC5 in controlling myelinogenesis.

Sentinels of chromatin: chromodomain helicase DNA-binding proteins in development and disease

Chromatin is highly dynamic, undergoing continuous global changes in its structure and type of histone and DNA modifications governed by processes such as transcription, repair, replication, and recombination. Members of the chromodomain helicase DNA-binding (CHD) family of enzymes are ATP-dependent chromatin remodelers that are intimately involved in the regulation of chromatin dynamics, altering nucleosomal structure and DNA accessibility. Genetic studies in yeast, fruit flies, zebrafish, and mice underscore essential roles of CHD enzymes in regulating cellular fate and identity, as well as proper embryonic development. With the advent of next-generation sequencing, evidence is emerging that these enzymes are subjected to frequent DNA copy number alterations or mutations and show aberrant expression in malignancies and other human diseases. As such, they might prove to be valuable biomarkers or targets for therapeutic intervention.

Early-life midazolam exposure persistently changes chromatin accessibility to impair adult hippocampal neurogenesis and cognition

Linkage between early-life exposure to anesthesia and subsequent learning disabilities is of great concern to children and their families. Here we show that early-life exposure to midazolam (MDZ), a widely used drug in pediatric anesthesia, persistently alters chromatin accessibility and the expression of quiescence-associated genes in neural stem cells (NSCs) in the mouse hippocampus. The alterations led to a sustained restriction of NSC proliferation toward adulthood, resulting in a reduction of neurogenesis that was associated with the impairment of hippocampal-dependent memory functions. Moreover, we found that voluntary exercise restored hippocampal neurogenesis, normalized the MDZ-perturbed transcriptome, and ameliorated cognitive ability in MDZ-exposed mice. Our findings thus explain how pediatric anesthesia provokes long-term adverse effects on brain function and provide a possible therapeutic strategy for countering them.

Microglial ASD-related genes are involved in oligodendrocyte differentiation

Autism spectrum disorders (ASD) are associated with mutations of chromodomain-helicase DNA-binding protein 8 (Chd8) and tuberous sclerosis complex 2 (Tsc2). Although these ASD-related genes are detected in glial cells such as microglia, the effect of Chd8 or Tsc2 deficiency on microglial functions and microglia-mediated brain development remains unclear. In this study, we investigated the role of microglial Chd8 and Tsc2 in cytokine expression, phagocytosis activity, and neuro/gliogenesis from neural stem cells (NSCs) in vitro. Chd8 or Tsc2 knockdown in microglia reduced insulin-like growth factor-1(Igf1) expression under lipopolysaccharide (LPS) stimulation. In addition, phagocytosis activity was inhibited by Tsc2 deficiency, microglia-mediated oligodendrocyte development was inhibited, in particular, the differentiation of oligodendrocyte precursor cells to oligodendrocytes was prevented by Chd8 or Tsc2 deficiency. These results suggest that ASD-related gene expression in microglia is involved in oligodendrocyte differentiation, which may contribute to the white matter pathology relating to ASD.

Using the lineage determinants Olig2 and Sox10 to explore transcriptional regulation of oligodendrocyte development

The transcription factors Olig2 and Sox10 jointly define oligodendroglial identity. Because of their continuous presence during development and in the differentiated state they shape the oligodendroglial regulatory network at all times. In this review, we exploit their eminent role and omnipresence to elaborate the central principles that organize the gene regulatory network in oligodendrocytes in such a way that it preserves its identity, but at the same time allows defined and stimulus-dependent changes that result in an ordered lineage progression, differentiation, and myelination. For this purpose, we outline the multiple functional and physical interactions and intricate cross-regulatory relationships with other transcription factors, such as Hes5, Id, and SoxD proteins, in oligodendrocyte precursors and Tcf7l2, Sip1, Nkx2.2, Zfp24, and Myrf during differentiation and myelination, and interpret them in the context of the regulatory network.

Cell of all trades: oligodendrocyte precursor cells in synaptic, vascular, and immune function

Oligodendrocyte precursor cells (OPCs) are not merely a transitory progenitor cell type, but rather a distinct and heterogeneous population of glia with various functions in the developing and adult central nervous system. In this review, we discuss the fate and function of OPCs in the brain beyond their contribution to myelination. OPCs are electrically sensitive, form synapses with neurons, support blood-brain barrier integrity, and mediate neuroinflammation. We explore how sex and age may influence OPC activity, and we review how OPC dysfunction may play a primary role in numerous neurological and neuropsychiatric diseases. Finally, we highlight areas of future research.

The phenomenal epigenome in neurodevelopmental disorders

Disruption of chromatin structure due to epimutations is a leading genetic etiology of neurodevelopmental disorders, collectively known as chromatinopathies. We show that there is an increasing level of convergence from the high diversity of genes that are affected by mutations to the molecular networks and pathways involving the respective proteins, the disrupted cellular and subcellular processes, and their consequence for higher order cellular network function. This convergence is ultimately reflected by specific phenotypic features shared across the various chromatinopathies. Based on these observations, we propose that the commonly disrupted molecular and cellular anomalies might provide a rational target for the development of symptomatic interventions for defined groups of genetically distinct neurodevelopmental disorders.

Oligodendrocyte dysfunction due to Chd8 mutation gives rise to behavioral deficits in mice

Mutations in the gene encoding the chromatin remodeler CHD8 are strongly associated with autism spectrum disorder (ASD). CHD8 haploinsufficiency also results in autistic phenotypes in humans and mice. Although myelination defects have been observed in individuals with ASD, whether oligodendrocyte dysfunction is responsible for autistic phenotypes has remained unknown. Here we show that reduced expression of CHD8 in oligodendrocytes gives rise to abnormal behavioral phenotypes in mice. CHD8 was found to regulate the expression of many myelination-related genes and to be required for oligodendrocyte maturation and myelination. Ablation of Chd8 specifically in oligodendrocytes of mice impaired myelination, slowed action potential propagation and resulted in behavioral deficits including increased social interaction and anxiety-like behavior, with similar effects being apparent in Chd8 heterozygous mutant mice. Our results thus indicate that CHD8 is essential for myelination and that dysfunction of oligodendrocytes as a result of CHD8 haploinsufficiency gives rise to several neuropsychiatric phenotypes.

The Mechanisms of CHD8 in Neurodevelopment and Autism Spectrum Disorders

Chromodomain-helicase-DNA-binding protein 8 (CHD8) has been identified as one of the genes with the strongest association with autism. The CHD8 protein is a transcriptional regulator that is expressed in nearly all cell types and has been implicated in multiple cellular processes, including cell cycle, cell adhesion, neuronal development, myelination, and synaptogenesis. Considering the central role of CHD8 in the genetics of autism, a deeper understanding of the physiological functions of CHD8 is important to understand the development of the autism phenotype and potential therapeutic targets. Different CHD8 mutant mouse models were developed to determine autism-like phenotypes and to fully understand their mechanisms. Here, we review the current knowledge on CHD8, with an emphasis on mechanistic lessons gained from animal models that have been studied.

The chromatin remodeler CHD8 governs hematopoietic stem/progenitor survival by regulating ATM-mediated P53 protein stability

The Chd8 gene encodes a member of the chromodomain helicase DNA-binding (CHD) family of SNF2H-like adenosine triphosphate (ATP)-dependent chromatin remodeler, the mutations of which define a subtype of autism spectrum disorders. Increasing evidence from recent studies indicates that ATP-dependent chromatin-remodeling genes are involved in the control of crucial gene-expression programs in hematopoietic stem/progenitor cell (HSPC) regulation. In this study, we identified CHD8 as a specific and essential regulator of normal hematopoiesis. Loss of Chd8 leads to severe anemia, pancytopenia, bone marrow failure, and engraftment failure related to a drastic depletion of HSPCs. CHD8 forms a complex with ATM and its deficiency increases chromatin accessibility and drives genomic instability in HSPCs causing an activation of ATM kinase that further stabilizes P53 protein by phosphorylation and leads to increased HSPC apoptosis. Deletion of P53 rescues the apoptotic defects of HSPCs and restores overall hematopoiesis in Chd8-/- mice. Our findings demonstrate that chromatin organization by CHD8 is uniquely necessary for the maintenance of hematopoiesis by integrating the ATM-P53-mediated survival of HSPCs.

MyelTracer: A Semi-Automated Software for Myelin g-Ratio Quantification

In the central and peripheral nervous systems, the myelin sheath promotes neuronal signal transduction. The thickness of the myelin sheath changes during development and in disease conditions like multiple sclerosis. Such changes are routinely detected using electron microscopy through g-ratio quantification. While g-ratio is one of the most critical measurements in myelin studies, a major drawback is that g-ratio quantification is extremely laborious and time-consuming. Here, we report the development and validation of MyelTracer, an installable, stand-alone software for semi-automated g-ratio quantification based on the Open Computer Vision Library (OpenCV). Compared with manual g-ratio quantification, using MyelTracer produces consistent results across multiple tissues and animal ages, as well as in remyelination after optic nerve crush, and reduces total quantification time by 40-60%. With g-ratio measurements via MyelTracer, a known hypomyelination phenotype can be detected in a Williams syndrome mouse model. MyelTracer is easy to use and freely available for Windows and Mac OS X (https://github.com/HarrisonAllen/MyelTracer).

Conditional Loss of BAF (mSWI/SNF) Scaffolding Subunits Affects Specification and Proliferation of Oligodendrocyte Precursors in Developing Mouse Forebrain

Oligodendrocytes are responsible for axon myelination in the brain and spinal cord. Generation of oligodendrocytes entails highly regulated multistage neurodevelopmental events, including proliferation, differentiation and maturation. The chromatin remodeling BAF (mSWI/SNF) complex is a notable regulator of neural development. In our previous studies, we determined the indispensability of the BAF complex scaffolding subunits BAF155 and BAF170 for neurogenesis, whereas their role in gliogenesis is unknown. Here, we show that the expression of BAF155 and BAF170 is essential for the genesis of oligodendrocytes during brain development. We report that the ablation of BAF155 and BAF170 in the dorsal telencephalic (dTel) neural progenitors or in oligodendrocyte-producing progenitors in the ventral telencephalon (vTel) in double-conditional knockout (dcKO) mouse mutants, perturbed the process of oligodendrogenesis. Molecular marker and cell cycle analyses revealed impairment of oligodendrocyte precursor specification and proliferation, as well as overt depletion of oligodendrocytes pool in dcKO mutants. Our findings unveil a central role of BAF155 and BAF170 in oligodendrogenesis, and thus substantiate the involvement of the BAF complex in the production of oligodendrocytes in the forebrain.

The CHD8/CHD7/Kismet family links blood-brain barrier glia and serotonin to ASD-associated sleep defects

Sleep disturbances in autism and neurodevelopmental disorders are common and adversely affect patient's quality of life, yet the underlying mechanisms are understudied. We found that individuals with mutations in CHD8, among the highest-confidence autism risk genes, or CHD7 suffer from disturbed sleep maintenance. These defects are recapitulated in Drosophila mutants affecting kismet, the sole CHD8/CHD7 ortholog. We show that Kismet is required in glia for early developmental and adult sleep architecture. This role localizes to subperineurial glia constituting the blood-brain barrier. We demonstrate that Kismet-related sleep disturbances are caused by high serotonin during development, paralleling a well-established but genetically unsolved autism endophenotype. Despite their developmental origin, Kismet's sleep architecture defects can be reversed in adulthood by a behavioral regime resembling human sleep restriction therapy. Our findings provide fundamental insights into glial regulation of sleep and propose a causal mechanistic link between the CHD8/CHD7/Kismet family, developmental hyperserotonemia, and autism-associated sleep disturbances.

Qki activates Srebp2-mediated cholesterol biosynthesis for maintenance of eye lens transparency

Defective cholesterol biosynthesis in eye lens cells is often associated with cataracts; however, how genes involved in cholesterol biosynthesis are regulated in lens cells remains unclear. Here, we show that Quaking (Qki) is required for the transcriptional activation of genes involved in cholesterol biosynthesis in the eye lens. At the transcriptome level, lens-specific Qki-deficient mice present downregulation of genes associated with the cholesterol biosynthesis pathway, resulting in a significant reduction of total cholesterol level in the eye lens. Mice with Qki depletion in lens epithelium display progressive accumulation of protein aggregates, eventually leading to cataracts. Notably, these defects are attenuated by topical sterol administration. Mechanistically, we demonstrate that Qki enhances cholesterol biosynthesis by recruiting Srebp2 and Pol II in the promoter regions of cholesterol biosynthesis genes. Supporting its function as a transcription co-activator, we show that Qki directly interacts with single-stranded DNA. In conclusion, we propose that Qki-Srebp2-mediated cholesterol biosynthesis is essential for maintaining the cholesterol level that protects lens from cataract development.