Mammalian achaete-scute homolog 1 is transiently expressed by spatially restricted subsets of early neuroepithelial and neural crest cells

Stratification of enterochromaffin cells by single-cell expression analysis

Dynamic interactions between gut mucosal cells and the external environment are essential to maintain gut homeostasis. Enterochromaffin (EC) cells transduce both chemical and mechanical signals and produce 5-hydroxytryptamine (5-HT) to mediate disparate physiological responses. However, the molecular and cellular basis for functional diversity of ECs remains to be adequately defined. Here, we integrated single-cell transcriptomics with spatial image analysis to identify fourteen EC clusters that are topographically organized along the gut. Subtypes predicted to be sensitive to the chemical environment and mechanical forces were identified that express distinct transcription factors and hormones. A Piezo2 + population in the distal colon was endowed with a distinctive neuronal signature. Using a combination of genetic, chemogenetic and pharmacological approaches, we demonstrated Piezo2 + ECs are required for normal colon motility. Our study constructs a molecular map for ECs and offers a framework for deconvoluting EC cells with pleiotropic functions.

ASCL1 Is Involved in the Pathogenesis of Schizophrenia by Regulation of Genes Related to Cell Proliferation, Neuronal Signature Formation, and Neuroplasticity

Schizophrenia (SZ) is a common psychiatric neurodevelopmental disorder with a complex genetic architecture. Genome-wide association studies indicate the involvement of several transcription factors, including ASCL1, in the pathogenesis of SZ. We aimed to identify ASCL1-dependent cellular and molecular mechanisms associated with SZ. We used Capture-C, CRISPR/Cas9 systems and RNA-seq analysis to confirm the involvement of ASCL1 in SZ-associated pathogenesis, establish a mutant SH-SY5Y line with a functional ASCL1 knockout (ASCL1-del) and elucidate differentially expressed genes that may underlie ASCL1-dependent pathogenic mechanisms. Capture-C confirmed the spatial interaction of the ASCL1 promoter with SZ-associated loci. Transcriptome analysis showed that ASCL1 regulation may be through a negative feedback mechanism. ASCL1 dysfunction affects the expression of genes associated with the pathogenesis of SZ, as well as bipolar and depressive disorders. Genes differentially expressed in ASCL1-del are involved in cell mitosis, neuronal projection, neuropeptide signaling, and the formation of intercellular contacts, including the synapse. After retinoic acid (RA)-induced differentiation, ASCL1 activity is restricted to a small subset of genes involved in neuroplasticity. These data suggest that ASCL1 dysfunction promotes SZ development predominantly before the onset of neuronal differentiation by slowing cell proliferation and impeding the formation of neuronal signatures.

Harnessing the Power of Enteric Glial Cells' Plasticity and Multipotency for Advancing Regenerative Medicine

The enteric nervous system (ENS), known as the intrinsic nervous system of the gastrointestinal tract, is composed of a diverse array of neuronal and glial cell subtypes. Fascinating questions surrounding the generation of cellular diversity in the ENS have captivated ENS biologists for a considerable time, particularly with recent advancements in cell type-specific transcriptomics at both population and single-cell levels. However, the current focus of research in this field is predominantly restricted to the study of enteric neuron subtypes, while the investigation of enteric glia subtypes significantly lags behind. Despite this, enteric glial cells (EGCs) are increasingly recognized as equally important regulators of numerous bowel functions. Moreover, a subset of postnatal EGCs exhibits remarkable plasticity and multipotency, distinguishing them as critical entities in the context of advancing regenerative medicine. In this review, we aim to provide an updated overview of the current knowledge on this subject, while also identifying key questions that necessitate future exploration.

Ascl1-expressing cell differentiation in initially developed taste buds and taste organoids

Mammalian taste bud cells are composed of several distinct cell types and differentiated from surrounding tongue epithelial cells. However, the detailed mechanisms underlying their differentiation have yet to be elucidated. In the present study, we examined an Ascl1-expressing cell lineage using circumvallate papillae (CVP) of newborn mice and taste organoids (three-dimensional self-organized tissue cultures), which allow studying the differentiation of taste bud cells in fine detail ex vivo. Using lineage-tracing analysis, we observed that Ascl1 lineage cells expressed type II and III taste cell markers both CVP of newborn mice and taste organoids. However, the coexpression rate in type II cells was lower than that in type III cells. Furthermore, we found that the generation of the cells which express type II and III cell markers was suppressed in taste organoids lacking Ascl1-expressing cells. These findings suggest that Ascl1-expressing precursor cells can differentiate into both type III and a subset of type II taste cells.

Tip60-mediated H2A.Z acetylation promotes neuronal fate specification and bivalent gene activation

Cell lineage specification is accomplished by a concerted action of chromatin remodeling and tissue-specific transcription factors. However, the mechanisms that induce and maintain appropriate lineage-specific gene expression remain elusive. Here, we used an unbiased proteomics approach to characterize chromatin regulators that mediate the induction of neuronal cell fate. We found that Tip60 acetyltransferase is essential to establish neuronal cell identity partly via acetylation of the histone variant H2A.Z. Despite its tight correlation with gene expression and active chromatin, loss of H2A.Z acetylation had little effect on chromatin accessibility or transcription. Instead, loss of Tip60 and acetyl-H2A.Z interfered with H3K4me3 deposition and activation of a unique subset of silent, lineage-restricted genes characterized by a bivalent chromatin configuration at their promoters. Altogether, our results illuminate the mechanisms underlying bivalent chromatin activation and reveal that H2A.Z acetylation regulates neuronal fate specification by establishing epigenetic competence for bivalent gene activation and cell lineage transition.

The proneural transcription factor ASCL1 regulates cell proliferation and primes for differentiation in neuroblastoma

Neuroblastoma is believed to arise from sympathetic neuroblast precursors that fail to engage the neuronal differentiation programme, but instead become locked in a pro-proliferative developmental state. Achaete-scute homolog 1 (ASCL1) is a proneural master regulator of transcription which modulates both proliferation and differentiation of sympathetic neuroblast precursor cells during development, while its expression has been implicated in the maintenance of an oncogenic programme in MYCN-amplified neuroblastoma. However, the role of ASCL1 expression in neuroblastoma is not clear, especially as its levels vary considerably in different neuroblastoma cell lines. Here, we have investigated the role of ASCL1 in maintaining proliferation and controlling differentiation in both MYCN amplified and Anaplastic Lymphoma Kinase (ALK)-driven neuroblastoma cells. Using CRISPR deletion, we generated neuroblastoma cell lines lacking ASCL1 expression, and these grew more slowly than parental cells, indicating that ASCL1 contributes to rapid proliferation of MYCN amplified and non-amplified neuroblastoma cells. Genome-wide analysis after ASCL1 deletion revealed reduced expression of genes associated with neuronal differentiation, while chromatin accessibility at regulatory regions associated with differentiation genes was also attenuated by ASCL1 knock-out. In neuroblastoma, ASCL1 has been described as part of a core regulatory circuit of developmental regulators whose high expression is maintained by mutual cross-activation of a network of super enhancers and is further augmented by the activity of MYC/MYCN. Surprisingly, ASCL1 deletion had little effect on the transcription of CRC gene transcripts in these neuroblastoma cell lines, but the ability of MYC/MYCN and CRC component proteins, PHOX2B and GATA3, to bind to chromatin was compromised. Taken together, our results demonstrate several roles for endogenous ASCL1 in neuroblastoma cells: maintaining a highly proliferative phenotype, regulating DNA binding of the core regulatory circuit genes to chromatin, while also controlling accessibility and transcription of differentiation targets. Thus, we propose a model where ASCL1, a key developmental regulator of sympathetic neurogenesis, plays a pivotal role in maintaining proliferation while simultaneously priming cells for differentiation in neuroblastoma.

Elevated ASCL1 activity creates de novo regulatory elements associated with neuronal differentiation

Background

The pro-neural transcription factor ASCL1 is a master regulator of neurogenesis and a key factor necessary for the reprogramming of permissive cell types to neurons. Endogenously, ASCL1 expression is often associated with neuroblast stem-ness. Moreover, ASCL1-mediated reprogramming of fibroblasts to differentiated neurons is commonly achieved using artificially high levels of ASCL1 protein, where ASCL1 acts as an “on-target” pioneer factor. However, the genome-wide effects of enhancing ASCL1 activity in a permissive neurogenic environment has not been thoroughly investigated. Here, we overexpressed ASCL1 in the neuronally-permissive context of neuroblastoma (NB) cells where modest endogenous ASCL1 supports the neuroblast programme.

Results

Increasing ASCL1 in neuroblastoma cells both enhances binding at existing ASCL1 sites and also leads to creation of numerous additional, lower affinity binding sites. These extensive genome-wide changes in ASCL1 binding result in significant reprogramming of the NB transcriptome, redirecting it from a proliferative neuroblastic state towards one favouring neuronal differentiation. Mechanistically, ASCL1-mediated cell cycle exit and differentiation can be increased further by preventing its multi-site phosphorylation, which is associated with additional changes in genome-wide binding and gene activation profiles.

Conclusions

Our findings show that enhancing ASCL1 activity in a neurogenic environment both increases binding at endogenous ASCL1 sites and also results in additional binding to new low affinity sites that favours neuronal differentiation over the proliferating neuroblast programme supported by the endogenous protein. These findings have important implications for controlling processes of neurogenesis in cancer and cellular reprogramming.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08495-8.

Plasticity in Neuroblastoma Cell Identity Defines a Noradrenergic-to-Mesenchymal Transition (NMT)

Neuroblastoma, a pediatric cancer of the peripheral sympathetic nervous system, is characterized by an important clinical heterogeneity, and high-risk tumors are associated with a poor overall survival. Neuroblastoma cells may present with diverse morphological and biochemical properties in vitro, and seminal observations suggested that interconversion between two phenotypes called N-type and S-type may occur. In 2017, two main studies provided novel insights into these subtypes through the characterization of the transcriptomic and epigenetic landscapes of a panel of neuroblastoma cell lines. In this review, we focus on the available data that define neuroblastoma cell identity and propose to use the term noradrenergic (NOR) and mesenchymal (MES) to refer to these identities. We also address the question of transdifferentiation between both states and suggest that the plasticity between the NOR identity and the MES identity defines a noradrenergic-to-mesenchymal transition, reminiscent of but different from the well-established epithelial-to-mesenchymal transition.

The role of hormones and neurons in cardiomyocyte maturation

The heart undergoes profound morphological and functional changes as it continues to mature postnatally. However, this phase of cardiac development remains understudied. More recently, cardiac maturation research has attracted a lot of interest due to the need for more mature stem cell-derived cardiomyocytes for disease modeling, drug screening and heart regeneration. Additionally, neonatal heart injury models have been utilized to study heart regeneration, and factors regulating postnatal heart development have been associated with adult cardiac disease. Critical components of cardiac maturation are systemic and local biochemical cues. Specifically, cardiac innervation and the concentration of various metabolic hormones appear to increase perinatally and they have striking effects on cardiomyocytes. Here, we first report some of the key parameters of mature cardiomyocytes and then discuss the specific effects of neurons and hormonal cues on cardiomyocyte maturation. We focus primarily on the structural, electrophysiologic, metabolic, hypertrophic and hyperplastic effects of each factor. This review highlights the significance of underappreciated regulators of cardiac maturation and underscores the need for further research in this exciting field.

Neural pathways of olfactory kin imprinting and kin recognition in zebrafish

Teleost fish exhibit extraordinary cognitive skills that are comparable to those of mammals and birds. Kin recognition based on olfactory and visual imprinting requires neuronal circuits that were assumed to be necessarily dependent on the interaction of mammalian amygdala, hippocampus, and isocortex, the latter being a structure that teleost fish are lacking. We show that teleosts—beyond having a hippocampus and pallial amygdala homolog—also have subpallial amygdalar structures. In particular, we identify the medial amygdala and neural olfactory central circuits related to kin imprinting and kin recognition corresponding to an accessory olfactory system despite the absence of a separate vomeronasal organ.

Mash1-expressing cells may be relevant to type III cells and a subset of PLCβ2-positive cell differentiation in adult mouse taste buds

Mammalian taste bud cells have a limited lifespan and differentiate into type I, II, and III cells from basal cells (type IV cells) (postmitotic precursor cells). However, little is known regarding the cell lineage within taste buds. In this study, we investigated the cell fate of Mash1-positive precursor cells utilizing the Cre-loxP system to explore the differentiation of taste bud cells. We found that Mash1-expressing cells in Ascl1^CreERT2::CAG-floxed tdTomato mice differentiated into taste bud cells that expressed aromatic _L-amino acid decarboxylase (AADC) and carbonic anhydrase IV (CA4) (type III cell markers), but did not differentiate into most of gustducin (type II cell marker)-positive cells. Additionally, we found that Mash1-expressing cells could differentiate into phospholipase C β2 (PLCβ2)-positive cells, which have a shorter lifespan compared with AADC- and CA4-positive cells. These results suggest that Mash1-positive precursor cells could differentiate into type III cells, but not into most of type II cells, in the taste buds.

Retinoic Acid Accelerates the Specification of Enteric Neural Progenitors from In-Vitro-Derived Neural Crest

The enteric nervous system (ENS) is derived primarily from the vagal neural crest, a migratory multipotent cell population emerging from the dorsal neural tube between somites 1 and 7. Defects in the development and function of the ENS cause a range of enteric neuropathies, including Hirschsprung disease. Little is known about the signals that specify early ENS progenitors, limiting progress in the generation of enteric neurons from human pluripotent stem cells (hPSCs) to provide tools for disease modeling and regenerative medicine for enteric neuropathies. We describe the efficient and accelerated generation of ENS progenitors from hPSCs, revealing that retinoic acid is critical for the acquisition of vagal axial identity and early ENS progenitor specification. These ENS progenitors generate enteric neurons in vitro and, following in vivo transplantation, achieved long-term colonization of the ENS in adult mice. Thus, hPSC-derived ENS progenitors may provide the basis for cell therapy for defects in the ENS.

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Retinoic acid alters the axial identity of hPSC-derived neural crest cells

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ENS progenitor markers are upregulated in response to RA

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ENS progenitors are capable of generating enteric neurons in vitro

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hPSC ENS progenitors colonize the ENS of mice following long-term transplantation

Generation of inner ear hair cells by direct lineage conversion of primary somatic cells

The mechanoreceptive sensory hair cells in the inner ear are selectively vulnerable to numerous genetic and environmental insults. In mammals, hair cells lack regenerative capacity, and their death leads to permanent hearing loss and vestibular dysfunction. Their paucity and inaccessibility has limited the search for otoprotective and regenerative strategies. Growing hair cells in vitro would provide a route to overcome this experimental bottleneck. We report a combination of four transcription factors (Six1, Atoh1, Pou4f3, and Gfi1) that can convert mouse embryonic fibroblasts, adult tail-tip fibroblasts and postnatal supporting cells into induced hair cell-like cells (iHCs). iHCs exhibit hair cell-like morphology, transcriptomic and epigenetic profiles, electrophysiological properties, mechanosensory channel expression, and vulnerability to ototoxin in a high-content phenotypic screening system. Thus, direct reprogramming provides a platform to identify causes and treatments for hair cell loss, and may help identify future gene therapy approaches for restoring hearing.

Molecular Events Controlling Cessation of Trunk Neural Crest Migration and Onset of Differentiation

Neural crest cells (NCC) migrate extensively in vertebrate embryos to populate diverse derivatives including ganglia of the peripheral nervous system. Little is known about the molecular mechanisms that lead migrating trunk NCC to settle at selected sites in the embryo, ceasing their migration and initiating differentiation programs. To identify candidate genes involved in these processes, we profiled genes up-regulated in purified post-migratory compared with migratory NCC using a staged, macroarrayed cDNA library. A secondary screen of in situ hybridization revealed that many genes are specifically enhanced in neural crest-derived ganglia, including macrophage migration inhibitory factor (MIF), a ligand for CXCR4 receptor. Through in vivo and in vitro assays, we found that MIF functions as a potent chemoattractant for NCC. These results provide a molecular profile of genes expressed concomitant with gangliogenesis, thus, offering new markers and potential regulatory candidates involved in cessation of migration and onset of differentiation.

Developmental characterization of Zswim5 expression in the progenitor domains and tangential migration pathways of cortical interneurons in the mouse forebrain

GABAergic interneurons play an essential role in modulating cortical networks. The progenitor domains of cortical interneurons are localized in developing ventral forebrain, including the medial ganglionic eminence (MGE), caudal ganglionic eminence (CGE), preoptic area (POA), and preoptic hypothalamic border domain (POH). Here, we characterized the expression pattern of Zswim5, an MGE-enriched gene in the mouse forebrain. At E11.5-E13.5, prominent Zswim5 expression was detected in the subventricular zone (SVZ) of MGE, POA, and POH, but not CGE of ventral telencephalon where progenitors of cortical interneurons resided. At E15.5 and E17.5, Zswim5 expression remained in the MGE/pallidum primordium and ventral germinal zone. Zswim5 mRNA was markedly decreased after birth and was absent in the adult forebrain. Interestingly, the Zswim5 expression pattern resembled the tangential migration pathways of cortical interneurons. Zswim5-positive cells in the MGE appeared to migrate from the MGE through the SVZ of LGE to overlying neocortex. Indeed, Zswim5 was co-localized with Nkx2.1 and Lhx6, markers of progenitors and migratory cortical interneurons. Double labeling showed that Ascl1/Mash1-positive cells co-expressed Zswim5. Zswim5 expressing cells contained none or at most low levels of Ki67 but co-expressed Tuj1 in the SVZ of MGE. These results suggest that Zswim5 is immediately upregulated as progenitors exiting cell cycle become postmitotic. Given that recent studies have elucidated that the cell fate of cortical interneurons is determined shortly after becoming postmitotic, the timing of Zswim5 expression in early postmitotic interneurons suggests a potential role of Zswim5 in regulation of neurogenesis and tangential migration of cortical interneurons.

ASCL1 is a MYCN- and LMO1-dependent member of the adrenergic neuroblastoma core regulatory circuitry

A heritable polymorphism within regulatory sequences of the LMO1 gene is associated with its elevated expression and increased susceptibility to develop neuroblastoma, but the oncogenic pathways downstream of the LMO1 transcriptional co-regulatory protein are unknown. Our ChIP-seq and RNA-seq analyses reveal that a key gene directly regulated by LMO1 and MYCN is ASCL1, which encodes a basic helix-loop-helix transcription factor. Regulatory elements controlling ASCL1 expression are bound by LMO1, MYCN and the transcription factors GATA3, HAND2, PHOX2B, TBX2 and ISL1-all members of the adrenergic (ADRN) neuroblastoma core regulatory circuitry (CRC). ASCL1 is required for neuroblastoma cell growth and arrest of differentiation. ASCL1 and LMO1 directly regulate the expression of CRC genes, indicating that ASCL1 is a member and LMO1 is a coregulator of the ADRN neuroblastoma CRC.

Using transcription factors for direct reprogramming of neurons in vitro

Cell therapy offers great promises in replacing the neurons lost due to neurodegenerative diseases or injuries. However, a key challenge is the cellular source for transplantation which is often limited by donor availability. Direct reprogramming provides an exciting avenue to generate specialized neuron subtypes in vitro, which have the potential to be used for autologous transplantation, as well as generation of patient-specific disease models in the lab for drug discovery and testing gene therapy. Here we present a detailed review on transcription factors that promote direct reprogramming of specific neuronal subtypes with particular focus on glutamatergic, GABAergic, dopaminergic, sensory and retinal neurons. We will discuss the developmental role of master transcriptional regulators and specification factors for neuronal subtypes, and summarize their use in promoting direct reprogramming into different neuronal subtypes. Furthermore, we will discuss up-and-coming technologies that advance the cell reprogramming field, including the use of computational prediction of reprogramming factors, opportunity of cellular reprogramming using small chemicals and microRNA, as well as the exciting potential for applying direct reprogramming in vivo as a novel approach to promote neuro-regeneration within the body. Finally, we will highlight the clinical potential of direct reprogramming and discuss the hurdles that need to be overcome for clinical translation.

Small molecules enhance neurogenic differentiation of dental-derived adult stem cells

OBJECTIVE

Dental-derived stem cells originate from the embryonic neural crest, and exhibit high neurogenic potential. This study aimed to investigate whether a cocktail of eight small molecules (Valproic acid, CHIR99021, Repsox, Forskolin, SP600125, GO6983, Y-27632 and Dorsomorphin) can enhance the in vitro neurogenic differentiation of dental pulp stem cells (DPSCs), stem cells from apical papilla (SCAPs) and gingival mesenchymal stem cells (GMSCs), as a preliminary step towards clinical applications.

MATERIALS AND METHODS

Neural induction was carried out with a small molecule cocktail based two-step culture protocol, over a total duration of 14 days. At the 8 and 14 day timepoints, the cells were analyzed for expression of neural markers with immunocytochemistry, qRT-PCR and Western Blot. The Fluo 4-AM calcium flux assay was also performed after a further 14 days of neural maturation.

RESULTS

More pronounced morphological changes characteristic of the neural lineage (i.e. neuritogenesis) were observed in all three cell types treated with small molecules, as compared to the untreated controls. This was corroborated by the immunocytochemistry, qRT-PCR and western blot data, which showed upregulated expression of several early and mature neural markers in all three cell types treated with small molecules, versus the corresponding untreated controls. Finally, the Fluo-4 AM calcium flux assay showed consistently higher calcium transient (F/F_o) peaks for the small molecule-treated versus untreated control groups.

CONCLUSIONS

Small molecules can enhance the neurogenic differentiation of DPSCs, SCAPs and GMSCs, which offer much potential for therapeutic applications.

High Hes1 expression and resultant Ascl1 suppression regulate quiescent vs. active neural stem cells in the adult mouse brain

Sueda et al. show that in quiescent neural stem cells, Hes1 levels are oscillatory, although the peaks and troughs are higher than those in active neural stem cells, causing Ascl1 expression to be continuously suppressed.

Somatic stem/progenitor cells are active in embryonic tissues but quiescent in many adult tissues. The detailed mechanisms that regulate active versus quiescent stem cell states are largely unknown. In active neural stem cells, Hes1 expression oscillates and drives cyclic expression of the proneural gene Ascl1, which activates cell proliferation. Here, we found that in quiescent neural stem cells in the adult mouse brain, Hes1 levels are oscillatory, although the peaks and troughs are higher than those in active neural stem cells, causing Ascl1 expression to be continuously suppressed. Inactivation of Hes1 and its related genes up-regulates Ascl1 expression and increases neurogenesis. This causes rapid depletion of neural stem cells and premature termination of neurogenesis. Conversely, sustained Hes1 expression represses Ascl1, inhibits neurogenesis, and maintains quiescent neural stem cells. In contrast, induction of Ascl1 oscillations activates neural stem cells and increases neurogenesis in the adult mouse brain. Thus, Ascl1 oscillations, which normally depend on Hes1 oscillations, regulate the active state, while high Hes1 expression and resultant Ascl1 suppression promote quiescence in neural stem cells.

Schwann Cell Precursors Generate the Majority of Chromaffin Cells in Zuckerkandl Organ and Some Sympathetic Neurons in Paraganglia

In humans, neurosecretory chromaffin cells control a number of important bodily functions, including those related to stress response. Chromaffin cells appear as a distinct cell type at the beginning of midgestation and are the main cellular source of adrenalin and noradrenalin released into the blood stream. In mammals, two different chromaffin organs emerge at a close distance to each other, the adrenal gland and Zuckerkandl organ (ZO). These two structures are found in close proximity to the kidneys and dorsal aorta, in a region where paraganglioma, pheochromocytoma and neuroblastoma originate in the majority of clinical cases. Recent studies showed that the chromaffin cells comprising the adrenal medulla are largely derived from nerve-associated multipotent Schwann cell precursors (SCPs) arriving at the adrenal anlage with the preganglionic nerve fibers, whereas the migratory neural crest cells provide only minor contribution. However, the embryonic origin of the ZO, which differs from the adrenal medulla in a number of aspects, has not been studied in detail. The ZO is composed of chromaffin cells in direct contact with the dorsal aorta and the intraperitoneal cavity and disappears through an autophagy-mediated mechanism after birth. In contrast, the adrenal medulla remains throughout the entire life and furthermore, is covered by the adrenal cortex. Using a combination of lineage tracing strategies with nerve- and cell type-specific ablations, we reveal that the ZO is largely SCP-derived and forms in synchrony with progressively increasing innervation. Moreover, the ZO develops hand-in-hand with the adjacent sympathetic ganglia that coalesce around the dorsal aorta. Finally, we were able to provide evidence for a SCP-contribution to a small but significant proportion of sympathetic neurons of the posterior paraganglia. Thus, this cellular source complements the neural crest, which acts as a main source of sympathetic neurons. Our discovery of a nerve-dependent origin of chromaffin cells and some sympathoblasts may help to understand the origin of pheochromocytoma, paraganglioma and neuroblastoma, all of which are currently thought to be derived from the neural crest or committed sympathoadrenal precursors.

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.

Generation of Adrenal Chromaffin-like Cells from Human Pluripotent Stem Cells

Adrenomedullary chromaffin cells are catecholamine (CA)-producing cells originating from trunk neural crest (NC) via sympathoadrenal progenitors (SAPs). We generated NC and SAPs from human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) in vitro via BMP2/FGF2 exposure, ascertained by qPCR and immunoexpression of SOX10, ASCL1, TFAP2α, and PHOX2B, and by fluorescence-activated cell sorting selection for p75NTR and GD2, and confirmed their trunk-like HOX gene expression. We showed that continuing BMP4 and curtailing FGF2 in vitro, augmented with corticosteroid mimetic, induced these cells to upregulate the chromaffin cell-specific marker PNMT and other CA synthesis and storage markers, and we demonstrated noradrenaline and adrenaline by Faglu and high-performance liquid chromatography. We showed these human cells' SAP-like property of migration and differentiation into cells expressing chromaffin cell markers by implanting them into avian embryos in vivo and in chorio-allantoic membrane grafts. These cells have the potential for investigating differentiation of human chromaffin cells and for modeling diseases involving this cell type.

Atoh1 regulation in the cochlea: more than just transcription

More than 80% of all cases of deafness are related to the death or degeneration of cochlear hair cells and the associated spiral ganglion neurons, and a lack of regeneration of these cells leads to permanent hearing loss. Therefore, the regeneration of lost hair cells is an important goal for the treatment of deafness. Atoh1 is a basic helix-loop-helix (bHLH) transcription factor that is critical in both the development and regeneration of cochlear hair cells. Atoh1 is transcriptionally regulated by several signaling pathways, including Notch and Wnt signalings. At the post-translational level, it is regulated through the ubiquitin-proteasome pathway. In vitro and in vivo studies have revealed that manipulation of these signaling pathways not only controls development, but also leads to the regeneration of cochlear hair cells after damage. Recent progress toward understanding the signaling networks involved in hair cell development and regeneration has led to the development of new strategies to replace lost hair cells. This review focuses on our current understanding of the signaling pathways that regulate Atoh1 in the cochlea.

Evolution of the hypoxia-sensitive cells involved in amniote respiratory reflexes

The evolutionary origins of the hypoxia-sensitive cells that trigger amniote respiratory reflexes - carotid body glomus cells, and 'pulmonary neuroendocrine cells' (PNECs) - are obscure. Homology has been proposed between glomus cells, which are neural crest-derived, and the hypoxia-sensitive 'neuroepithelial cells' (NECs) of fish gills, whose embryonic origin is unknown. NECs have also been likened to PNECs, which differentiate in situ within lung airway epithelia. Using genetic lineage-tracing and neural crest-deficient mutants in zebrafish, and physical fate-mapping in frog and lamprey, we find that NECs are not neural crest-derived, but endoderm-derived, like PNECs, whose endodermal origin we confirm. We discover neural crest-derived catecholaminergic cells associated with zebrafish pharyngeal arch blood vessels, and propose a new model for amniote hypoxia-sensitive cell evolution: endoderm-derived NECs were retained as PNECs, while the carotid body evolved via the aggregation of neural crest-derived catecholaminergic (chromaffin) cells already associated with blood vessels in anamniote pharyngeal arches.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

Achaete-Scute Homolog 1 Expression Controls Cellular Differentiation of Neuroblastoma

Neuroblastoma, the major cause of infant cancer deaths, results from fast proliferation of undifferentiated neuroblasts. Treatment of high-risk neuroblastoma includes differentiation with retinoic acid (RA); however, the resistance of many of these tumors to RA-induced differentiation poses a considerable challenge. Human achaete-scute homolog 1 (hASH1) is a proneural basic helix-loop-helix transcription factor essential for neurogenesis and is often upregulated in neuroblastoma. Here, we identified a novel function for hASH1 in regulating the differentiation phenotype of neuroblastoma cells. Global analysis of 986 human neuroblastoma datasets revealed a negative correlation between hASH1 and neuron differentiation that was independent of the N-myc (MYCN) oncogene. Using RA to induce neuron differentiation in two neuroblastoma cell lines displaying high and low levels of hASH1 expression, we confirmed the link between hASH1 expression and the differentiation defective phenotype, which was reversed by silencing hASH1 or by hypoxic preconditioning. We further show that hASH1 suppresses neuronal differentiation by inhibiting transcription at the RA receptor element. Collectively, our data indicate hASH1 to be key for understanding neuroblastoma resistance to differentiation therapy and pave the way for hASH1-targeted therapies for augmenting the response of neuroblastoma to differentiation therapy.

Depletion of the Fragile X Mental Retardation Protein in Embryonic Stem Cells Alters the Kinetics of Neurogenesis

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and a leading cause of autism. FXS is due to the silencing of the Fragile X Mental Retardation Protein (FMRP), an RNA binding protein mainly involved in translational control, dendritic spine morphology and synaptic plasticity. Despite extensive studies, there is currently no cure for FXS. With the purpose to decipher the initial molecular events leading to this pathology, we developed a stem-cell-based disease model by knocking-down the expression of Fmr1 in mouse embryonic stem cells (ESCs). Repressing FMRP in ESCs increased the expression of amyloid precursor protein (APP) and Ascl1. When inducing neuronal differentiation, βIII-tubulin, p27^kip1 , NeuN, and NeuroD1 were upregulated, leading to an accelerated neuronal differentiation that was partially compensated at later stages. Interestingly, we observed that neurogenesis is also accelerated in the embryonic brain of Fmr1-knockout mice, indicating that our cellular model recapitulates the molecular alterations present in vivo. Importantly, we rescued the main phenotype of the Fmr1 knockdown cell line, not only by reintroducing FMRP but also by pharmacologically targeting APP processing, showing the role of this protein in the pathophysiology of FXS during the earliest steps of neurogenesis. Our work allows to define an early therapeutic window but also to identify more effective molecules for treating this disorder. Stem Cells 2017;35:374-385.

Part and Parcel of the Cardiac Autonomic Nerve System: Unravelling Its Cellular Building Blocks during Development

The autonomic nervous system (cANS) is essential for proper heart function, and complications such as heart failure, arrhythmias and even sudden cardiac death are associated with an altered cANS function. A changed innervation state may underlie (part of) the atrial and ventricular arrhythmias observed after myocardial infarction. In other cardiac diseases, such as congenital heart disease, autonomic dysfunction may be related to disease outcome. This is also the case after heart transplantation, when the heart is denervated. Interest in the origin of the autonomic nerve system has renewed since the role of autonomic function in disease progression was recognized, and some plasticity in autonomic regeneration is evident. As with many pathological processes, autonomic dysfunction based on pathological innervation may be a partial recapitulation of the early development of innervation. As such, insight into the development of cardiac innervation and an understanding of the cellular background contributing to cardiac innervation during different phases of development is required. This review describes the development of the cANS and focuses on the cellular contributions, either directly by delivering cells or indirectly by secretion of necessary factors or cell-derivatives.

Ascl1 Is Required for the Development of Specific Neuronal Subtypes in the Enteric Nervous System

The enteric nervous system (ENS) is organized into neural circuits within the gastrointestinal wall where it controls the peristaltic movements, secretion, and blood flow. Although proper gut function relies on the complex neuronal composition of the ENS, little is known about the transcriptional networks that regulate the diversification into different classes of enteric neurons and glia during development. Here we redefine the role of Ascl1 (Mash1), one of the few regulatory transcription factors described during ENS development. We show that enteric glia and all enteric neuronal subtypes appear to be derived from Ascl1-expressing progenitor cells. In the gut of Ascl1(-/-) mutant mice, neurogenesis is delayed and reduced, and posterior gliogenesis impaired. The ratio of neurons expressing Calbindin, TH, and VIP is selectively decreased while, for instance, 5-HT(+) neurons, which previously were believed to be Ascl1-dependent, are formed in normal numbers. Essentially the same differentiation defects are observed in Ascl1(KINgn2) transgenic mutants, where the proneural activity of Ngn2 replaces Ascl1, demonstrating that Ascl1 is required for the acquisition of specific enteric neuronal subtype features independent of its role in neurogenesis. In this study, we provide novel insights into the expression and function of Ascl1 in the differentiation process of specific neuronal subtypes during ENS development.

SIGNIFICANCE STATEMENT

The molecular mechanisms underlying the generation of different neuronal subtypes during development of the enteric nervous system are poorly understood despite its pivotal function in gut motility and involvement in gastrointestinal pathology. This report identifies novel roles for the transcription factor Ascl1 in enteric gliogenesis and neurogenesis. Moreover, independent of its proneurogenic activity, Ascl1 is required for the normal expression of specific enteric neuronal subtype characteristics. Distinct enteric neuronal subtypes are formed in a temporally defined order, and we observe that the early-born 5-HT(+) neurons are generated in Ascl1(-/-) mutants, despite the delayed neurogenesis. Enteric nervous system progenitor cells may therefore possess strong intrinsic control over their specification at the initial waves of neurogenesis.

Combining Suppression of Stemness with Lineage-Specific Induction Leads to Conversion of Pluripotent Cells into Functional Neurons

Sox2 is a key player in the maintenance of pluripotency and stemness, and thus inhibition of its function would abrogate the stemness of pluripotent cells and induce differentiation into several types of cells. Herein we describe a strategy that relies on a combination of Sox2 inhibition with lineage-specific induction to promote efficient and selective differentiation of pluripotent P19 cells into neurons. When P19 cells transduced with Skp protein, an inhibitor of Sox2, are incubated with a neurogenesis inducer, the cells are selectively converted into neurons that generate depolarization-induced sodium currents and action potentials. This finding indicates that the differentiated neurons are electrophysiologically active. Signaling pathway studies lead us to conclude that a combination of Skp with the neurogenesis inducer enhances neurogenesis in P19 cells by activating Wnt and Notch pathways. The present differentiation protocol could be valuable to selectively generate functionally active neurons from pluripotent cells.

The LIM-homeobox transcription factor Isl1 plays crucial roles in the development of multiple arcuate nucleus neurons

Neurons in the hypothalamic arcuate nucleus relay and translate important cues from the periphery into the central nervous system. However, the gene regulatory program directing their development remains poorly understood. Here, we report that the LIM-homeodomain transcription factor Isl1 is expressed in several subpopulations of developing arcuate neurons and plays crucial roles in their fate specification. Mice with conditional deletion of the Isl1 gene in developing hypothalamus display severe deficits in both feeding and linear growth. Consistent with these results, their arcuate nucleus fails to express key fate markers of Isl1-expressing neurons that regulate feeding and growth. These include the orexigenic neuropeptides AgRP and NPY for specifying AgRP-neurons, the anorexigenic neuropeptide αMSH for POMC-neurons, and two growth-stimulatory peptides, growth hormone-releasing hormone (GHRH) for GHRH-neurons and somatostatin (Sst) for Sst-neurons. Finally, we show that Isl1 directly enhances the expression of AgRP by cooperating with the key orexigenic transcription factors glucocorticoid receptor and brain-specific homeobox factor. Our results identify Isl1 as a crucial transcription factor that plays essential roles in the gene regulatory program directing development of multiple arcuate neuronal subpopulations.

Direct lineage reprogramming via pioneer factors; a detour through developmental gene regulatory networks

Although many approaches have been employed to generate defined fate in vitro, the resultant cells often appear developmentally immature or incompletely specified, limiting their utility. Growing evidence suggests that current methods of direct lineage conversion may rely on the transition through a developmental intermediate. Here, I hypothesize that complete conversion between cell fates is more probable and feasible via reversion to a developmentally immature state. I posit that this is due to the role of pioneer transcription factors in engaging silent, unmarked chromatin and activating hierarchical gene regulatory networks responsible for embryonic patterning. Understanding these developmental contexts will be essential for the precise engineering of cell identity.

Exhaustive identification of human class II basic helix-loop-helix proteins by virtual library screening

Cellular proliferation, specification and differentiation in developing tissues are tightly coordinated by groups of transcription factors in response to extrinsic and intrinsic signals. Furthermore, renewable pools of stem cells in adult tissues are subject to similar regulation. Basic helix-loop-helix (bHLH) proteins are a group of transcription factors that exert such a determinative influence on a variety of developmental pathways from C. elegans to humans, and we wished to exclusively identify novel members from within the whole human bHLH family. We have, therefore, developed an 'empirical custom fingerprint', to define the class II bHLH domain and exclusively identify these proteins in silico. We have identified nine previously uncharacterised human class II proteins, four of which were novel, by interrogating conceptual translations of the GenBank HTGS database. RT-PCR and mammalian 2-hybrid analysis of a subset of the factors demonstrated that they were indeed expressed, and were able to interact with an appropriate binding partner in vitro. Thus, we are now approaching an almost complete listing of human class II bHLH factors.

The molecular and cellular signatures of the mouse eminentia thalami support its role as a signalling centre in the developing forebrain

The mammalian eminentia thalami (EmT) (or thalamic eminence) is an embryonic forebrain structure of unknown function. Here, we examined the molecular and cellular properties of the mouse EmT. We first studied mRNA expression of signalling molecules and found that the EmT is a structure, rich in expression of secreted factors, with Wnts being the most abundantly detected. We then examined whether EmT tissue could induce cell fate changes when grafted ectopically. For this, we transplanted EmT tissue from a tau-GFP mouse to the ventral telencephalon of a wild type host, a telencephalic region where Wnt signalling is not normally active but which we showed in culture experiments is competent to respond to Wnts. We observed that the EmT was able to induce in adjacent ventral telencephalic cells ectopic expression of Lef1, a transcriptional activator and a target gene of the Wnt/β-catenin pathway. These Lef1-positive;GFP-negative cells expressed the telencephalic marker Foxg1 but not Ascl1, which is normally expressed by ventral telencephalic cells. These results suggest that the EmT has the capacity to activate Wnt/β-catenin signalling in the ventral telencephalon and to suppress ventral telencephalic gene expression. Altogether, our data support a role of the EmT as a signalling centre in the developing mouse forebrain.

Intraoperative hypertensive crisis due to a catecholamine-secreting esthesioneuroblastoma

BACKGROUND

Although uncommon, esthesioneuroblastomas may produce clinically significant amounts of catecholamines.

METHODS

We report a patient with a catecholamine-secreting esthesioneuroblastoma who developed an intraoperative hypertensive crisis.

RESULTS

A patient with a history of hypertension was referred to our skull base center for management of a residual esthesioneuroblastoma. A staged endonasal endoscopic approach was planned. At the conclusion of the first stage, a hypertensive crisis occurred. Workup revealed elevated levels of serum and urinary catecholamines. The patient was treated with alpha adrenoceptor blockade before the second stage. Serum catecholamine levels after this second stage were normal. On immunohistochemical analysis, the tumor cells were found to be positive for tyrosine hydroxylase, the rate limiting enzyme in catecholamine synthesis, and achaete-scute homologue 1, a transcription factor essential in the development of olfactory and sympathetic neurons.

CONCLUSION

Catecholamine production should be considered in the differential of unexpected extreme hypertension during surgical resection of esthesioneuroblastoma.

Shutdown of achaete-scute homolog-1 expression by heterogeneous nuclear ribonucleoprotein (hnRNP)-A2/B1 in hypoxia

The basic helix-loop-helix transcription factor hASH1, encoded by the ASCL1 gene, plays an important role in neurogenesis and tumor development. Recent findings indicate that local oxygen tension is a critical determinant for the progression of neuroblastomas. Here we investigated the molecular mechanisms underlying the oxygen-dependent expression of hASH1 in neuroblastoma cells. Exposure of human neuroblastoma-derived Kelly cells to 1% O2 significantly decreased ASCL1 mRNA and hASH1 protein levels. Using reporter gene assays, we show that the response of hASH1 to hypoxia is mediated mainly by post-transcriptional inhibition via the ASCL1 mRNA 5'- and 3'-UTRs, whereas additional inhibition of the ASCL1 promoter was observed under prolonged hypoxia. By RNA pulldown experiments followed by MALDI/TOF-MS analysis, we identified heterogeneous nuclear ribonucleoprotein (hnRNP)-A2/B1 and hnRNP-R as interactors binding directly to the ASCL1 mRNA 5'- and 3'-UTRs and influencing its expression. We further demonstrate that hnRNP-A2/B1 is a key positive regulator of ASCL1, findings that were also confirmed by analysis of a large compilation of gene expression data. Our data suggest that a prominent down-regulation of hnRNP-A2/B1 during hypoxia is associated with the post-transcriptional suppression of hASH1 synthesis. This novel post-transcriptional mechanism for regulating hASH1 levels will have important implications in neural cell fate development and disease.

Generation of induced neuronal cells by the single reprogramming factor ASCL1

Direct conversion of nonneural cells to functional neurons holds great promise for neurological disease modeling and regenerative medicine. We previously reported rapid reprogramming of mouse embryonic fibroblasts (MEFs) into mature induced neuronal (iN) cells by forced expression of three transcription factors: ASCL1, MYT1L, and BRN2. Here, we show that ASCL1 alone is sufficient to generate functional iN cells from mouse and human fibroblasts and embryonic stem cells, indicating that ASCL1 is the key driver of iN cell reprogramming in different cell contexts and that the role of MYT1L and BRN2 is primarily to enhance the neuronal maturation process. ASCL1-induced single-factor neurons (1F-iN) expressed mature neuronal markers, exhibited typical passive and active intrinsic membrane properties, and formed functional pre- and postsynaptic structures. Surprisingly, ASCL1-induced iN cells were predominantly excitatory, demonstrating that ASCL1 is permissive but alone not deterministic for the inhibitory neuronal lineage.

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ASCL1 alone generates functional neurons from fibroblast and embryonic stem cells

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ASCL1-induced 1F-iN cells display slow maturation kinetics

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ASCL1 overexpression induces endogenous expression of Myt1l and Brn2

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ASCL1-induced 1F-iN cells are predominantly excitatory

Expression of GAD67 and Dlx5 in the taste buds of mice genetically lacking Mash1

It has been reported that a subset of type III taste cells express glutamate decarboxylase (GAD)67, which is a molecule that synthesizes gamma-aminobutyric acid (GABA), and that Mash1 could be a potential regulator of the development of GABAnergic neurons via Dlx transcription factors in the central nervous system. In this study, we investigated the expression of GAD67 and Dlx in the embryonic taste buds of the soft palate and circumvallate papilla using Mash1 knockout (KO)/GAD67-GFP knock-in mice. In the wild-type animal, a subset of type III taste cells contained GAD67 in the taste buds of the soft palate and the developing circumvallate papilla, whereas GAD67-expressing taste bud cells were missing from Mash1 KO mice. A subset of type III cells expressed mRNA for Dlx5 in the wild-type animals, whereas Dlx5-expressing cells were not evident in the apical part of the circumvallate papilla and taste buds in the soft palate of Mash1 KO mice. Our results suggest that Mash1 is required for the expression of GAD67 and Dlx5 in taste bud cells.

Proneural bHLH genes in development and disease

Proneural genes encode evolutionarily conserved basic-helix-loop-helix transcription factors. In Drosophila, proneural genes are required and sufficient to confer a neural identity onto naïve ectodermal cells, inducing delamination and subsequent neuronal differentiation. In vertebrates, proneural genes are expressed in cells that already have a neural identity, but they are still required and sufficient to initiate neurogenesis. In all organisms, proneural genes control neurogenesis by regulating Notch-mediated lateral inhibition and initiating the expression of downstream differentiation genes. The general mode of proneural gene function has thus been elucidated. However, the regulatory mechanisms that spatially and temporally control proneural gene function are only beginning to be deciphered. Understanding how proneural gene function is regulated is essential, as aberrant proneural gene expression has recently been linked to a variety of human diseases-ranging from cancer to neuropsychiatric illnesses and diabetes. Recent insights into proneural gene function in development and disease are highlighted herein.

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.

The ascl1a and dlx genes have a regulatory role in the development of GABAergic interneurons in the zebrafish diencephalon

During development of the mouse forebrain interneurons, the Dlx genes play a key role in a gene regulatory network (GRN) that leads to the GABAergic phenotype. Here, we have examined the regulatory relationships between the ascl1a, dlx, and gad1b genes in the zebrafish forebrain. Expression of ascl1a overlaps with dlx1a in the telencephalon and diencephalon during early forebrain development. The loss of Ascl1a function results in a loss of dlx expression, and subsequent losses of dlx5a and gad1b expression in the diencephalic prethalamus and hypothalamus. Loss of Dlx1a and Dlx2a function, and, to a lesser extent, of Dlx5a and Dlx6a, impairs gad1b expression in the prethalamus and hypothalamus. We conclude that dlx1a/2a act downstream of ascl1a but upstream of dlx5a/dlx6a and gad1b to activate GABAergic specification. This pathway is conserved in the diencephalon, but has diverged between mammals and teleosts in the telencephalon.

Loss of ascl1a prevents secretory cell differentiation within the zebrafish intestinal epithelium resulting in a loss of distal intestinal motility

The vertebrate intestinal epithelium is renewed continuously from stem cells at the base of the crypt in mammals or base of the fold in fish over the life of the organism. As stem cells divide, newly formed epithelial cells make an initial choice between a secretory or enterocyte fate. This choice has previously been demonstrated to involve Notch signaling as well as Atonal and Her transcription factors in both embryogenesis and adults. Here, we demonstrate that in contrast to the atoh1 in mammals, ascl1a is responsible for formation of secretory cells in zebrafish. ascl1a-/- embryos lack all intestinal epithelial secretory cells and instead differentiate into enterocytes. ascl1a-/- embryos also fail to induce intestinal epithelial expression of deltaD suggesting that ascl1a plays a role in initiation of Notch signaling. Inhibition of Notch signaling increases the number of ascl1a and deltaD expressing intestinal epithelial cells as well as the number of developing secretory cells during two specific time periods: between 30 and 34hpf and again between 64 and 74hpf. Loss of enteroendocrine products results in loss of anterograde motility in ascl1a-/- embryos. 5HT produced by enterochromaffin cells is critical in motility and secretion within the intestine. We find that addition of exogenous 5HT to ascl1a-/- embryos at near physiological levels (measured by differential pulse voltammetry) induce anterograde motility at similar levels to wild type velocity, distance, and frequency. Removal or doubling the concentration of 5HT in WT embryos does not significantly affect anterograde motility, suggesting that the loss of additional enteroendocrine products in ascl1a-/- embryos also contributes to intestinal motility. Thus, zebrafish intestinal epithelial cells appear to have a common secretory progenitor from which all subtypes form. Loss of enteroendocrine cells reveals the critical need for enteroendocrine products in maintenance of normal intestinal motility.

Isolation, characterization, and differentiation of progenitor cells from human adult adrenal medulla

Chromaffin cells, sympathetic neurons of the dorsal ganglia, and the intermediate small intensely fluorescent cells derive from a common neural crest progenitor cell. Contrary to the closely related sympathetic nervous system, within the adult adrenal medulla a subpopulation of undifferentiated progenitor cells persists, and recently, we established a method to isolate and differentiate these progenitor cells from adult bovine adrenals. However, no studies have elucidated the existence of adrenal progenitor cells within the human adrenal medulla. Here we describe the isolation, characterization, and differentiation of chromaffin progenitor cells obtained from adult human adrenals. Human chromaffin progenitor cells were cultured in low-attachment conditions for 10-12 days as free-floating spheres in the presence of fibroblast growth factor-2 (FGF-2) and epidermal growth factor. These primary human chromosphere cultures were characterized by the expression of several progenitor markers, including nestin, CD133, Notch1, nerve growth factor receptor, Snai2, Sox9, Sox10, Phox2b, and Ascl1 on the molecular level and of Sox9 on the immunohistochemical level. In opposition, phenylethanolamine N-methyltransferase (PNMT), a marker for differentiated chromaffin cells, significantly decreased after 12 days in culture. Moreover, when plated on poly-l-lysine/laminin-coated slides in the presence of FGF-2, human chromaffin progenitor cells were able to differentiate into two distinct neuron-like cell types, tyrosine hydroxylase (TH)(+)/β-3-tubulin(+) cells and TH(-)/β-3-tubulin(+) cells, and into chromaffin cells (TH(+)/PNMT(+)). This study demonstrates the presence of progenitor cells in the human adrenal medulla and reveals their potential use in regenerative medicine, especially in the treatment of neuroendocrine and neurodegenerative diseases.

The multiple roles of the cyclin-dependent kinase inhibitory protein p57(KIP2) in cerebral cortical neurogenesis

The members of the CIP/KIP family of cyclin-dependent kinase (CDK) inhibitory proteins (CKIs), including p57(KIP2), p27(KIP1), and p21(CIP1), block the progression of the cell cycle by binding and inhibiting cyclin/CDK complexes of the G1 phase. In addition to this well-characterized function, p57(KIP2) and p27(KIP1) have been shown to participate in an increasing number of other important cellular processes including cell fate and differentiation, cell motility and migration, and cell death/survival, both in peripheral and central nervous systems. Increasing evidence over the past few years has characterized the functions of the newest CIP/KIP member p57(KIP2) in orchestrating cell proliferation, differentiation, and migration during neurogenesis. Here, we focus our discussion on the multiple roles played by p57(KIP2) during cortical development, making comparisons to p27(KIP1) as well as the INK4 family of CKIs.

Autonomic neurocristopathy-associated mutations in PHOX2B dysregulate Sox10 expression

The most common forms of neurocristopathy in the autonomic nervous system are Hirschsprung disease (HSCR), resulting in congenital loss of enteric ganglia, and neuroblastoma (NB), childhood tumors originating from the sympathetic ganglia and adrenal medulla. The risk for these diseases dramatically increases in patients with congenital central hypoventilation syndrome (CCHS) harboring a nonpolyalanine repeat expansion mutation of the Paired-like homeobox 2b (PHOX2B) gene, but the molecular mechanism of pathogenesis remains unknown. We found that introducing nonpolyalanine repeat expansion mutation of the PHOX2B into the mouse Phox2b locus recapitulates the clinical features of the CCHS associated with HSCR and NB. In mutant embryos, enteric and sympathetic ganglion progenitors showed sustained sex-determining region Y (SRY) box10 (Sox10) expression, with impaired proliferation and biased differentiation toward the glial lineage. Nonpolyalanine repeat expansion mutation of PHOX2B reduced transactivation of wild-type PHOX2B on its known target, dopamine β-hydroxylase (DBH), in a dominant-negative fashion. Moreover, the introduced mutation converted the transcriptional effect of PHOX2B on a Sox10 enhancer from repression to transactivation. Collectively, these data reveal that nonpolyalanine repeat expansion mutation of PHOX2B is both a dominant-negative and gain-of-function mutation. Our results also demonstrate that Sox10 regulation by PHOX2B is pivotal for the development and pathogenesis of the autonomic ganglia.