Chromatin-remodeling factor Brg1 is required for Schwann cell differentiation and myelination

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.

The SWI/SNF Complex in Neural Crest Cell Development and Disease

While the neural crest cell population gives rise to an extraordinary array of derivatives, including elements of the craniofacial skeleton, skin pigmentation, and peripheral nervous system, it is today increasingly recognized that Schwann cell precursors are also multipotent. Two mammalian paralogs of the SWI/SNF (switch/sucrose nonfermentable) chromatin-remodeling complexes, BAF (Brg1-associated factors) and PBAF (polybromo-associated BAF), are critical for neural crest specification during normal mammalian development. There is increasing evidence that pathogenic variants in components of the BAF and PBAF complexes play central roles in the pathogenesis of neural crest-derived tumors. Transgenic mouse models demonstrate a temporal window early in development where pathogenic variants in Smarcb1 result in the formation of aggressive, poorly differentiated tumors, such as rhabdoid tumors. By contrast, later in development, homozygous inactivation of Smarcb1 requires additional pathogenic variants in tumor suppressor genes to drive the development of differentiated adult neoplasms derived from the neural crest, which have a comparatively good prognosis in humans.

Stepwise activities of mSWI/SNF family chromatin remodeling complexes direct T cell activation and exhaustion

Highly coordinated changes in gene expression underlie T cell activation and exhaustion. However, the mechanisms by which such programs are regulated and how these may be targeted for therapeutic benefit remain poorly understood. Here, we comprehensively profile the genomic occupancy of mSWI/SNF chromatin remodeling complexes throughout acute and chronic T cell stimulation, finding that stepwise changes in localization over transcription factor binding sites direct site-specific chromatin accessibility and gene activation leading to distinct phenotypes. Notably, perturbation of mSWI/SNF complexes using genetic and clinically relevant chemical strategies enhances the persistence of T cells with attenuated exhaustion hallmarks and increased memory features in vitro and in vivo. Finally, pharmacologic mSWI/SNF inhibition improves CAR-T expansion and results in improved anti-tumor control in vivo. These findings reveal the central role of mSWI/SNF complexes in the coordination of T cell activation and exhaustion and nominate small-molecule-based strategies for the improvement of current immunotherapy protocols.

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.

Development of myelinating glia: An overview

Myelin is essential to nervous system function, playing roles in saltatory conduction and trophic support. Oligodendrocytes (OLs) and Schwann cells (SCs) form myelin in the central and peripheral nervous systems respectively and follow different developmental paths. OLs are neural stem-cell derived and follow an intrinsic developmental program resulting in a largely irreversible differentiation state. During embryonic development, OL precursor cells (OPCs) are produced in distinct waves originating from different locations in the central nervous system, with a subset developing into myelinating OLs. OPCs remain evenly distributed throughout life, providing a population of responsive, multifunctional cells with the capacity to remyelinate after injury. SCs derive from the neural crest, are highly dependent on extrinsic signals, and have plastic differentiation states. SC precursors (SCPs) are produced in early embryonic nerve structures and differentiate into multipotent immature SCs (iSCs), which initiate radial sorting and differentiate into myelinating and non-myelinating SCs. Differentiated SCs retain the capacity to radically change phenotypes in response to external signals, including becoming repair SCs, which drive peripheral regeneration. While several transcription factors and myelin components are common between OLs and SCs, their differentiation mechanisms are highly distinct, owing to their unique lineages and their respective environments. In addition, both OLs and SCs respond to neuronal activity and regulate nervous system output in reciprocal manners, possibly through different pathways. Here, we outline their basic developmental programs, mechanisms regulating their differentiation, and recent advances in the field.

KLF9 and KLF13 transcription factors boost myelin gene expression in oligodendrocytes as partners of SOX10 and MYRF

Differentiated oligodendrocytes produce myelin and thereby ensure rapid nerve impulse conduction and efficient information processing in the vertebrate central nervous system. The Krüppel-like transcription factor KLF9 enhances oligodendrocyte differentiation in culture, but appears dispensable in vivo. Its mode of action and role within the oligodendroglial gene regulatory network are unclear. Here we show that KLF9 shares its expression in differentiating oligodendrocytes with the closely related KLF13 protein. Both KLF9 and KLF13 bind to regulatory regions of genes that are important for oligodendrocyte differentiation and equally recognized by the central differentiation promoting transcription factors SOX10 and MYRF. KLF9 and KLF13 physically interact and synergistically activate oligodendrocyte-specific regulatory regions with SOX10 and MYRF. Similar to KLF9, KLF13 promotes differentiation and myelination in primary oligodendroglial cultures. Oligodendrocyte differentiation is also altered in KLF13-deficient mice as demonstrated by a transiently reduced myelin gene expression during the first postnatal week. Considering mouse phenotypes, the similarities in expression pattern and genomic binding and the behaviour in functional assays, KLF9 and KLF13 are important and largely redundant components of the gene regulatory network in charge of oligodendrocyte differentiation and myelination.

Transcription factor SS18L1 regulates the proliferation, migration and differentiation of Schwann cells in peripheral nerve injury

Transcription factors bind to specific DNA sequences, modulate the transcription of target genes, and regulate various biological processes, including peripheral nerve regeneration. Our previous analysis showed that SS18L1, a gene encoding the transcription factor SS18-like protein 1, was differentially expressed in the distal sciatic nerve stumps after rat sciatic nerve transection injury, but its effect on peripheral nerve injury has not been reported. In the current study, we isolated and cultured primary Schwann cells, and examined the role of SS18L1 for the biological functions of the cells. Depletion of SS18L1 by siRNA in Schwann cells enhanced cell proliferation and inhibited cell migration, as determined by EdU assay and transwell migration assay, respectively. In addition, silencing of SS18L1 inhibited Schwann cell differentiation induced by HRG and cAMP. Bioinformatics analyses revealed an interaction network of SS18L1, including DF2, SMARCD1, SMARCA4, and SMARCE1, which may be implicated in the regulatory functions of SS18L1 on the proliferation, migration and differentiation of Schwann cells. In conclusion, our results revealed a temporal expression profile of SS18L1 in peripheral nerve injury and its potential roles during the process of nerve recovery.

SOX2 transcription factor binding and function

The transcription factor SOX2 is a vital regulator of stem cell activity in various developing and adult tissues. Mounting evidence has demonstrated the importance of SOX2 in regulating the induction and maintenance of stemness as well as in controlling cell proliferation, lineage decisions and differentiation. Recent studies have revealed that the ability of SOX2 to regulate these stem cell features involves its function as a pioneer factor, with the capacity to target nucleosomal DNA, modulate chromatin accessibility and prepare silent genes for subsequent activation. Moreover, although SOX2 binds to similar DNA motifs in different stem cells, its multifaceted and cell type-specific functions are reliant on context-dependent features. These cell type-specific properties include variations in partner factor availability and SOX2 protein expression levels. In this Primer, we discuss recent findings that have increased our understanding of how SOX2 executes its versatile functions as a master regulator of stem cell activities.

ACTL6a coordinates axonal caliber recognition and myelination in the peripheral nerve

Cells elaborate transcriptional programs in response to external signals. In the peripheral nerves, Schwann cells (SC) sort axons of given caliber and start the process of wrapping their membrane around them. We identify Actin-like protein 6a (ACTL6a), part of SWI/SNF chromatin remodeling complex, as critical for the integration of axonal caliber recognition with the transcriptional program of myelination. Nuclear levels of ACTL6A in SC are increased by contact with large caliber axons or nanofibers, and result in the eviction of repressive histone marks to facilitate myelination. Without Actl6a the SC are unable to coordinate caliber recognition and myelin production. Peripheral nerves in knockout mice display defective radial sorting, hypo-myelination of large caliber axons, and redundant myelin around small caliber axons, resulting in a clinical motor phenotype. Overall, this suggests that ACTL6A is a key component of the machinery integrating external signals for proper myelination of the peripheral nerve.

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ACTL6a levels in Schwann cells respond to stiffness and caliber of PLA nanofibers

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ACTL6a integrates axonal caliber recognition signals with Schwann cells transcriptome

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ACTL6a null mice have thin myelin on large axons and redundant myelin on small axons

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Mice lacking ACTL6a in Schwann cells show severe clinical symptoms

SWI/SNF Chromatin Remodeling Enzymes in Melanoma

Melanoma is an aggressive malignancy that arises from the transformation of melanocytes on the skin, mucosal membranes, and uvea of the eye. SWI/SNF chromatin remodeling enzymes are multi-subunit complexes that play important roles in the development of the melanocyte lineage and in the response to ultraviolet radiation, a key environmental risk factor for developing cutaneous melanoma. Exome sequencing has revealed frequent loss of function mutations in genes encoding SWI/SNF subunits in melanoma. However, some SWI/SNF subunits have also been demonstrated to have pro-tumorigenic roles in melanoma and to affect sensitivity to therapeutics. This review summarizes studies that have implicated SWI/SNF components in melanomagenesis and have evaluated how SWI/SNF subunits modulate the response to current therapeutics.

Role of the Pbrm1 subunit and the PBAF complex in Schwann cell development

Myelin sheath formation in the peripheral nervous system and the ensuing saltatory conduction rely on differentiated Schwann cells. We have previously shown that transition of Schwann cells from an immature into a differentiated state requires Brg1 that serves as the central energy generating subunit in two related SWI/SNF-type chromatin remodelers, the BAF and the PBAF complex. Here we used conditional deletion of Pbrm1 to selectively interfere with the PBAF complex in Schwann cells. Despite efficient loss of Pbrm1 early during lineage progression, we failed to detect any substantial alterations in the number, proliferation or survival of immature Schwann cells as well as in their rate and timing of terminal differentiation. As a consequence, postnatal myelin formation in peripheral nerves appeared normal. There were no inflammatory alterations in the nerve or other signs of a peripheral neuropathy. We conclude from our study that Pbrm1 and very likely the PBAF complex are dispensable for proper Schwann cell development and that Schwann cell defects previously observed upon Brg1 deletion are mostly attributable to altered or absent function of the BAF complex.

Reprogramming of the epigenome in neurodevelopmental disorders

The etiology of neurodevelopmental disorders (NDDs) remains a challenge for researchers. Human brain development is tightly regulated and sensitive to cellular alterations caused by endogenous or exogenous factors. Intriguingly, the surge of clinical sequencing studies has revealed that many of these disorders are monogenic and monoallelic. Notably, chromatin regulation has emerged as highly dysregulated in NDDs, with many syndromes demonstrating phenotypic overlap, such as intellectual disabilities, with one another. Here we discuss epigenetic writers, erasers, readers, remodelers, and even histones mutated in NDD patients, predicted to affect gene regulation. Moreover, this review focuses on disorders associated with mutations in enzymes involved in histone acetylation and methylation, and it highlights syndromes involving chromatin remodeling complexes. Finally, we explore recently discovered histone germline mutations and their pathogenic outcome on neurological function. Epigenetic regulators are mutated at every level of chromatin organization. Throughout this review, we discuss mechanistic investigations, as well as various animal and iPSC models of these disorders and their usefulness in determining pathomechanism and potential therapeutics. Understanding the mechanism of these mutations will illuminate common pathways between disorders. Ultimately, classifying these disorders based on their effects on the epigenome will not only aid in prognosis in patients but will aid in understanding the role of epigenetic machinery throughout neurodevelopment.

Chromatin Remodelers Interact with Eya1 and Six2 to Target Enhancers to Control Nephron Progenitor Cell Maintenance

BACKGROUND

Eya1 is a critical regulator of nephron progenitor cell specification and interacts with Six2 to promote NPC self-renewal. Haploinsufficiency of these genes causes kidney hypoplasia. However, how the Eya1-centered network operates remains unknown.

METHODS

We engineered a 2×HA-3×Flag-Eya1 knock-in mouse line and performed coimmunoprecipitation with anti-HA or -Flag to precipitate the multitagged-Eya1 and its associated proteins. Loss-of-function, transcriptome profiling, and genome-wide binding analyses for Eya1's interacting chromatin-remodeling ATPase Brg1 were carried out. We assayed the activity of the cis-regulatory elements co-occupied by Brg1/Six2 in vivo.

RESULTS

Eya1 and Six2 interact with the Brg1-based SWI/SNF complex during kidney development. Knockout of Brg1 results in failure of metanephric mesenchyme formation and depletion of nephron progenitors, which has been linked to loss of Eya1 expression. Transcriptional profiling shows conspicuous downregulation of important regulators for nephrogenesis in Brg1-deficient cells, including Lin28, Pbx1, and Dchs1-Fat4 signaling, but upregulation of podocyte lineage, oncogenic, and cell death-inducing genes, many of which Brg1 targets. Genome-wide binding analysis identifies Brg1 occupancy to a distal enhancer of Eya1 that drives nephron progenitor-specific expression. We demonstrate that Brg1 enrichment to two distal intronic enhancers of Pbx1 and a proximal promoter region of Mycn requires Six2 activity and that these Brg1/Six2-bound enhancers govern nephron progenitor-specific expression in response to Six2 activity.

CONCLUSIONS

Our results reveal an essential role for Brg1, its downstream pathways, and its interaction with Eya1-Six2 in mediating the fine balance among the self-renewal, differentiation, and survival of nephron progenitors.

Peripheral Nerve Development and the Pathogenesis of Peripheral Neuropathy: the Sorting Point

Nerve development requires a coordinated sequence of events and steps to be accomplished for the generation of functional peripheral nerves to convey sensory and motor signals. Any abnormality during development may result in pathological structure and function of the nerve, which evolves in peripheral neuropathy. In this review, we will briefly describe different steps of nerve development while we will mostly focus on the molecular mechanisms involved in radial sorting of axons, one of these nerve developmental steps. We will summarize current knowledge of molecular pathways so far reported in radial sorting and their possible interactions. Finally, we will describe how disruption of these pathways may result in human neuropathies.

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.

Brahma-related gene 1 has time-specific roles during brain and eye development

During development, gene expression is tightly controlled to facilitate the generation of the diverse cell types that form the central nervous system. Brahma-related gene 1 (Brg1, also known as Smarca4) is the catalytic subunit of the SWItch/sucrose nonfermentable (SWI/SNF) chromatin remodeling complex that regulates transcription. We investigated the role of Brg1 between embryonic day 6.5 (E6.5) and E14.5 in Sox2-positive neural stem cells (NSCs). Being without major consequences at E6.5 and E14.5, loss of Brg1 between E7.5 and E12.5 resulted in the formation of rosette-like structures in the subventricular zone, as well as morphological alterations and enlargement of neural retina (NR). Additionally, Brg1-deficient cells showed decreased survival in vitro and in vivo. Furthermore, we uncovered distinct changes in gene expression upon Brg1 loss, pointing towards impaired neuron functions, especially those involving synaptic communication and altered composition of the extracellular matrix. Comparison with mice deficient for integrase interactor 1 (Ini1, also known as Smarcb1) revealed that the enlarged NR was Brg1 specific and was not caused by a general dysfunction of the SWI/SNF complex. These results suggest a crucial role for Brg1 in NSCs during brain and eye development.

Genetic Events and Signaling Mechanisms Underlying Schwann Cell Fate in Development and Cancer

In this review, we describe Schwann cell development from embryonic neural crest cells to terminally differentiated myelinated and nonmyelinated mature Schwann cells. We focus on the genetic drivers and signaling mechanisms mediating decisions to proliferate versus differentiate during Schwann cell development, highlighting pathways that overlap with Schwann cell development and are dysregulated in tumorigenesis. We conclude by considering how our knowledge of the events underlying Schwann cell development and mouse models of schwannoma, neurofibroma, and malignant peripheral nerve sheath tumor can inform novel therapeutic strategies for patients with cancers derived from Schwann cell lineages.

Schwann cell development: From neural crest to myelin sheath

Vertebrate nervous system function requires glial cells, including myelinating glia that insulate axons and provide trophic support that allows for efficient signal propagation by neurons. In vertebrate peripheral nervous systems, neural crest-derived glial cells known as Schwann cells (SCs) generate myelin by encompassing and iteratively wrapping membrane around single axon segments. SC gliogenesis and neurogenesis are intimately linked and governed by a complex molecular environment that shapes their developmental trajectory. Changes in this external milieu drive developing SCs through a series of distinct morphological and transcriptional stages from the neural crest to a variety of glial derivatives, including the myelinating sublineage. Cues originate from the extracellular matrix, adjacent axons, and the developing SC basal lamina to trigger intracellular signaling cascades and gene expression changes that specify stages and transitions in SC development. Here, we integrate the findings from in vitro neuron-glia co-culture experiments with in vivo studies investigating SC development, particularly in zebrafish and mouse, to highlight critical factors that specify SC fate. Ultimately, we connect classic biochemical and mutant studies with modern genetic and visualization tools that have elucidated the dynamics of SC development. This article is categorized under: Signaling Pathways > Cell Fate Signaling Nervous System Development > Vertebrates: Regional Development.

Egr2-guided histone H2B monoubiquitination is required for peripheral nervous system myelination

Schwann cells are the nerve ensheathing cells of the peripheral nervous system. Absence, loss and malfunction of Schwann cells or their myelin sheaths lead to peripheral neuropathies such as Charcot-Marie-Tooth disease in humans. During Schwann cell development and myelination chromatin is dramatically modified. However, impact and functional relevance of these modifications are poorly understood. Here, we analyzed histone H2B monoubiquitination as one such chromatin modification by conditionally deleting the Rnf40 subunit of the responsible E3 ligase in mice. Rnf40-deficient Schwann cells were arrested immediately before myelination or generated abnormally thin, unstable myelin, resulting in a peripheral neuropathy characterized by hypomyelination and progressive axonal degeneration. By combining sequencing techniques with functional studies we show that H2B monoubiquitination does not influence global gene expression patterns, but instead ensures selective high expression of myelin and lipid biosynthesis genes and proper repression of immaturity genes. This requires the specific recruitment of the Rnf40-containing E3 ligase by Egr2, the central transcriptional regulator of peripheral myelination, to its target genes. Our study identifies histone ubiquitination as essential for Schwann cell myelination and unravels new disease-relevant links between chromatin modifications and transcription factors in the underlying regulatory network.

COMPASS and SWI/SNF complexes in development and disease

The Trithorax group (TrxG) of proteins is a large family of epigenetic regulators that form multiprotein complexes to counteract repressive developmental gene expression programmes established by the Polycomb group of proteins and to promote and maintain an active state of gene expression. Recent studies are providing new insights into how two crucial families of the TrxG - the COMPASS family of histone H3 lysine 4 methyltransferases and the SWI/SNF family of chromatin remodelling complexes - regulate gene expression and developmental programmes, and how misregulation of their activities through genetic abnormalities leads to pathologies such as developmental disorders and malignancies.

Transcription factor Tcf4 is the preferred heterodimerization partner for Olig2 in oligodendrocytes and required for differentiation

Development of oligodendrocytes and myelin formation in the vertebrate central nervous system is under control of several basic helix-loop-helix transcription factors such as Olig2, Ascl1, Hes5 and the Id proteins. The class I basic helix-loop-helix proteins Tcf3, Tcf4 and Tcf12 represent potential heterodimerization partners and functional modulators for all, but have not been investigated in oligodendrocytes so far. Using mouse mutants, organotypic slice and primary cell cultures we here show that Tcf4 is required in a cell-autonomous manner for proper terminal differentiation and myelination in vivo and ex vivo. Partial compensation is provided by the paralogous Tcf3, but not Tcf12. On the mechanistic level Tcf4 was identified as the preferred heterodimerization partner of the central regulator of oligodendrocyte development Olig2. Both genetic studies in the mouse as well as functional studies on enhancer regions of myelin genes confirmed the relevance of this physical interaction for oligodendrocyte differentiation. Considering that alterations in TCF4 are associated with syndromic and non-syndromic forms of intellectual disability, schizophrenia and autism in humans, our findings point to the possibility of an oligodendroglial contribution to these disorders.

SOX10-regulated promoter use defines isoform-specific gene expression in Schwann cells

Background

Multicellular organisms adopt various strategies to tailor gene expression to cellular contexts including the employment of multiple promoters (and the associated transcription start sites (TSSs)) at a single locus that encodes distinct gene isoforms. Schwann cells—the myelinating cells of the peripheral nervous system (PNS)—exhibit a specialized gene expression profile directed by the transcription factor SOX10, which is essential for PNS myelination. SOX10 regulates promoter elements associated with unique TSSs and gene isoforms at several target loci, implicating SOX10-mediated, isoform-specific gene expression in Schwann cell function. Here, we report on genome-wide efforts to identify SOX10-regulated promoters and TSSs in Schwann cells to prioritize genes and isoforms for further study.

Results

We performed global TSS analyses and mined previously reported ChIP-seq datasets to assess the activity of SOX10-bound promoters in three models: (i) an adult mammalian nerve; (ii) differentiating primary Schwann cells, and (iii) cultured Schwann cells with ablated SOX10 function. We explored specific characteristics of SOX10-dependent TSSs, which provides confidence in defining them as SOX10 targets. Finally, we performed functional studies to validate our findings at four previously unreported SOX10 target loci: ARPC1A, CHN2, DDR1, and GAS7. These findings suggest roles for the associated SOX10-regulated gene products in PNS myelination.

Conclusions

In sum, we provide comprehensive computational and functional assessments of SOX10-regulated TSS use in Schwann cells. The data presented in this study will stimulate functional studies on the specific mRNA and protein isoforms that SOX10 regulates, which will improve our understanding of myelination in the peripheral nerve.

Chromatin remodelers in oligodendroglia

Oligodendrocytes, the myelinating cells in the vertebrate central nervous system, produce myelin sheaths to enable saltatory propagation of action potentials. The process of oligodendrocyte myelination entails a stepwise progression from precursor specification to differentiation, which is coordinated by a series of transcriptional and chromatin remodeling events. ATP-dependent chromatin remodeling enzymes, which utilize ATP as an energy source to control chromatin dynamics and regulate the accessibility of chromatin to transcriptional regulators, are critical for oligodendrocyte lineage development and regeneration. In this review, we focus on the latest insights into the spatial and temporal specificity of chromatin remodelers during oligodendrocyte development, myelinogenesis, and regeneration. We will also bring together various plausible mechanisms by which lineage specific transcriptional regulators coordinate with chromatin remodeling factors for programming genomic landscapes to specifically modulate these different processes during developmental myelination and remyelination upon injury.

Myrf guides target gene selection of transcription factor Sox10 during oligodendroglial development

Oligodendrocytes generate myelin in the vertebrate central nervous system and thus ensure rapid propagation of neuronal activity. Their development is controlled by a network of transcription factors that function as determinants of cell identity or as temporally restricted stage-specific regulators. The continuously expressed Sox10 and Myrf, a factor induced during late development, are particularly important for terminal differentiation. How these factors function together mechanistically and influence each other, is not well understood. Here we show that Myrf not only cooperates with Sox10 during the induction of genes required for differentiation and myelin formation. Myrf also inhibits the activity of Sox10 on genes that are essential during earlier phases of oligodendroglial development. By characterization of the exact DNA-binding requirements of Myrf, we furthermore show that cooperative activation is a consequence of joint binding of Sox10 and Myrf to the same regulatory regions. In contrast, inhibition of Sox10-dependent gene activation occurs on genes that lack Myrf binding sites and likely involves physical interaction between Myrf and Sox10 followed by sequestration. These two opposite activities allow Myrf to redirect Sox10 from genes that it activates in oligodendrocyte precursor cells to genes that need to be induced during terminal differentiation.

Advances in Transcription Factors Related to Neuroglial Cell Reprogramming

Neuroglial cells have a high level of plasticity, and many types of these cells are present in the nervous system. Neuroglial cells provide diverse therapeutic targets for neurological diseases and injury repair. Cell reprogramming technology provides an efficient pathway for cell transformation during neural regeneration, while transcription factor-mediated reprogramming can facilitate the understanding of how neuroglial cells mature into functional neurons and promote neurological function recovery.

Chromatin remodeler Ep400 ensures oligodendrocyte survival and is required for myelination in the vertebrate central nervous system

Differentiating oligodendrocytes generate myelin to ensure rapid saltatory conduction in the vertebrate central nervous system. Although oligodendroglial differentiation and myelination are accompanied by dramatic chromatin reorganizations, previously studied chromatin remodelers had only limited direct effects on the process. To study the functional significance of chromatin changes for myelination and identify relevant remodelers, we deleted Ep400, the central ATP-hydrolyzing subunit of the TIP60/EP400 complex, at defined times of mouse oligodendrocyte development. Whereas Ep400-deficient oligodendrocyte precursors develop normally, terminal differentiation and myelination are dramatically impaired. Mechanistically, Ep400 interacts with transcription factor Sox10, binds to regulatory regions of the Myrf gene and is required to induce this central transcriptional regulator of the myelination program. In addition to reduced and aberrant myelin formation, oligodendrocytes exhibit increased DNA damage and apoptosis so that numbers never reach wildtype levels during the short lifespan of Ep400-deficient mice. Ep400 deletion in already mature oligodendrocytes remains phenotypically inapparent arguing that Ep400 is dispensable for myelin maintenance. Given its essential function in myelin formation, modulation of Ep400 activity may be beneficial in conditions such as multiple sclerosis where this process is compromised.

The transcription factor prospero homeobox protein 1 is a direct target of SoxC proteins during developmental vertebrate neurogenesis

The high-mobility-group domain containing SoxC transcription factors Sox4 and Sox11 are expressed and required in the vertebrate central nervous system in neuronal precursors and neuroblasts. To identify genes that are widely regulated by SoxC proteins during vertebrate neurogenesis we generated expression profiles from developing mouse brain and chicken neural tube with reduced SoxC expression and found the transcription factor prospero homeobox protein 1 (Prox1) strongly down-regulated under both conditions. This led us to hypothesize that Prox1 expression depends on SoxC proteins in the developing central nervous system of mouse and chicken. By combining luciferase reporter assays and over-expression in the chicken neural tube with in vivo and in vitro binding studies, we identify the Prox1 gene promoter and two upstream enhancers at -44 kb and -40 kb relative to the transcription start as regulatory regions that are bound and activated by SoxC proteins. This argues that Prox1 is a direct target gene of SoxC proteins during neurogenesis. Electroporations in the chicken neural tube furthermore show that Prox1 activates a subset of SoxC target genes, whereas it has no effects on others. We propose that the transcriptional control of Prox1 by SoxC proteins may ensure coupling of two types of transcription factors that are both required during early neurogenesis, but have at least in part distinct functions. Open Data: Materials are available on https://cos.io/our-services/open-science-badges/ https://osf.io/93n6m/.

Ep400 deficiency in Schwann cells causes persistent expression of early developmental regulators and peripheral neuropathy

Schwann cells ensure efficient nerve impulse conduction in the peripheral nervous system. Their development is accompanied by defined chromatin changes, including variant histone deposition and redistribution. To study the importance of variant histones for Schwann cell development, we altered their genomic distribution by conditionally deleting Ep400, the central subunit of the Tip60/Ep400 complex. Ep400 absence causes peripheral neuropathy in mice, characterized by terminal differentiation defects in myelinating and non-myelinating Schwann cells and immune cell activation. Variant histone H2A.Z is differently distributed throughout the genome and remains at promoters of Tfap2a, Pax3 and other transcriptional regulator genes with transient function at earlier developmental stages. Tfap2a deletion in Ep400-deficient Schwann cells causes a partial rescue arguing that continued expression of early regulators mediates the phenotypic defects. Our results show that proper genomic distribution of variant histones is essential for Schwann cell differentiation, and assign importance to Ep400-containing chromatin remodelers in the process.

The Ep400 chromatin remodeler determines genomic distribution of variant histones. In the current study, the authors show that loss of Ep400 in Schwann cells leads to aberrant expression of developmental regulators, and a peripheral neuropathy phenotype.

Chromatin remodellers Brg1 and Bptf are required for normal gene expression and progression of oncogenic Braf-driven mouse melanoma

Somatic oncogenic mutation of BRAF coupled with inactivation of PTEN constitute a frequent combination of genomic alterations driving the development of human melanoma. Mice genetically engineered to conditionally express oncogenic Braf^V600E and inactivate Pten in melanocytes following tamoxifen treatment rapidly develop melanoma. While early-stage melanomas comprised melanin-pigmented Mitf and Dct-expressing cells, expression of these and other melanocyte identity genes was lost in later stage tumours that showed histological and molecular characteristics of de-differentiated neural crest type cells. Melanocyte identity genes displayed loss of active chromatin marks and RNA polymerase II and gain of heterochromatin marks, indicating epigenetic reprogramming during tumour progression. Nevertheless, late-stage tumour cells grown in culture re-expressed Mitf, and melanocyte markers and Mitf together with Sox10 coregulated a large number of genes essential for their growth. In this melanoma model, somatic inactivation that the catalytic Brg1 (Smarca4) subunit of the SWI/SNF complex and the scaffolding Bptf subunit of the NuRF complex delayed tumour formation and deregulated large and overlapping gene expression programs essential for normal tumour cell growth. Moreover, we show that Brg1 and Bptf coregulated many genes together with Mitf and Sox10. Together these transcription factors and chromatin remodelling complexes orchestrate essential gene expression programs in mouse melanoma cells.

Transcriptional control of myelination and remyelination

Myelination is an evolutionary recent differentiation program that has been independently acquired in vertebrates by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. Therefore, it is not surprising that regulating transcription factors differ substantially between both cell types. However, overall principles are similar as transcriptional control in Schwann cells and oligodendrocytes combines lineage determining and stage-specific factors in complex regulatory networks. Myelination does not only occur during development, but also as remyelination in the adult. In line with the different conditions during developmental myelination and remyelination and the distinctive properties of Schwann cells and oligodendrocytes, transcriptional regulation of remyelination exhibits unique features and differs between the two cell types. This review gives an overview of the current state in the field.

Individual components of the SWI/SNF chromatin remodelling complex have distinct roles in memory neurons of the Drosophila mushroom body

Technology has led to rapid progress in the identification of genes involved in neurodevelopmental disorders such as intellectual disability (ID), but our functional understanding of the causative genes is lagging. Here, we show that the SWI/SNF chromatin remodelling complex is one of the most over-represented cellular components disrupted in ID. We investigated the role of individual subunits of this large protein complex using targeted RNA interference in post-mitotic memory-forming neurons of the Drosophila mushroom body (MB). Knockdown flies were tested for defects in MB morphology, short-term memory and long-term memory. Using this approach, we identified distinct roles for individual subunits of the Drosophila SWI/SNF complex. Bap60, Snr1 and E(y)3 are required for pruning of the MBγ neurons during pupal morphogenesis, while Brm and Osa are required for survival of MBγ axons during ageing. We used the courtship conditioning assay to test the effect of MB-specific SWI/SNF knockdown on short- and long-term memory. Several subunits, including Brm, Bap60, Snr1 and E(y)3, were required in the MB for both short- and long-term memory. In contrast, Osa knockdown only reduced long-term memory. Our results suggest that individual components of the SWI/SNF complex have different roles in the regulation of structural plasticity, survival and functionality of post-mitotic MB neurons. This study highlights the many possible processes that might be disrupted in SWI/SNF-related ID disorders. Our broad phenotypic characterization provides a starting point for understanding SWI/SNF-mediated gene regulatory mechanisms that are important for development and function of post-mitotic neurons.

Summary: The SWI/SNF chromatin remodelling complex is the most over-represented protein complex in the intellectual disability. Different components of this complex have distinct roles in development and function of memory-forming neurons in the Drosophila mushroom body.

BAF60A mediates interactions between the microphthalmia-associated transcription factor and the BRG1-containing SWI/SNF complex during melanocyte differentiation

SWI/SNF chromatin remodeling enzymes are multisubunit complexes that contain one of two catalytic subunits, BRG1 or BRM and 9-11 additional subunits called BRG1 or BRM-associated factors (BAFs). BRG1 interacts with the microphthalmia-associated transcription factor (MITF) and is required for melanocyte development in vitro and in vivo. The subunits of SWI/SNF that mediate interactions between BRG1 and MITF have not been elucidated. Three mutually exclusive isoforms of a 60-kDa subunit (BAF60A, B, or C) often facilitate interactions with transcription factors during lineage specification. We tested the hypothesis that a BAF60 subunit promotes interactions between MITF and the BRG1-containing SWI/SNF complex. We found that MITF can physically interact with BAF60A, BAF60B, and BAF60C. The interaction between MITF and BAF60A required the basic helix-loop-helix domain of MITF. Recombinant BAF60A pulled down recombinant MITF, suggesting that the interaction can occur in the absence of other SWI/SNF subunits and other transcriptional regulators of the melanocyte lineage. Depletion of BAF60A in differentiating melanoblasts inhibited melanin synthesis and expression of MITF target genes. MITF promoted BAF60A recruitment to melanocyte-specific promoters, and BAF60A was required to promote BRG1 recruitment and chromatin remodeling. Thus, BAF60A promotes interactions between MITF and the SWI/SNF complex and is required for melanocyte differentiation.

Dysregulation of NAD+ Metabolism Induces a Schwann Cell Dedifferentiation Program

The Schwann cell (SC) is the major component of the peripheral nervous system (PNS) that provides metabolic and functional support for peripheral axons. The emerging roles of SC mitochondrial function for PNS development and axonal stability indicate the importance of SC metabolism in nerve function and in peripheral neuropathies associated with metabolic disorders. Nicotinamide adenine dinucleotide (NAD⁺) is a crucial molecule in the regulation of cellular metabolism and redox homeostasis. Here, we investigated the roles of NAD⁺ metabolism in SC functions in vivo by mutating NAMPT, the rate-limiting enzyme of NAD⁺ biosynthesis, specifically in SCs. NAMPT SC knock-out male and female mice (NAMPT SCKO mice) had delayed SC maturation in development and developed hypomyelinating peripheral neuropathy without axon degeneration or decreased SC survival. JUN, a master regulator of SC dedifferentiation, is elevated in NAMPT SCKO SCs, suggesting that decreased NAD⁺ levels cause them to arrest at an immature stage. Nicotinic acid administration rescues the NAD⁺ decline and reverses the SC maturation defect and the development of peripheral neuropathy, indicating the central role of NAD⁺ in PNS development. Upon nicotinic acid withdrawal in adulthood, NAMPT SCKO mice showed rapid and severe peripheral neuropathy and activation of ERK/MEK/JUN signaling, which in turn promotes SC dedifferentiation. These data demonstrate the importance of NAD⁺ metabolism in SC maturation and nerve development and maintenance and suggest that altered SC NAD⁺ metabolism could underlie neuropathies associated with diabetes and aging.SIGNIFICANCE STATEMENT In this study, we showed that Schwann cell differentiation status is critically dependent on NAD⁺ homeostasis. Aberrant regulation of NAD⁺ biosynthesis via NAMPT deletion results in a blockade of Schwann cell maturation during development and severe peripheral neuropathy without significant axon loss. The phenotype can be rescued by supplementation with nicotinic acid; however, withdrawal of nicotinic acid leads to Schwann cell dedifferentiation, myelination defects, and death. These results provide new therapeutic possibilities for peripheral neuropathies associated with NAD⁺ decline during aging or diabetes.

ATP-Dependent Chromatin Remodeling During Cortical Neurogenesis

The generation of individual neurons (neurogenesis) during cortical development occurs in discrete steps that are subtly regulated and orchestrated to ensure normal histogenesis and function of the cortex. Notably, various gene expression programs are known to critically drive many facets of neurogenesis with a high level of specificity during brain development. Typically, precise regulation of gene expression patterns ensures that key events like proliferation and differentiation of neural progenitors, specification of neuronal subtypes, as well as migration and maturation of neurons in the developing cortex occur properly. ATP-dependent chromatin remodeling complexes regulate gene expression through utilization of energy from ATP hydrolysis to reorganize chromatin structure. These chromatin remodeling complexes are characteristically multimeric, with some capable of adopting functionally distinct conformations via subunit reconstitution to perform specific roles in major aspects of cortical neurogenesis. In this review, we highlight the functions of such chromatin remodelers during cortical development. We also bring together various proposed mechanisms by which ATP-dependent chromatin remodelers function individually or in concert, to specifically modulate vital steps in cortical neurogenesis.

Nfat/calcineurin signaling promotes oligodendrocyte differentiation and myelination by transcription factor network tuning

Oligodendrocytes produce myelin for rapid transmission and saltatory conduction of action potentials in the vertebrate central nervous system. Activation of the myelination program requires several transcription factors including Sox10, Olig2, and Nkx2.2. Functional interactions among them are poorly understood and important components of the regulatory network are still unknown. Here, we identify Nfat proteins as Sox10 targets and regulators of oligodendroglial differentiation in rodents and humans. Overall levels and nuclear fraction increase during differentiation. Inhibition of Nfat activity impedes oligodendrocyte differentiation in vitro and in vivo. On a molecular level, Nfat proteins cooperate with Sox10 to relieve reciprocal repression of Olig2 and Nkx2.2 as precondition for oligodendroglial differentiation and myelination. As Nfat activity depends on calcium-dependent activation of calcineurin signaling, regulatory network and oligodendroglial differentiation become sensitive to calcium signals. NFAT proteins are also detected in human oligodendrocytes, downregulated in active multiple sclerosis lesions and thus likely relevant in demyelinating disease.

Epigenetic modifications-insight into oligodendrocyte lineage progression, regeneration, and disease

Myelination by oligodendrocytes in the central nervous system permits high-fidelity saltatory conduction from neuronal cell bodies to axon terminals. Dysmyelinating and demyelinating disorders impair normal nervous system functions. Consequently, an understanding of oligodendrocyte differentiation that moves beyond the genetic code into the field of epigenetics is essential. Chromatin reprogramming is critical for steering stage-specific differentiation processes during oligodendrocyte development. Fine temporal control of chromatin remodeling through ATP-dependent chromatin remodelers and sequential histone modifiers shapes a chromatin regulatory landscape conducive to oligodendrocyte fate specification, lineage differentiation, and maintenance of cell identity. In this Review, we will focus on the biological functions of ATP-dependent chromatin remodelers and histone deacetylases in myelinating oligodendrocyte development and implications for myelin regeneration in neurodegenerative diseases.

Epigenetic Control of Schwann Cells

The journey of Schwann cells from their origin in the neural crest to their ensheathment and myelination of peripheral nerves is a remarkable one. Their apparent static function in enabling saltatory conduction of mature nerve is not only vital for long-term health of peripheral nerve but also belies an innate capacity of terminally differentiated Schwann cells to radically alter their differentiation status in the face of nerve injury. The transition from migrating neural crest cells to nerve ensheathment, and then myelination of large diameter axons has been characterized extensively and several of the transcriptional networks have been identified. However, transcription factors must also modify chromatin structure during Schwann cell maturation and this review will focus on chromatin modification machinery that is involved in promoting the transition to, and maintenance of, myelinating Schwann cells. In addition, Schwann cells are known to play important regenerative roles after peripheral nerve injury, and information on epigenomic reprogramming of the Schwann cell genome has emerged. Characterization of epigenomic requirements for myelin maintenance and Schwann cell responses to injury will be vital in understanding how the various Schwann cell functions can be optimized to maintain and repair peripheral nerve function.

Miz1 Controls Schwann Cell Proliferation via H3K36me2 Demethylase Kdm8 to Prevent Peripheral Nerve Demyelination

Schwann cell differentiation and myelination depends on chromatin remodeling, histone acetylation, and methylation, which all affect Schwann cell proliferation. We previously reported that the deletion of the POZ (POxvirus and Zinc finger) domain of the transcription factor Miz1 (Myc-interacting zinc finger protein; encoded by Zbtb17) in mouse Schwann cells (Miz1ΔPOZ) causes a neuropathy at 90 d after birth [postnatal day (P) 90], with a subsequent spontaneous regeneration. Here we show that RNA sequencing from Miz1ΔPOZ and control animals at P30 revealed a set of upregulated genes with a strong correlation to cell-cycle regulation. Consistently, a subset of Schwann cells did not exit the cell cycle as observed in control animals and the growth fraction increased over time. From the RNAseq gene list, two direct Miz1 target genes were identified, one of which encodes the histone H3K36^me2 demethylase Kdm8. We show that the expression of Kdm8 is repressed by Miz1 and that its release in Miz1ΔPOZ cells induces a decrease of H3K36^me2, especially in deregulated cell-cycle-related genes. The linkage between elevated Kdm8 expression, hypomethylation of H3K36 at cell-cycle-relevant genes, and the subsequent re-entering of adult Schwann cells into the cell cycle suggests that the release of Kdm8 repression in the absence of a functional Miz1 is a central issue in the development of the Miz1ΔPOZ phenotype.SIGNIFICANCE STATEMENT The deletion of the Miz1 (Myc-interacting zinc finger protein 1) POZ (POxvirus and Zinc finger) domain in Schwann cells causes a neuropathy. Here we report sustained Schwann cell proliferation caused by an increased expression of the direct Miz1 target gene Kdm8, encoding a H3K36me2 demethylase. Hence, the demethylation of H3K36 is linked to the pathogenesis of a neuropathy.

BRG1 interacts with SOX10 to establish the melanocyte lineage and to promote differentiation

Mutations in SOX10 cause neurocristopathies which display varying degrees of hypopigmentation. Using a sensitized mutagenesis screen, we identified Smarca4 as a modifier gene that exacerbates the phenotypic severity of Sox10 haplo-insufficient mice. Conditional deletion of Smarca4 in SOX10 expressing cells resulted in reduced numbers of cranial and ventral trunk melanoblasts. To define the requirement for the Smarca4 -encoded BRG1 subunit of the SWI/SNF chromatin remodeling complex, we employed in vitro models of melanocyte differentiation in which induction of melanocyte-specific gene expression is closely linked to chromatin alterations. We found that BRG1 was required for expression of Dct, Tyrp1 and Tyr, genes that are regulated by SOX10 and MITF and for chromatin remodeling at distal and proximal regulatory sites. SOX10 was found to physically interact with BRG1 in differentiating melanocytes and binding of SOX10 to the Tyrp1 distal enhancer temporally coincided with recruitment of BRG1. Our data show that SOX10 cooperates with MITF to facilitate BRG1 binding to distal enhancers of melanocyte-specific genes. Thus, BRG1 is a SOX10 co-activator, required to establish the melanocyte lineage and promote expression of genes important for melanocyte function.

ATP-dependent chromatin remodeling during mammalian development

Precise gene expression ensures proper stem and progenitor cell differentiation, lineage commitment and organogenesis during mammalian development. ATP-dependent chromatin-remodeling complexes utilize the energy from ATP hydrolysis to reorganize chromatin and, hence, regulate gene expression. These complexes contain diverse subunits that together provide a multitude of functions, from early embryogenesis through cell differentiation and development into various adult tissues. Here, we review the functions of chromatin remodelers and their different subunits during mammalian development. We discuss the mechanisms by which chromatin remodelers function and highlight their specificities during mammalian cell differentiation and organogenesis.

Chromatin-remodeling enzymes in control of Schwann cell development, maintenance and plasticity

Gene regulation is essential for cellular differentiation and plasticity. Schwann cells (SCs), the myelinating glia of the peripheral nervous system (PNS), develop from neural crest cells to mature myelinating SCs and can at early developmental stage differentiate into various cell types. After a PNS lesion, SCs can also convert into repair cells that guide and stimulate axonal regrowth, and remyelinate regenerated axons. What controls their development and versatile nature? Several recent studies highlight the key roles of chromatin modifiers in these processes, allowing SCs to regulate their gene expression profile and thereby acquire or change their identity and quickly react to their environment.

Regulatory Logic Underlying Diversification of the Neural Crest

The neural crest is a transient, multipotent population of cells that arises at the border of the developing nervous system. After closure of the neural tube, these cells undergo an epithelial-to-mesenchymal transition (EMT) to delaminate and migrate, often to distant locations in the embryo. Neural crest cells give rise to a diverse array of derivatives including neurons and glia of the peripheral nervous system, melanocytes, and bone and cartilage of the face. A gene regulatory network (GRN) controls the specification, delamination, migration, and differentiation of this fascinating cell type. With increasing technological advances, direct linkages within the neural crest GRN are being uncovered. The underlying circuitry is useful for understanding important topics such as reprogramming, evolution, and disease.

Epigenomic Regulation of Schwann Cell Reprogramming in Peripheral Nerve Injury

The rapid and dynamic transcriptional changes of Schwann cells in response to injury are critical to peripheral nerve repair, yet the epigenomic reprograming that leads to the induction of injury-activated genes has not been characterized. Polycomb Repressive Complex 2 (PRC2) catalyzes the trimethylation of lysine 27 of histone H3 (H3K27me3), which produces a transcriptionally repressive chromatin environment. We find that many promoters and/or gene bodies of injury-activated genes of mature rat nerves are occupied with H3K27me3. In contrast, the majority of distal enhancers that gain H3K27 acetylation after injury are not repressed by H3K27 methylation before injury, which is normally observed in developmentally poised enhancers. Injury induces demethylation of H3K27 in many genes, such as Sonic hedgehog (Shh), which is silenced throughout Schwann cell development before injury. In addition, experiments using a Schwann cell-specific mouse knock-out of the Eed subunit of PRC2 indicate that demethylation is a rate-limiting step in the activation of such genes. We also show that some transcription start sites of H3K27me3-repressed injury genes of uninjured nerves are bound with a mark of active promoters H3K4me3, for example, Shh and Gdnf, and the reduction of H3K27me3 results in increased trimethylation of H3K4. Our findings identify reversal of polycomb repression as a key step in gene activation after injury.

SIGNIFICANCE STATEMENT

Peripheral nerve regeneration after injury is dependent upon implementation of a novel genetic program in Schwann cells that supports axonal survival and regeneration. Identifying means to enhance Schwann cell reprogramming after nerve injury could be used to foster effective remyelination in the treatment of demyelinating disorders and in identifying pathways involved in regenerative process of myelination. Although recent progress has identified transcriptional determinants of successful reprogramming of the Schwann cell transcriptome after nerve injury, our results have highlighted a novel epigenomic pathway in which reversal of the Polycomb pathway of repressive histone methylation is required for activation of a significant number of injury-induced genes.

Chromatin Remodeling BAF (SWI/SNF) Complexes in Neural Development and Disorders

The ATP-dependent BRG1/BRM associated factor (BAF) chromatin remodeling complexes are crucial in regulating gene expression by controlling chromatin dynamics. Over the last decade, it has become increasingly clear that during neural development in mammals, distinct ontogenetic stage-specific BAF complexes derived from combinatorial assembly of their subunits are formed in neural progenitors and post-mitotic neural cells. Proper functioning of the BAF complexes plays critical roles in neural development, including the establishment and maintenance of neural fates and functionality. Indeed, recent human exome sequencing and genome-wide association studies have revealed that mutations in BAF complex subunits are linked to neurodevelopmental disorders such as Coffin-Siris syndrome, Nicolaides-Baraitser syndrome, Kleefstra's syndrome spectrum, Hirschsprung's disease, autism spectrum disorder, and schizophrenia. In this review, we focus on the latest insights into the functions of BAF complexes during neural development and the plausible mechanistic basis of how mutations in known BAF subunits are associated with certain neurodevelopmental disorders.

A mutation in the Tubb4a gene leads to microtubule accumulation with hypomyelination and demyelination

OBJECTIVE

Our goal was to define the genetic cause of the profound hypomyelination in the taiep rat model and determine its relevance to human white matter disease.

METHODS

Based on previous localization of the taiep mutation to rat chromosome 9, we tested whether the mutation resided within the Tubb4a (β-tubulin 4A) gene, because mutations in the TUBB4A gene have been described in patients with central nervous system hypomyelination. To determine whether accumulation of microtubules led to progressive demyelination, we analyzed the spinal cord and optic nerves of 2-year-old rats by light and electron microscopy. Cerebral white matter from a patient with TUBB4A Asn414Lys mutation and magnetic resonance imaging evidence of severe hypomyelination were studied similarly.

RESULTS

As the taiep rat ages, there is progressive loss of myelin in the brain and dorsal column of the spinal cord associated with increased oligodendrocyte numbers with accumulation of microtubules. This accumulation involved the entire cell body and distal processes of oligodendrocytes, but there was no accumulation of microtubules in axons. A single point mutation in Tubb4a (p.Ala302Thr) was found in homozygous taiep samples. A similar hypomyelination associated with increased oligodendrocyte numbers and arrays of microtubules in oligodendrocytes was demonstrated in the human patient sample.

INTERPRETATION

The taiep rat is the first animal model of TUBB4 mutations in humans and a novel system in which to test the mechanism of microtubule accumulation. The finding of microtubule accumulation in a patient with a TUBB4A mutation and leukodystrophy confirms the usefulness of taiep as a model of the human disease. Ann Neurol 2017;81:690-702.

[Epigenetic regulation of gene expression in malignant melanoma]

Malignant melanoma is a highly aggressive cancer with a propensity for early metastasis. Melanocyte transformation results predominantly from oncogenic mutations in BRAF, NRAS or NF1 leading to constitutive activation of the MAP kinase pathway driving cell proliferation and second site mutations such as loss of CDKN1A, or PTEN or activating mutations in the beta-catenin pathway that allow escape from oncogene induced senescence. Nevertheless, irrespective of the nature of the driver mutations, melanoma cell physiology is strongly regulated by transcription factors and epigenetic mechanisms. MITF (Microphthalmia-associated Transcription Factor) and SOX10 are two major transcription factors that regulate both normal melanocyte and melanoma cell physiology. Using a combination of mouse genetics, biochemistry and high throughput genomics we have identified cofactors for MITF and addressed the mechanisms by which MITF, SOX10 and their cofactors regulate gene expression in melanocytes and melanoma.