X-MyT1, a Xenopus C2HC-type zinc finger protein with a regulatory function in neuronal differentiation

Synaptotagmins family affect glucose transport in retinal pigment epithelial cells through their ubiquitination-mediated degradation and glucose transporter-1 regulation

BACKGROUND

Synaptotagmins (SYTs) are a family of 17 membrane transporters that function as calcium ion sensors during the release of Ca2+-dependent neurotransmitters and hormones. However, few studies have reported whether members of the SYT family play a role in glucose uptake in diabetic retinopathy (DR) through Ca2+/glucose transporter-1 (GLUT1) and the possible regulatory mechanism of SYTs.

AIM

To elucidate the role of the SYT family in the regulation of glucose transport in retinal pigment epithelial cells and explore its potential as a therapeutic target for the clinical management of DR.

METHODS

DR was induced by streptozotocin in C57BL/6J mice and by high glucose medium in human retinal pigment epithelial cells (ARPE-19). Bioinformatics analysis, reverse transcriptase-polymerase chain reaction, Western blot, flow cytometry, ELISA, HE staining, and TUNEL staining were used for analysis.

RESULTS

Six differentially expressed proteins (SYT2, SYT3, SYT4, SYT7, SYT11, and SYT13) were found between the DR and control groups, and SYT4 was highly expressed. Hyperglycemia induces SYT4 overexpression, manipulates Ca2+ influx to induce GLUT1 fusion with the plasma membrane, promotes abnormal expression of the glucose transporter GLUT1 and excessive glucose uptake, induces ARPE-19 cell apoptosis, and promotes DR progression. Parkin deficiency inhibits the proteasomal degradation of SYT4 in DR, resulting in SYT4 accumulation and enhanced GLUT1 fusion with the plasma membrane, and these effects were blocked by oe-Parkin treatment. Moreover, dysregulation of the myelin transcription factor 1 (Myt1)-induced transcription of SYT4 in DR further activated the SYT4-mediated stimulus-secretion coupling process, and this process was inhibited in the oe-MYT1-treated group.

CONCLUSION

Our study reveals the key role of SYT4 in regulating glucose transport in retinal pigment epithelial cells during the pathogenesis of DR and the underlying mechanism and suggests potential therapeutic targets for clinical DR.

A multiplexed, single-cell sequencing screen identifies compounds that increase neurogenic reprogramming of murine Muller glia

Retinal degeneration in mammals causes permanent loss of vision, due to an inability to regenerate naturally. Some non-mammalian vertebrates show robust regeneration, via Muller glia (MG). We have recently made significant progress in stimulating adult mouse MG to regenerate functional neurons by transgenic expression of the proneural transcription factor Ascl1. While these results showed that MG can serve as an endogenous source of neuronal replacement, the efficacy of this process is limited. With the goal of improving this in mammals, we designed a small molecule screen using sci-Plex, a method to multiplex up to thousands of single nucleus RNA-seq conditions into a single experiment. We used this technology to screen a library of 92 compounds, identified, and validated two that promote neurogenesis in vivo. Our results demonstrate that high-throughput single-cell molecular profiling can substantially improve the discovery process for molecules and pathways that can stimulate neural regeneration and further demonstrate the potential for this approach to restore vision in patients with retinal disease.

MYT1L in the making: emerging insights on functions of a neurodevelopmental disorder gene

Large scale human genetic studies have shown that loss of function (LoF) mutations in MYT1L are implicated in neurodevelopmental disorders (NDDs). Here, we provide an overview of the growing number of published MYT1L patient cases, and summarize prior studies in cells, zebrafish, and mice, both to understand MYT1L's molecular and cellular role during brain development and consider how its dysfunction can lead to NDDs. We integrate the conclusions from these studies and highlight conflicting findings to reassess the current model of the role of MYT1L as a transcriptional activator and/or repressor based on the biological context. Finally, we highlight additional functional studies that are needed to understand the molecular mechanisms underlying pathophysiology and propose key questions to guide future preclinical studies.

Partitioning of trace elements in the tissues of Mediterranean mussels (Mytilus galloprovincialis) sampled from industrial sites along the Algerian coast

This research was aimed at evaluating the seasonal partitioning of Zn, Se, As, Cu, and Co in the tissues of Mytilus galloprovincialis sampled at two industrial sites along the Algerian coast. Adult mussels were seasonally collected from two sites over the course of a whole year. The gills, digestive glands, gonads, and remaining soft tissues were analyzed through ICP-MS. The observations led to identifying metals ranges (μg g^-1_{Dry Weight}) of 67.17-395.51 (Zn), 2.18-12.74 (Se), 7.81-28.61 (As), 3.32-155.91 (Cu), and 0.10-3.59 (Co) in the various tissues. The highest concentrations were found in the digestive glands and gills as compared to the gonads and remaining soft tissues. Distinct patterns of metals partitioning were found: indeed, As and Co concentrations were higher in the digestive glands, while Se and Zn concentrations were higher in the gills. Many of the mussels samples resulted contaminated, therefore potentially posing a considerable health risk to consumers.

Genetic control of retinal ganglion cell genesis

Retinal ganglion cells (RGCs) are the only projection neurons in the neural retina. They receive and integrate visual signals from upstream retinal neurons in the visual circuitry and transmit them to the brain. The function of RGCs is performed by the approximately 40 RGC types projecting to various central brain targets. RGCs are the first cell type to form during retinogenesis. The specification and differentiation of the RGC lineage is a stepwise process; a hierarchical gene regulatory network controlling the RGC lineage has been identified and continues to be elaborated. Recent studies with single-cell transcriptomics have led to unprecedented new insights into their types and developmental trajectory. In this review, we summarize our current understanding of the functions and relationships of the many regulators of the specification and differentiation of the RGC lineage. We emphasize the roles of these key transcription factors and pathways in different developmental steps, including the transition from retinal progenitor cells (RPCs) to RGCs, RGC differentiation, generation of diverse RGC types, and central projection of the RGC axons. We discuss critical issues that remain to be addressed for a comprehensive understanding of these different aspects of RGC genesis and emerging technologies, including single-cell techniques, novel genetic tools and resources, and high-throughput genome editing and screening assays, which can be leveraged in future studies.

Segregation of brain and organizer precursors is differentially regulated by Nodal signaling at blastula stage

The blastula Chordin- and Noggin-expressing (BCNE) center comprises animal-dorsal and marginal-dorsal cells of the amphibian blastula and contains the precursors of the brain and the gastrula organizer. Previous findings suggested that the BCNE behaves as a homogeneous cell population that only depends on nuclear β-catenin activity but does not require Nodal and later segregates into its descendants during gastrulation. In contrast to previous findings, in this work, we show that the BCNE does not behave as a homogeneous cell population in response to Nodal antagonists. In fact, we found that chordin.1 expression in a marginal subpopulation of notochordal precursors indeed requires Nodal input. We also establish that an animal BCNE subpopulation of cells that express both, chordin.1 and sox2 (a marker of pluripotent neuroectodermal cells), and gives rise to most of the brain, persisted at blastula stage after blocking Nodal. Therefore, Nodal signaling is required to define a population of chordin.1+ cells and to restrict the recruitment of brain precursors within the BCNE as early as at blastula stage. We discuss our findings in Xenopus in comparison to other vertebrate models, uncovering similitudes in early brain induction and delimitation through Nodal signaling.

This article has an associated First Person interview with the first author of the paper.

Summary: Nodal signaling is involved in the delimitation of the blastula cell populations that give rise to the brain and axial mesoderm in Xenopus.

Myt Transcription Factors Prevent Stress-Response Gene Overactivation to Enable Postnatal Pancreatic β Cell Proliferation, Function, and Survival

Although cellular stress response is important for maintaining function and survival, overactivation of late-stage stress effectors cause dysfunction and death. We show that the myelin transcription factors (TFs) Myt1 (Nzf2), Myt2 (Myt1l, Nztf1, and Png-1), and Myt3 (St18 and Nzf3) prevent such overactivation in islet β cells. Thus, we found that co-inactivating the Myt TFs in mouse pancreatic progenitors compromised postnatal β cell function, proliferation, and survival, preceded by upregulation of late-stage stress-response genes activating transcription factors (e.g., Atf4) and heat-shock proteins (Hsps). Myt1 binds putative enhancers of Atf4 and Hsps, whose overexpression largely recapitulated the Myt-mutant phenotypes. Moreover, Myt(MYT)-TF levels were upregulated in mouse and human β cells during metabolic stress-induced compensation but downregulated in dysfunctional type 2 diabetic (T2D) human β cells. Lastly, MYT knockdown caused stress-gene overactivation and death in human EndoC-βH1 cells. These findings suggest that Myt TFs are essential restrictors of stress-response overactivity.

MYT1 attenuates neuroblastoma cell differentiation by interacting with the LSD1/CoREST complex

Impaired neuronal differentiation is a feature of neuroblastoma tumorigenesis, and the differentiation grade of neuroblastoma tumors is associated with patient prognosis. Detailed understanding of the molecular mechanisms underlying neuroblastoma differentiation will facilitate the development of effective treatment strategies. Recent studies have shown that myelin transcription factor 1 (MYT1) promotes vertebrate neurogenesis by regulating gene expression. We performed quantitative analysis of neuroblastoma samples, which revealed that MYT1 was differentially expressed among neuroblastoma patients with different pathological diagnoses. Analysis of clinical data showed that MYT1 overexpression was associated with a significantly shorter 3-year overall survival rate and poor differentiation in neuroblastoma specimens. MYT1 knockdown inhibited proliferation and promoted the expression of multiple differentiation-associated proteins. Integrated omics data indicated that many genes involved in neuro-differentiation were regulated by MYT1. Interestingly, many of these genes are targets of the REST complex; therefore, we further identified the physical interaction of MYT1 with LSD1/CoREST. Depletion of LSD1 or inhibition of LSD1 by ORY-1001 decreased MYT1 expression, providing an alternative approach to target MYT1. Taken together, our results indicate that MYT1 significantly attenuates cell differentiation by interacting with the LSD1/CoREST complex. MYT1 is, therefore, a promising therapeutic target for enhancing the neurite-inducing effect of retinoic acid and for inhibiting the growth of neuroblastoma.

A Myt1 family transcription factor defines neuronal fate by repressing non-neuronal genes

Cellular differentiation requires both activation of target cell transcriptional programs and repression of non-target cell programs. The Myt1 family of zinc finger transcription factors contributes to fibroblast to neuron reprogramming in vitro. Here, we show that ztf-11 (Zinc-finger Transcription Factor-11), the sole Caenorhabditis elegans Myt1 homolog, is required for neurogenesis in multiple neuronal lineages from previously differentiated epithelial cells, including a neuron generated by a developmental epithelial-to-neuronal transdifferentiation event. ztf-11 is exclusively expressed in all neuronal precursors with remarkable specificity at single-cell resolution. Loss of ztf-11 leads to upregulation of non-neuronal genes and reduced neurogenesis. Ectopic expression of ztf-11 in epidermal lineages is sufficient to produce additional neurons. ZTF-11 functions together with the MuvB corepressor complex to suppress the activation of non-neuronal genes in neurons. These results dovetail with the ability of Myt1l (Myt1-like) to drive neuronal transdifferentiation in vitro in vertebrate systems. Together, we identified an evolutionarily conserved mechanism to specify neuronal cell fate by repressing non-neuronal genes.

Silencing the alternative

The transcription factor ztf-11 promotes neuronal differentiation by repressing other cell fates in the nematode worm C. elegans.

Myt1 and Myt1l transcription factors limit proliferation in GBM cells by repressing YAP1 expression

Myelin transcription factor 1 (Myt1) and Myt1l (Myt1-like) are zinc finger transcription factors that regulate neuronal differentiation. Reduced Myt1l expression has been implicated in glioblastoma (GBM), and the related St18 was originally identified as a potential tumor suppressor for breast cancer. We previously analyzed changes in gene expression in a human GBM cell line with re-expression of either Myt1 or Myt1l. This revealed largely overlapping gene expression changes, suggesting similar function in these cells. Here we show that re-expression of Myt1 or Myt1l reduces proliferation in two different GBM cell lines, activates gene expression programs associated with neuronal differentiation, and limits expression of proliferative and epithelial to mesenchymal transition gene-sets. Consistent with this, expression of both MYT1 and MYT1L is lower in more aggressive glioma sub-types. Examination of the gene expression changes in cells expressing Myt1 or Myt1l suggests that both repress expression of the YAP1 transcriptional coactivator, which functions primarily in the Hippo signaling pathway. Expression of YAP1 and its target genes is reduced in Myt-expressing cells, and there is an inverse correlation between YAP1 and MYT1/MYT1L expression in human brain cancer datasets. Proliferation of GBM cell lines is reduced by lowering YAP1 expression and increased with YAP1 over-expression, which overcomes the anti-proliferative effect of Myt1/Myt1l expression. Finally we show that reducing YAP1 expression in a GBM cell line slows the growth of orthotopic tumor xenografts. Together, our data suggest that Myt1 and Myt1l directly repress expression of YAP1, a protein which promotes proliferation and GBM growth.

The N terminus of Ascl1 underlies differing proneural activity of mouse and Xenopus Ascl1 proteins

The proneural basic-helix-loop-helix (bHLH) transcription factor Ascl1 is a master regulator of neurogenesis in both central and peripheral nervous systems in vivo, and is a central driver of neuronal reprogramming in vitro. Over the last three decades, assaying primary neuron formation in Xenopus embryos in response to transcription factor overexpression has contributed to our understanding of the roles and regulation of proneural proteins like Ascl1, with homologues from different species usually exhibiting similar functional effects. Here we demonstrate that the mouse Ascl1 protein is twice as active as the Xenopus protein in inducing neural-β-tubulin expression in Xenopus embryos, despite there being little difference in protein accumulation or ability to undergo phosphorylation, two properties known to influence Ascl1 function. This superior activity of the mouse compared to the Xenopus protein is dependent on the presence of the non-conserved N terminal region of the protein, and indicates species-specific regulation that may necessitate care when interpreting results in cross-species experiments.

Analysis of transcriptional activity by the Myt1 and Myt1l transcription factors

Myt1 and Myt1l (Myelin transcription factor 1, and Myt1-like) are members of a small family of closely related zinc finger transcription factors, characterized by two clusters of C2HC zinc fingers. Both are widely expressed during early embryogenesis, but are largely restricted to expression within the brain in the adult. Myt1l, as part of a three transcription factor mix, can reprogram fibroblasts to neurons and plays a role in maintaining neuronal identity. Previous analyses have indicated roles in both transcriptional activation and repression and suggested that Myt1 and Myt1l may have opposing functions in gene expression. We show that when targeted to DNA via multiple copies of the consensus Myt1/Myt1l binding site Myt1 represses transcription, whereas Myt1l activates. By targeting via a heterologous DNA binding domain we mapped an activation function in Myt1l to an amino-terminal region that is poorly conserved in Myt1. However, genome wide analyses of the effects of Myt1 and Myt1l expression in a glioblastoma cell line suggest that the two proteins have largely similar effects on endogenous gene expression. Transcriptional repression is likely mediated by binding to DNA via the known consensus site, whereas this site is not associated with the transcriptional start sites of genes with higher expression in the presence of Myt1 or Myt1l. This work suggests that these two proteins function similarly, despite differences observed in analyses based on synthetic reporter constructs.

C8orf46 homolog encodes a novel protein Vexin that is required for neurogenesis in Xenopus laevis

Neural basic helix-loop helix (bHLH) transcription factors promote progenitor cell differentiation by activation of downstream target genes that coordinate neuronal differentiation. Here we characterize a neural bHLH target gene in Xenopus laevis, vexin (vxn; previously sbt1), that is homologous to human c8orf46 and is conserved across vertebrate species. C8orf46 has been implicated in cancer progression, but its function is unknown. Vxn is transiently expressed in differentiating progenitors in the developing central nervous system (CNS), and is required for neurogenesis in the neural plate and retina. Its function is conserved, since overexpression of either Xenopus or mouse vxn expands primary neurogenesis and promotes early retinal cell differentiation in cooperation with neural bHLH factors. Vxn protein is localized to the cell membrane and the nucleus, but functions in the nucleus to promote neural differentiation. Vxn inhibits cell proliferation, and works with the cyclin-dependent kinase inhibitor p27Xic1 (cdkn1b) to enhance neurogenesis and increase levels of the proneural protein Neurog2. We propose that vxn provides a key link between neural bHLH activity and execution of the neurogenic program.

Direct Interactions Between Gli3, Wnt8b, and Fgfs Underlie Patterning of the Dorsal Telencephalon

A key step in the development of the cerebral cortex is a patterning process, which subdivides the telencephalon into several molecularly distinct domains and is critical for cortical arealization. This process is dependent on a complex network of interactions between signaling molecules of the Fgf and Wnt gene families and the Gli3 transcription factor gene, but a better knowledge of the molecular basis of the interplay between these factors is required to gain a deeper understanding of the genetic circuitry underlying telencephalic patterning. Using DNA-binding and reporter gene assays, we here investigate the possibility that Gli3 and these signaling molecules interact by directly regulating each other's expression. We show that Fgf signaling is required for Wnt8b enhancer activity in the cortical hem, whereas Wnt/β-catenin signaling represses Fgf17 forebrain enhancer activity. In contrast, Fgf and Wnt/β-catenin signaling cooperate to regulate Gli3 expression. Taken together, these findings indicate that mutual interactions between Gli3, Wnt8b, and Fgf17 are crucial elements of the balance between these factors thereby conferring robustness to the patterning process. Hence, our study provides a framework for understanding the genetic circuitry underlying telencephalic patterning and how defects in this process can affect the formation of cortical areas.

RNA helicase Mov10 is essential for gastrulation and central nervous system development

BACKGROUND

Mov10 is an RNA helicase that modulates access of Argonaute 2 to microRNA recognition elements in mRNAs. We examined the role of Mov10 in Xenopus laevis development and show a critical role for Mov10 in gastrulation and in the development of the central nervous system (CNS).

RESULTS

Knockdown of maternal Mov10 in Xenopus embryos using a translation blocking morpholino led to defects in gastrulation and the development of notochord and paraxial mesoderm, and a failure to neurulate. RNA sequencing of the Mov10 knockdown embryos showed significant upregulation of many mRNAs when compared with controls at stage 10.5 (including those related to the cytoskeleton, adhesion, and extracellular matrix, which are involved in those morphogenetic processes). Additionally, the degradation of the miR-427 target mRNA, cyclin A1, was delayed in the Mov10 knockdowns. These defects suggest that Mov10's role in miRNA-mediated regulation of the maternal to zygotic transition could lead to pleiotropic effects that cause the gastrulation defects. Additionally, the knockdown of zygotic Mov10 showed that it was necessary for normal head, eye, and brain development in Xenopus consistent with a recent study in the mouse.

CONCLUSIONS

Mov10 is essential for gastrulation and normal CNS development. Developmental Dynamics 247:660-671, 2018. © 2017 Wiley Periodicals, Inc.

MyT1 Counteracts the Neural Progenitor Program to Promote Vertebrate Neurogenesis

The generation of neurons from neural stem cells requires large-scale changes in gene expression that are controlled to a large extent by proneural transcription factors, such as Ascl1. While recent studies have characterized the differentiation genes activated by proneural factors, less is known on the mechanisms that suppress progenitor cell identity. Here, we show that Ascl1 induces the transcription factor MyT1 while promoting neuronal differentiation. We combined functional studies of MyT1 during neurogenesis with the characterization of its transcriptional program. MyT1 binding is associated with repression of gene transcription in neural progenitor cells. It promotes neuronal differentiation by counteracting the inhibitory activity of Notch signaling at multiple levels, targeting the Notch1 receptor and many of its downstream targets. These include regulators of the neural progenitor program, such as Hes1, Sox2, Id3, and Olig1. Thus, Ascl1 suppresses Notch signaling cell-autonomously via MyT1, coupling neuronal differentiation with repression of the progenitor fate.

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MyT1 promotes neurogenesis downstream Ascl1

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MyT1 represses Notch1 receptor and many of its downstream target genes

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MyT1 represses Hes1 expression by direct DNA binding and competition with RBPJ

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Ascl1 suppresses Notch signaling cell-autonomously while promoting differentiation

KDM3A-mediated demethylation of histone H3 lysine 9 facilitates the chromatin binding of Neurog2 during neurogenesis

Neurog2 is a crucial regulator of neuronal fate specification and differentiation in vivo and in vitro. However, it remains unclear how Neurog2 transactivates neuronal genes that are silenced by repressive chromatin. Here, we provide evidence that the histone H3 lysine 9 demethylase KDM3A facilitates the Xenopus Neurog2 (formerly known as Xngnr1) chromatin accessibility during neuronal transcription. Loss-of-function analyses reveal that KDM3A is not required for the transition of naive ectoderm to neural progenitor cells but is essential for primary neuron formation. ChIP series followed by qPCR analyses reveal that Neurog2 promotes the removal of the repressive H3K9me2 marks and addition of active histone marks, including H3K27ac and H3K4me3, at the NeuroD1 and Tubb2b promoters; this activity depends on the presence of KDM3A because Neurog2, via its C-terminal domain, interacts with KDM3A. Interestingly, KDM3A is dispensable for the neuronal transcription initiated by Ascl1, a proneural factor related to neurogenin in the bHLH family. In summary, our findings uncover a crucial role for histone H3K9 demethylation during Neurog2-mediated neuronal transcription and help in the understanding of the different activities of Neurog2 and Ascl1 in initiating neuronal development.

Myt1l safeguards neuronal identity by actively repressing many non-neuronal fates

Normal differentiation and induced reprogramming require the activation of target cell programs and silencing of donor cell programs. In reprogramming, the same factors are often used to reprogram many different donor cell types. As most developmental repressors, such as RE1-silencing transcription factor (REST) and Groucho (also known as TLE), are considered lineage-specific repressors, it remains unclear how identical combinations of transcription factors can silence so many different donor programs. Distinct lineage repressors would have to be induced in different donor cell types. Here, by studying the reprogramming of mouse fibroblasts to neurons, we found that the pan neuron-specific transcription factor Myt1-like (Myt1l) exerts its pro-neuronal function by direct repression of many different somatic lineage programs except the neuronal program. The repressive function of Myt1l is mediated via recruitment of a complex containing Sin3b by binding to a previously uncharacterized N-terminal domain. In agreement with its repressive function, the genomic binding sites of Myt1l are similar in neurons and fibroblasts and are preferentially in an open chromatin configuration. The Notch signalling pathway is repressed by Myt1l through silencing of several members, including Hes1. Acute knockdown of Myt1l in the developing mouse brain mimicked a Notch gain-of-function phenotype, suggesting that Myt1l allows newborn neurons to escape Notch activation during normal development. Depletion of Myt1l in primary postmitotic neurons de-repressed non-neuronal programs and impaired neuronal gene expression and function, indicating that many somatic lineage programs are actively and persistently repressed by Myt1l to maintain neuronal identity. It is now tempting to speculate that similar 'many-but-one' lineage repressors exist for other cell fates; such repressors, in combination with lineage-specific activators, would be prime candidates for use in reprogramming additional cell types.

Enhancer Analysis Unveils Genetic Interactions between TLX and SOX2 in Neural Stem Cells and In Vivo Reprogramming

The orphan nuclear receptor TLX is a master regulator of postnatal neural stem cell (NSC) self-renewal and neurogenesis; however, it remains unclear how TLX expression is precisely regulated in these tissue-specific stem cells. Here, we show that a highly conserved cis-element within the Tlx locus functions to drive gene expression in NSCs. We demonstrate that the transcription factors SOX2 and MYT1 specifically interact with this genomic element to directly regulate Tlx enhancer activity in vivo. Knockdown experiments further reveal that SOX2 dominantly controls endogenous expression of TLX, whereas MYT1 only plays a modulatory role. Importantly, TLX is essential for SOX2-mediated in vivo reprogramming of astrocytes and itself is also sufficient to induce neurogenesis in the adult striatum. Together, these findings unveil functional genetic interactions among transcription factors that are critical to NSCs and in vivo cell reprogramming.

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An evolutionarily conserved enhancer drives Tlx expression in neural stem cells

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SOX2 directly activates the identified enhancer and Tlx expression

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SOX2-mediated in vivo reprogramming of astrocytes to neuroblasts requires TLX

Mutations in MYT1, encoding the myelin transcription factor 1, are a rare cause of OAVS

BACKGROUND

Oculo-auriculo-vertebral spectrum (OAVS) is a developmental disorder involving first and second branchial arches derivatives, mainly characterised by asymmetric ear anomalies, hemifacial microsomia, ocular defects and vertebral malformations. Although numerous chromosomal abnormalities have been associated with OAVS, no causative gene has been identified so far.

OBJECTIVES

We aimed to identify the first causative gene for OAVS.

METHODS

As sporadic cases are mostly described in Goldenhar syndrome, we have performed whole exome sequencing (WES) on selected affected individuals and their unaffected parents, looking for de novo mutations. Candidate gene was tested through transient knockdown experiment in zebrafish using a morpholino-based approach. A functional test was developed in cell culture in order to assess deleterious consequences of mutations.

RESULTS

By WES, we identified a heterozygous nonsense mutation in one patient in the myelin transcription factor 1 (MYT1) gene. Further, we detected one heterozygous missense mutation in another patient among a cohort of 169 patients with OAVS. This gene encodes the MYT1. Functional studies by transient knockdown of myt1a, homologue of MYT1 in zebrafish, led to specific craniofacial cartilage alterations. Treatment with all-trans retinoic acid (RA), a known teratogenic agent causing OAVS, led to an upregulation of cellular endogenous MYT1 expression. Additionally, cellular wild-type MYT1 overexpression induced a downregulation of RA receptor β (RARB), whereas mutated MYT1 did not.

CONCLUSION

We report MYT1 as the first gene implicated in OAVS, within the RA signalling pathway.

Assessing Primary Neurogenesis in Xenopus Embryos Using Immunostaining

Primary neurogenesis is a dynamic and complex process during embryonic development that sets up the initial layout of the central nervous system. During this process, a portion of neural stem cells undergo differentiation and give rise to the first populations of differentiated primary neurons within the nascent central nervous system. Several vertebrate model organisms have been used to explore the mechanisms of neural cell fate specification, patterning, and differentiation. Among these is the African clawed frog, Xenopus, which provides a powerful system for investigating the molecular and cellular mechanisms responsible for primary neurogenesis due to its rapid and accessible development and ease of embryological and molecular manipulations. Here, we present a convenient and rapid method to observe the different populations of neuronal cells within Xenopus central nervous system. Using antibody staining and immunofluorescence on sections of Xenopus embryos, we are able to observe the locations of neural stem cells and differentiated primary neurons during primary neurogenesis.

Analysis of neural progenitors from embryogenesis to juvenile adult in Xenopus laevis reveals biphasic neurogenesis and continuous lengthening of the cell cycle

Xenopus laevis is a prominent model system for studying neural development, but our understanding of the long-term temporal dynamics of neurogenesis remains incomplete. Here, we present the first continuous description of neurogenesis in X. laevis, covering the entire period of development from the specification of neural ectoderm during gastrulation to juvenile frog. We have used molecular markers to identify progenitors and neurons, short-term bromodeoxyuridine (BrdU) incorporation to map the generation of newborn neurons and dual pulse S-phase labelling to characterise changes in their cell cycle length. Our study revealed the persistence of Sox3-positive progenitor cells from the earliest stages of neural development through to the juvenile adult. Two periods of intense neuronal generation were observed, confirming the existence of primary and secondary waves of neurogenesis, punctuated by a period of quiescence before metamorphosis and culminating in another period of quiescence in the young adult. Analysis of multiple parameters indicates that neural progenitors alternate between global phases of differentiation and amplification and that, regardless of their behaviour, their cell cycle lengthens monotonically during development, at least at the population level.

A decade from discovery to therapy: Lingo-1, the dark horse in neurological and psychiatric disorders

Leucine-rich repeat and immunoglobulin domain-containing protein (Lingo-1) is a potent negative regulator of neuron and oligodendrocyte survival, neurite extension, axon regeneration, oligodendrocyte differentiation, axonal myelination and functional recovery; all processes highly implicated in numerous brain-related functions. Although playing a major role in developmental brain functions, the potential application of Lingo-1 as a therapeutic target for the treatment of neurological disorders has so far been under-estimated. A number of preclinical studies have shown that various methods of antagonizing Lingo-1 results in neuronal and oligodendroglial survival, axonal growth and remyelination; however to date literature has only detailed applications of Lingo-1 targeted therapeutics with a focus primarily on myelination disorders such as multiple sclerosis and spinal cord injury; omitting important information regarding Lingo-1 signaling co-factors. Here, we provide for the first time a complete and thorough review of the implications of Lingo-1 signaling in a wide range of neurological and psychiatric disorders, and critically examine its potential as a novel therapeutic target for these disorders.

Multi-site phosphorylation regulates NeuroD4 activity during primary neurogenesis: a conserved mechanism amongst proneural proteins

Background

Basic Helix Loop Helix (bHLH) proneural transcription factors are master regulators of neurogenesis that act at multiple stages in this process. We have previously demonstrated that multi-site phosphorylation of two members of the proneural protein family, Ngn2 and Ascl1, limits their ability to drive neuronal differentiation when cyclin-dependent kinase levels are high, as would be found in rapidly cycling cells. Here we investigate potential phospho-regulation of proneural protein NeuroD4 (also known as Xath3), the Xenopus homologue of Math3/NeuroM, that functions downstream of Ngn2 in the neurogenic cascade.

Results

Using the developing Xenopus embryo system, we show that NeuroD4 is expressed and phosphorylated during primary neurogenesis, and this phosphorylation limits its ability to drive neuronal differentiation. Phosphorylation of up to six serine/threonine-proline sites contributes additively to regulation of NeuroD4 proneural activity without altering neuronal subtype specification, and number rather than location of available phospho-sites is the key for limiting NeuroD4 activity. Mechanistically, a phospho-mutant NeuroD4 displays increased protein stability and enhanced chromatin binding relative to wild-type NeuroD4, resulting in transcriptional up-regulation of a range of target genes that further promote neuronal differentiation.

Conclusions

Multi-site phosphorylation on serine/threonine-proline pairs is a widely conserved mechanism of limiting proneural protein activity, where it is the number of phosphorylated sites, rather than their location that determines protein activity. Hence, multi-site phosphorylation is very well suited to allow co-ordination of proneural protein activity with the cellular proline-directed kinase environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-015-0044-8) contains supplementary material, which is available to authorized users.

Ascl1 Coordinately Regulates Gene Expression and the Chromatin Landscape during Neurogenesis

The proneural transcription factor Ascl1 coordinates gene expression in both proliferating and differentiating progenitors along the neuronal lineage. Here, we used a cellular model of neurogenesis to investigate how Ascl1 interacts with the chromatin landscape to regulate gene expression when promoting neuronal differentiation. We find that Ascl1 binding occurs mostly at distal enhancers and is associated with activation of gene transcription. Surprisingly, the accessibility of Ascl1 to its binding sites in neural stem/progenitor cells remains largely unchanged throughout their differentiation, as Ascl1 targets regions of both readily accessible and closed chromatin in proliferating cells. Moreover, binding of Ascl1 often precedes an increase in chromatin accessibility and the appearance of new regions of open chromatin, associated with de novo gene expression during differentiation. Our results reveal a function of Ascl1 in promoting chromatin accessibility during neurogenesis, linking the chromatin landscape at Ascl1 target regions with the temporal progression of its transcriptional program.

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Genome-wide binding of Ascl1 correlates with transcription activation

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Ascl1 can bind to both open and closed chromatin in proliferating cells

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Ascl1 promotes local chromatin accessibility at its target sites

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Chromatin dynamics at Ascl1 sites regulates temporal progression of its program

Fezf2 promotes neuronal differentiation through localised activation of Wnt/β-catenin signalling during forebrain development

Brain regionalisation, neuronal subtype diversification and circuit connectivity are crucial events in the establishment of higher cognitive functions. Here we report the requirement for the transcriptional repressor Fezf2 for proper differentiation of neural progenitor cells during the development of the Xenopus forebrain. Depletion of Fezf2 induces apoptosis in postmitotic neural progenitors, with concomitant reduction in forebrain size and neuronal differentiation. Mechanistically, we found that Fezf2 stimulates neuronal differentiation by promoting Wnt/β-catenin signalling in the developing forebrain. In addition, we show that Fezf2 promotes activation of Wnt/β-catenin signalling by repressing the expression of two negative regulators of Wnt signalling, namely lhx2 and lhx9. Our findings suggest that Fezf2 plays an essential role in controlling when and where neuronal differentiation occurs within the developing forebrain and that it does so by promoting local Wnt/β-catenin signalling via a double-repressor model.

Sox21 regulates the progression of neuronal differentiation in a dose-dependent manner

Members of the SoxB transcription factor family play critical roles in the regulation of neurogenesis. The SoxB1 proteins are required for the induction and maintenance of a proliferating neural progenitor population in numerous vertebrates, however the role of the SoxB2 protein, Sox21, is less clear due to conflicting results. To clarify the role of Sox21 in neurogenesis, we examined its function in the Xenopus neural plate. Here we report that misexpression of Sox21 expands the neural progenitor domain, and represses neuron formation by binding to Neurogenin (Ngn2) and blocking its function. Conversely, we found that Sox21 is also required for neuron formation, as cells lacking Sox21 undergo cell death and thus are unable to differentiate. Together our data indicate that Sox21 plays more than one role in neurogenesis, where a threshold level is required for cell viability and normal differentiation of neurons, but a higher concentration of Sox21 inhibits neuron formation and instead promotes progenitor maintenance.

The rice gene OsZFP6 functions in multiple stress tolerance responses in yeast and Arabidopsis

The role of zinc finger proteins in organismal stress conditions has been widely reported. However, little is known concerning the function of CCHC-type zinc finger proteins in rice. In this study, OsZFP6, a rice CCHC-type zinc finger protein 6 gene, was cloned from rice using RT-PCR. The OsZFP6 protein contains 305 amino acids and a conserved zinc finger domain and is localised to the nucleus. Southern blot analysis revealed that a single copy was encoded in the rice genome. OsZFP6 expression was increased by abiotic stress, including salt (NaCl), alkali (NaHCO3) and H2O2 treatment. When OsZFP6 was transformed into yeast, the transgenic yeast showed significantly increased resistance to NaHCO3 compared to the control. Moreover, Arabidopsis transgenic plants overexpressing OsZFP6 were more tolerant to both NaHCO3 and H2O2 treatments. Overall, we uncovered a role for OsZFP6 in abiotic stress responses and identified OsZFP6 as a putatively useful gene for developing crops with increased alkali and H2O2 tolerance.

The phosphorylation status of Ascl1 is a key determinant of neuronal differentiation and maturation in vivo and in vitro

Generation of neurons from patient fibroblasts using a combination of developmentally defined transcription factors has great potential in disease modelling, as well as ultimately for use in regeneration and repair. However, generation of physiologically mature neurons in vitro remains problematic. Here we demonstrate the cell-cycle-dependent phosphorylation of a key reprogramming transcription factor, Ascl1, on multiple serine-proline sites. This multisite phosphorylation is a crucial regulator of the ability of Ascl1 to drive neuronal differentiation and maturation in vivo in the developing embryo; a phosphomutant form of Ascl1 shows substantially enhanced neuronal induction activity in Xenopus embryos. Mechanistically, we see that this un(der)phosphorylated Ascl1 is resistant to inhibition by both cyclin-dependent kinase activity and Notch signalling, both of which normally limit its neurogenic potential. Ascl1 is a central component of reprogramming transcription factor cocktails to generate neurons from human fibroblasts; the use of phosphomutant Ascl1 in place of the wild-type protein significantly promotes neuronal maturity after human fibroblast reprogramming in vitro. These results demonstrate that cell-cycle-dependent post-translational modification of proneural proteins directly regulates neuronal differentiation in vivo during development, and that this regulatory mechanism can be harnessed to promote maturation of neurons obtained by transdifferentiation of human cells in vitro.

Cell cycle regulation of proliferation versus differentiation in the central nervous system

Formation of the central nervous system requires a period of extensive progenitor cell proliferation, accompanied or closely followed by differentiation; the balance between these two processes in various regions of the central nervous system gives rise to differential growth and cellular diversity. The correlation between cell cycle lengthening and differentiation has been reported across several types of cell lineage and from diverse model organisms, both in vivo and in vitro. Furthermore, different cell fates might be determined during different phases of the preceding cell cycle, indicating direct cell cycle influences on both early lineage commitment and terminal cell fate decisions. Significant advances have been made in the last decade and have revealed multi-directional interactions between the molecular machinery regulating the processes of cell proliferation and neuronal differentiation. Here, we first introduce the modes of proliferation in neural progenitor cells and summarise evidence linking cell cycle length and neuronal differentiation. Second, we describe the manner in which components of the cell cycle machinery can have additional and, sometimes, cell-cycle-independent roles in directly regulating neurogenesis. Finally, we discuss the way that differentiation factors, such as proneural bHLH proteins, can promote either progenitor maintenance or differentiation according to the cellular environment. These intricate connections contribute to precise coordination and the ultimate division versus differentiation decision.

Spatiotemporal expression pattern of Myt/NZF family zinc finger transcription factors during mouse nervous system development

BACKGROUND

Three members of the Myt/NZF family of transcription factors are involved in many processes of vertebrate development. Several studies have reported that Myt1/NZF-2 has a regulatory function in the development of cultured oligodendrocyte progenitors or in neuronal differentiation during Xenopus primary neurogenesis. However, little is known about the proper function of Myt/NZF family proteins during mammalian nervous system development. To assess the possible function of Myt/NZF transcription factors in mammalian neuronal differentiation, we determined the comparative spatial and temporal expression patterns of all three types of Myt/NZF family genes in the embryonic mouse nervous system using quantitative reverse transcriptase polymerase chain reaction and in situ hybridization.

RESULTS

All three Myt/NZF family genes were extensively expressed in developing mouse nervous tissues, and their expression was transient. NZF-1 was expressed later in post-mitotic neurons. NZF-2 was initially expressed in neuronal cells a little earlier than NZF-3. NZF-3 was initially expressed in neuronal cells, just after proliferation was complete.

CONCLUSION

These expression patterns suggest that the expression of NZF family genes is spatially and temporally regulated, and each Myt/NZF family gene may have a regulatory function in a specific phase during neuronal differentiation.

Acetylcholine-metabolizing butyrylcholinesterase (BCHE) copy number and single nucleotide polymorphisms and their role in attention-deficit/hyperactivity syndrome

A previous genome-wide screen for copy number variations (CNVs) in attention deficit/hyperactivity disorder (ADHD) revealed a de novo chromosome 3q26.1 deletion in one of the patients. Candidate genes at this locus include the acetylcholine-metabolizing butyrylcholinesterase (BCHE) expressing gene (OMIM #177400), which is of particular interest. The present study investigates the hypothesis that the heterozygous deletion of the BCHE gene is associated with adult ADHD (aADHD). Ina first step, we screened 348 aADHD patients and 352 controls for stretches of loss of heterozygosity (LOH) across the entire BCHE gene to screen for the deletion. Our second aim was to clarify whether BCHE single nucleotide polymorphisms (SNPs) themselves influence the risk towards ADHD. Putative functional consequences of associated SNPs as well as their un-typed proxies were predicted by several bioinformatic tools. 96 individuals displayed entirely homozygous genotype reads in all 12 examined SNPs, making them possible candidates to harbor a heterozygous BCHE deletion. DNA from these 96 probands was further analyzed by real-time PCR using a BCHE-specific CNV assay. However, no deletion was found. Of the 12 tag SNPs that passed inclusion criteria, rs4680612 and rs829508 were significantly associated with aADHD, as their minor alleles occurred more often in cases than in controls (p = 0.018 and p = 0.039, respectively). The risk variant rs4680612 is located in the transcriptional control region of the gene and predicted to disrupt a binding site for MYT-1, which has previously been associated with mental disorders. However, when examining a second independent adult ADHD sample of 353 cases, the association did not replicate. When looking up the deletion in three genome-wide screens for CNV in ADHD and combining it with the present study, it became apparent that 3 from a total of 1030 ADHD patients, but none of 5787 controls, featured a deletion of the BCHE promoter region including rs4680612 (p = 0.00004). Taken together, there are several lines of evidence suggesting a potential involvement of BCHE in the etiopathology of ADHD, as a rare hemizygous deletion as well as a common SNP in the same region are associated with disease, although with different penetrance. Both variations result in the disruption of the binding site of the transcription factor MYT-1 suggesting epistatic effects of BCHE and MYT-1 in the pathogenesis of ADHD. As we were not able to replicate the SNP association, our findings should be considered preliminary and call for larger studies in extended phenotypes.

Hierarchical mechanisms for direct reprogramming of fibroblasts to neurons

Direct lineage reprogramming is a promising approach for human disease modeling and regenerative medicine, with poorly understood mechanisms. Here, we reveal a hierarchical mechanism in the direct conversion of fibroblasts into induced neuronal (iN) cells mediated by the transcription factors Ascl1, Brn2, and Myt1l. Ascl1 acts as an "on-target" pioneer factor by immediately occupying most cognate genomic sites in fibroblasts. In contrast, Brn2 and Myt1l do not access fibroblast chromatin productively on their own; instead, Ascl1 recruits Brn2 to Ascl1 sites genome wide. A unique trivalent chromatin signature in the host cells predicts the permissiveness for Ascl1 pioneering activity among different cell types. Finally, we identified Zfp238 as a key Ascl1 target gene that can partially substitute for Ascl1 during iN cell reprogramming. Thus, a precise match between pioneer factors and the chromatin context at key target genes is determinative for transdifferentiation to neurons and likely other cell types.

NumbL is essential for Xenopus primary neurogenesis

Background

Members of the vertebrate Numb family of cell fate determinants serve multiple functions throughout early embryogenesis, including an essential role in the development of the nervous system. The Numb proteins interact with various partner proteins and correspondingly participate in multiple cellular activities, including inhibition of the Notch pathway.

Results

Here, we describe the expression characteristics of Numb and Numblike (NumbL) during Xenopus development and characterize the function of NumbL during primary neurogenesis. NumbL, in contrast to Numb, is expressed in the territories of primary neurogenesis and is positively regulated by the Neurogenin family of proneural transcription factors. Knockdown of NumbL afforded a complete loss of primary neurons and did not lead to an increase in Notch signaling in the open neural plate. Furthermore, we provide evidence that interaction of NumbL with the AP-2 complex is required for NumbL function during primary neurogenesis.

Conclusion

We demonstrate an essential role of NumbL during Xenopus primary neurogenesis and provide evidence for a Notch-independent function of NumbL in this context.

A structural analysis of DNA binding by myelin transcription factor 1 double zinc fingers

Myelin transcription factor 1 (MyT1/NZF2), a member of the neural zinc-finger (NZF) protein family, is a transcription factor that plays a central role in the developing central nervous system. It has also recently been shown that, in combination with two other transcription factors, the highly similar paralog MyT1L is able to direct the differentiation of murine and human stem cells into functional neurons. MyT1 contains seven zinc fingers (ZFs) that are highly conserved throughout the protein and throughout the NZF family. We recently presented a model for the interaction of the fifth ZF of MyT1 with a DNA sequence derived from the promoter of the retinoic acid receptor (RARE) gene. Here, we have used NMR spectroscopy, in combination with surface plasmon resonance and data-driven molecular docking, to delineate the mechanism of DNA binding for double ZF polypeptides derived from MyT1. Our data indicate that a two-ZF unit interacts with the major groove of the entire RARE motif and that both fingers bind in an identical manner and with overall two-fold rotational symmetry, consistent with the palindromic nature of the target DNA. Several key residues located in one of the irregular loops of the ZFs are utilized to achieve specific binding. Analysis of the human and mouse genomes based on our structural data reveals three putative MyT1 target genes involved in neuronal development.

ERF and ETV3L are retinoic acid-inducible repressors required for primary neurogenesis

Cells in the developing neural tissue demonstrate an exquisite balance between proliferation and differentiation. Retinoic acid (RA) is required for neuronal differentiation by promoting expression of proneural and neurogenic genes. We show that RA acts early in the neurogenic pathway by inhibiting expression of neural progenitor markers Geminin and Foxd4l1, thereby promoting differentiation. Our screen for RA target genes in early Xenopus development identified Ets2 Repressor Factor (Erf) and the closely related ETS repressors Etv3 and Etv3-like (Etv3l). Erf and Etv3l are RA responsive and inhibit the action of ETS genes downstream of FGF signaling, placing them at the intersection of RA and growth factor signaling. We hypothesized that RA regulates primary neurogenesis by inducing Erf and Etv3l to antagonize proliferative signals. Loss-of-function analysis showed that Erf and Etv3l are required to inhibit proliferation of neural progenitors to allow differentiation, whereas overexpression of Erf led to an increase in the number of primary neurons. Therefore, these RA-induced ETS repressors are key components of the proliferation-differentiation switch during primary neurogenesis in vivo.

Interhemispheric asymmetry of olfactory input-dependent neuronal specification in the adult brain

The vertebrate brain is anatomically and functionally asymmetric. The left and right cerebral hemispheres harbor neural stem cell niches at the ventricular-subventricular zone (V-SVZ) of the ventricular walls, where new neurons are continuously generated throughout life. However, any interhemispheric asymmetry of neural stem cell niches remains unclear. We performed gene-trap screens in adult zebrafish to identify genes that are differentially expressed in the two hemispheres and found that adult-born neurons expressing the neural zinc-finger protein Myt1 exist predominantly in the left V-SVZ. This lateralization could be reversed by left olfactory sensory deprivation-induced inactivation of Notch signaling. The olfactory behavioral preference for attractive amino acids was also impaired by sensory deprivation of the left olfactory system, but not of the right olfactory system. Our findings suggest that olfactory input generates interhemispheric differences in the fate of adult-born neurons in the zebrafish brain.

Switching metal ion coordination and DNA Recognition in a Tandem CCHHC-type zinc finger peptide

Neural Zinc Finger Factor-1 (NZF-1) and Myelin Transcription Factor 1 (MyT1) are two homologous nonclassical zinc finger (ZF) proteins that are involved in the development of the central nervous system (CNS). Both NZF-1 and MyT1 contain multiple ZF domains, each of which contains an absolutely conserved Cys2His2Cys motif. All three cysteines and the second histidine have been shown to coordinate Zn(II); however, the role of the first histidine remains unresolved. Using a functional form of NZF-1 that contains two ZF domains (NZF-1-F2F3), mutant proteins in which each histidine was sequentially mutated to a phenylalanine were prepared to determine the role(s) of the histidine residues in DNA recognition. When the first histidine is mutated, the protein binds Zn(II) in an analogous manner to the native protein. Surprisingly, this mutant does not bind to target DNA (β-RAR), suggesting that the noncoordinating histidine is critical for sequence selective DNA recognition. The first histidine will coordinate Zn(II) when the second histidine is mutated; however, the overall fold of the protein is perturbed leading to abrogation of DNA binding. NZF-1-F2F3 selectively binds to a specific DNA target sequence (from β-RAR) with high affinity (nM); while its homologue MyT1 (MyT1-F2F3), which is 92% identical to NZF-1-F2F3, binds to this same DNA sequence nonspecifically. A single, nonconserved amino acid residue in NZF-1-F2F3 is shown to be responsible for this high affinity DNA binding to β-RAR. When this residue (arginine) is engineered into the MyT1-F2F3 sequence, the affinity for β-RAR DNA increases.

Transcriptional regulation of the peripheral nervous system in Ciona intestinalis

The formation of the sensory organs and cells that make up the peripheral nervous system (PNS) relies on the activity of transcription factors encoded by proneural genes (PNGs). Although PNGs have been identified in the nervous systems of both vertebrates and invertebrates, the complexity of their interactions has complicated efforts to understand their function in the context of their underlying regulatory networks. To gain insight into the regulatory network of PNG activity in chordates, we investigated the roles played by PNG homologs in regulating PNS development of the invertebrate chordate Ciona intestinalis. We discovered that in Ciona, MyT1, Pou4, Atonal, and NeuroD-like are expressed in a sequential regulatory cascade in the developing epidermal sensory neurons (ESNs) of the PNS and act downstream of Notch signaling, which negatively regulates these genes and the number of ESNs along the tail midlines. Transgenic embryos mis-expressing any of these proneural genes in the epidermis produced ectopic midline ESNs. In transgenic embryos mis-expressing Pou4, and MyT1 to a lesser extent, numerous ESNs were produced outside of the embryonic midlines. In addition we found that the microRNA miR-124, which inhibits Notch signaling in ESNs, is activated downstream of all the proneural factors we tested, suggesting that these genes operate collectively in a regulatory network. Interestingly, these factors are encoded by the same genes that have recently been demonstrated to convert fibroblasts into neurons. Our findings suggest the ascidian PNS can serve as an in vivo model to study the underlying regulatory mechanisms that enable the conversion of cells into sensory neurons.

Leveling Waddington: the emergence of direct programming and the loss of cell fate hierarchies

For decades, Waddington's concept of the 'epigenetic landscape' has served as an educative hierarchical model to illustrate the progressive restriction of cell differentiation potential during normal development. While still being highly valuable in the context of normal development, the Waddington model falls short of accommodating recent breakthroughs in cell programming. The advent of induced pluripotent stem (iPS) cells and advances in direct cell fate conversion (also known as transdifferentiation) suggest that somatic and pluripotent cell fates can be interconverted without transiting through distinct hierarchies. We propose a non-hierarchical 'epigenetic disc' model to explain such cell fate transitions, which provides an alternative landscape for modelling cell programming and reprogramming.

The transcription factor Myt3 acts as a pro-survival factor in β-cells

Aims/Hypothesis

We previously identified the transcription factor Myt3 as specifically expressed in pancreatic islets. Here, we sought to determine the expression and regulation of Myt3 in islets and to determine its significance in regulating islet function and survival.

Methods

Myt3 expression was determined in embryonic pancreas and adult islets by qPCR and immunohistochemistry. ChIP-seq, ChIP-qPCR and luciferase assays were used to evaluate regulation of Myt3 expression. Suppression of Myt3 was used to evaluate gene expression, insulin secretion and apoptosis in islets.

Results

We show that Myt3 is the most abundant MYT family member in adult islets and that it is expressed in all the major endocrine cell types in the pancreas after E18.5. We demonstrate that Myt3 expression is directly regulated by Foxa2, Pdx1, and Neurod1, which are critical to normal β-cell development and function, and that Ngn3 induces Myt3 expression through alterations in the Myt3 promoter chromatin state. Further, we show that Myt3 expression is sensitive to both glucose and cytokine exposure. Of specific interest, suppressing Myt3 expression reduces insulin content and increases β-cell apoptosis, at least in part, due to reduced Pdx1, Mafa, Il-6, Bcl-xl, c-Iap2 and Igfr1 levels, while over-expression of Myt3 protects islets from cytokine induced apoptosis.

Conclusion/Interpretation

We have identified Myt3 as a novel transcriptional regulator with a critical role in β-cell survival. These data are an important step in clarifying the regulatory networks responsible for β-cell survival, and point to Myt3 as a potential therapeutic target for improving functional β-cell mass.

Scratch2 modulates neurogenesis and cell migration through antagonism of bHLH proteins in the developing neocortex

Scratch genes (Scrt) are neural-specific zinc-finger transcription factors (TFs) with an unknown function in the developing brain. Here, we show that, in addition to the reported expression of mammalian Scrt2 in postmitotic differentiating and mature neurons in the developing and early postnatal brain, Scrt2 is also localized in subsets of mitotic and neurogenic radial glial (RGP) and intermediate (IP) progenitors, as well as in their descendants-postmitotic IPs and differentiating neurons at the border subventricular/intermediate zone. Conditional activation of transgenic Scrt2 in cortical progenitors in mice promotes neuronal differentiation by favoring the direct mode of neurogenesis of RGPs at the onset of neurogenesis, at the expense of IP generation. Neuronal amplification via indirect IP neurogenesis is thereby extenuated, leading to a mild postnatal reduction of cortical thickness. Forced in vivo overexpression of Scrt2 suppressed the generation of IPs from RGPs and caused a delay in the radial migration of upper layer neurons toward the cortical plate. Mechanistically, our results indicate that Scrt2 negatively regulates the transcriptional activation of the basic helix loop helix TFs Ngn2/NeuroD1 on E-box containing common target genes, including Rnd2, a well-known major effector for migrational defects in developing cortex. Altogether, these findings reveal a modulatory role of Scrt2 protein in cortical neurogenesis and neuronal migration.

The Xenopus doublesex-related gene Dmrt5 is required for olfactory placode neurogenesis

The Dmrt (doublesex and mab-3 related transcription factor) genes encode a large family of evolutionarily conserved transcription factors whose function in sex specific differentiation has been well studied in all animal lineages. In vertebrates, their function is not restricted to the developing gonads. For example, Xenopus Dmrt4 is essential for neurogenesis in the olfactory system. Here we have isolated and characterized Xenopus Dmrt5 and found that it is coexpressed with Dmrt4 in the developing olfactory placodes. As Dmrt4, Dmrt5 is positively regulated in the ectoderm by neural inducers and negatively by proneural factors. Both Dmrt5 and Dmrt4 genes are also activated by the combined action of the transcription factor Otx2, broadly transcribed in the head ectoderm and of Notch signaling, activated in the anterior neural ridge. As for Dmrt4, knockdown of Dmrt5 impairs neurogenesis in the embryonic olfactory system and in neuralized animal caps. Conversely, its overexpression promotes neuronal differentiation in animal caps, a property that requires the conserved C-terminal DMA and DMB domains. We also found that the sea anenome Dmrt4/5 related gene NvDmrtb also induces neurogenesis in Xenopus animal caps and that conversely, its knockdown in Nematostella reduces elav-1 positive neurons. Together, our data identify Dmrt5 as a novel important regulator of neurogenesis whose function overlaps with that of Dmrt4 during Xenopus olfactory system development. They also suggest that Dmrt may have had a role in neurogenesis in the last common ancestor of cnidarians and bilaterians.

Regulation of neuronal differentiation at the neurogenic wavefront

Signaling mediated by the Delta/Notch system controls the process of lateral inhibition, known to regulate neurogenesis in metazoans. Lateral inhibition takes place in equivalence groups formed by cells having equal capacity to differentiate, and it results in the singling out of precursors, which subsequently become neurons. During normal development, areas of active neurogenesis spread through non-neurogenic regions in response to specific morphogens, giving rise to neurogenic wavefronts. Close contact of these wavefronts with non-neurogenic cells is expected to affect lateral inhibition. Therefore, a mechanism should exist in these regions to prevent disturbances of the lateral inhibitory process. Focusing on the developing chick retina, we show that Dll1 is widely expressed by non-neurogenic precursors located at the periphery of this tissue, a region lacking Notch1, lFng, and differentiation-related gene expression. We investigated the role of this Dll1 expression through mathematical modeling. Our analysis predicts that the absence of Dll1 ahead of the neurogenic wavefront results in reduced robustness of the lateral inhibition process, often linked to enhanced neurogenesis and the presence of morphological alterations of the wavefront itself. These predictions are consistent with previous observations in the retina of mice in which Dll1 is conditionally mutated. The predictive capacity of our mathematical model was confirmed further by mimicking published results on the perturbation of morphogenetic furrow progression in the eye imaginal disc of Drosophila. Altogether, we propose that Notch-independent Delta expression ahead of the neurogenic wavefront is required to avoid perturbations in lateral inhibition and wavefront progression, thus optimizing the neurogenic process.

Direct lineage conversions: unnatural but useful?

Classic experiments such as somatic cell nuclear transfer into oocytes and cell fusion demonstrated that differentiated cells are not irreversibly committed to their fate. More recent work has built on these conclusions and discovered defined factors that directly induce one specific cell type from another, which may be as distantly related as cells from different germ layers. This suggests the possibility that any specific cell type may be directly converted into any other if the appropriate reprogramming factors are known. Direct lineage conversion could provide important new sources of human cells for modeling disease processes or for cellular-replacement therapies. For future applications, it will be critical to carefully determine the fidelity of reprogramming and to develop methods for robustly and efficiently generating human cell types of interest.