The bHLH protein NEUROGENIN 2 is a determination factor for epibranchial placode-derived sensory neurons

Reprogramming of astrocytes and glioma cells into neurons for central nervous system repair and glioblastoma therapy

Central nervous system (CNS) damage is usually irreversible owing to the limited regenerative capability of neurons. Following CNS injury, astrocytes are reactively activated and are the key cells involved in post-injury repair mechanisms. Consequently, research on the reprogramming of reactive astrocytes into neurons could provide new directions for the restoration of neural function after CNS injury and in the promotion of recovery in various neurodegenerative diseases. This review aims to provide an overview of the means through which reactive astrocytes around lesions can be reprogrammed into neurons, to elucidate the intrinsic connection between the two cell types from a neurogenesis perspective, and to summarize what is known about the neurotranscription factors, small-molecule compounds and MicroRNA that play major roles in astrocyte reprogramming. As the malignant proliferation of astrocytes promotes the development of glioblastoma multiforme (GBM), this review also examines the research advances on and the theoretical basis for the reprogramming of GBM cells into neurons and discusses the advantages of such approaches over traditional treatment modalities. This comprehensive review provides new insights into the field of GBM therapy and theoretical insights into the mechanisms of neurological recovery following neurological injury and in GBM treatment.

Mapping oto-pharyngeal development in a human inner ear organoid model

Inner ear development requires the coordination of cell types from distinct epithelial, mesenchymal and neuronal lineages. Although we have learned much from animal models, many details about human inner ear development remain elusive. We recently developed an in vitro model of human inner ear organogenesis using pluripotent stem cells in a 3D culture, fostering the growth of a sensorineural circuit, including hair cells and neurons. Despite previously characterizing some cell types, many remain undefined. This study aimed to chart the in vitro development timeline of the inner ear organoid to understand the mechanisms at play. Using single-cell RNA sequencing at ten stages during the first 36 days of differentiation, we tracked the evolution from pluripotency to various ear cell types after exposure to specific signaling modulators. Our findings showcase gene expression that influences differentiation, identifying a plethora of ectodermal and mesenchymal cell types. We also discern aspects of the organoid model consistent with in vivo development, while highlighting potential discrepancies. Our study establishes the Inner Ear Organoid Developmental Atlas (IODA), offering deeper insights into human biology and improving inner ear tissue differentiation.

Early Steps towards Hearing: Placodes and Sensory Development

Sensorineural hearing loss is the most prevalent sensory deficit in humans. Most cases of hearing loss are due to the degeneration of key structures of the sensory pathway in the cochlea, such as the sensory hair cells, the primary auditory neurons, and their synaptic connection to the hair cells. Different cell-based strategies to replace damaged inner ear neurosensory tissue aiming at the restoration of regeneration or functional recovery are currently the subject of intensive research. Most of these cell-based treatment approaches require experimental in vitro models that rely on a fine understanding of the earliest morphogenetic steps that underlie the in vivo development of the inner ear since its initial induction from a common otic-epibranchial territory. This knowledge will be applied to various proposed experimental cell replacement strategies to either address the feasibility or identify novel therapeutic options for sensorineural hearing loss. In this review, we describe how ear and epibranchial placode development can be recapitulated by focusing on the cellular transformations that occur as the inner ear is converted from a thickening of the surface ectoderm next to the hindbrain known as the otic placode to an otocyst embedded in the head mesenchyme. Finally, we will highlight otic and epibranchial placode development and morphogenetic events towards progenitors of the inner ear and their neurosensory cell derivatives.

Natural variation in gene expression and viral susceptibility revealed by neural progenitor cell villages

Human genome variation contributes to diversity in neurodevelopmental outcomes and vulnerabilities; recognizing the underlying molecular and cellular mechanisms will require scalable approaches. Here, we describe a "cell village" experimental platform we used to analyze genetic, molecular, and phenotypic heterogeneity across neural progenitor cells from 44 human donors cultured in a shared in vitro environment using algorithms (Dropulation and Census-seq) to assign cells and phenotypes to individual donors. Through rapid induction of human stem cell-derived neural progenitor cells, measurements of natural genetic variation, and CRISPR-Cas9 genetic perturbations, we identified a common variant that regulates antiviral IFITM3 expression and explains most inter-individual variation in susceptibility to the Zika virus. We also detected expression QTLs corresponding to GWAS loci for brain traits and discovered novel disease-relevant regulators of progenitor proliferation and differentiation such as CACHD1. This approach provides scalable ways to elucidate the effects of genes and genetic variation on cellular phenotypes.

Neurogenin 2 and Neuronal Differentiation 1 Control Proper Development of the Chick Trigeminal Ganglion and Its Nerve Branches

The trigeminal ganglion contains the cell bodies of sensory neurons comprising cranial nerve V, which relays information related to pain, touch, and temperature from the face and head to the brain. Like other cranial ganglia, the trigeminal ganglion is composed of neuronal derivatives of two critical embryonic cell types, neural crest and placode cells. Neurogenesis within the cranial ganglia is promoted by Neurogenin 2 (Neurog2), which is expressed in trigeminal placode cells and their neuronal derivatives, and transcriptionally activates neuronal differentiation genes such as Neuronal Differentiation 1 (NeuroD1). Little is known, however, about the role of Neurog2 and NeuroD1 during chick trigeminal gangliogenesis. To address this, we depleted Neurog2 and NeuroD1 from trigeminal placode cells with morpholinos and demonstrated that Neurog2 and NeuroD1 influence trigeminal ganglion development. While knockdown of both Neurog2 and NeuroD1 affected innervation of the eye, Neurog2 and NeuroD1 had opposite effects on ophthalmic nerve branch organization. Taken together, our results highlight, for the first time, functional roles for Neurog2 and NeuroD1 during chick trigeminal gangliogenesis. These studies shed new light on the molecular mechanisms underlying trigeminal ganglion formation and may also provide insight into general cranial gangliogenesis and diseases of the peripheral nervous system.

A novel case of two siblings harbouring homozygous variant in the NEUROG1 gene with autism as an additional phenotype: a case report

INTRODUCTION

NEUROG1 gene is yet to be associated with a set of human phenotypes in the OMIM database. Three cases have previously been diagnosed with cranial dysinnervation due to biallelic variants in the NEUROG1 gene. This is the fourth and a novel report of a sibling pair harboring a homozygous variant in the NEUROG1 gene with autism as an additional phenotype. A brief review of the literature in conjunction with a genotype-phenotype correlation has been described. A potential hypothesis for the presence of the autistic phenotype in the present case has also been elucidated.

CASE PRESENTATION

A female aged 6 years and 9 months born to endogamous and phenotypically healthy parents was diagnosed with global developmental delay, autism spectrum disorder, hearing loss, corneal opacity and no eye blinking. Her MRI of the brain revealed mild peritrigonal white matter hyperintensity, and MRI and CT scan of the temporal bones showed abnormal cranial nerves. The proband's younger sister, aged 4-years, was similarly affected. Whole exome sequencing was performed in the proband, which revealed a novel homozygous, likely pathogenic, truncating frameshift variant, c.228_231dup (p.Thr78ProfsTer122) in exon 1 of the NEUROG1 gene (ENST00000314744.4). Segregation analysis by Sanger sequencing showed the proband and her younger sister to be homozygotes and their parents to be heterozygous carriers.

CONCLUSION

This is the fourth report across the globe with a variant identified in the NEUROG1 gene to be associated with cranial dysinnervation phenotype. An additional phenotype of autism in two female siblings was a novel observation. We provide a hypothetical framework which could explain the pleiotropic effect of a dysfunctional NEUROG1 protein leading to autism and posit it as a candidate for diagnosis of autism spectrum disorder with congenital cranial dysinnervation disorder.

Transcription factors regulating the specification of brainstem respiratory neurons

Breathing (or respiration) is an unconscious and complex motor behavior which neuronal drive emerges from the brainstem. In simplistic terms, respiratory motor activity comprises two phases, inspiration (uptake of oxygen, O2) and expiration (release of carbon dioxide, CO2). Breathing is not rigid, but instead highly adaptable to external and internal physiological demands of the organism. The neurons that generate, monitor, and adjust breathing patterns locate to two major brainstem structures, the pons and medulla oblongata. Extensive research over the last three decades has begun to identify the developmental origins of most brainstem neurons that control different aspects of breathing. This research has also elucidated the transcriptional control that secures the specification of brainstem respiratory neurons. In this review, we aim to summarize our current knowledge on the transcriptional regulation that operates during the specification of respiratory neurons, and we will highlight the cell lineages that contribute to the central respiratory circuit. Lastly, we will discuss on genetic disturbances altering transcription factor regulation and their impact in hypoventilation disorders in humans.

A developmental pathway for epithelial-to-motoneuron transformation in C. elegans

Motoneurons and motoneuron-like pancreatic β cells arise from radial glia and ductal cells, respectively, both tube-lining progenitors that share molecular regulators. To uncover programs underlying motoneuron formation, we studied a similar, cell-division-independent transformation of the C. elegans tube-lining Y cell into the PDA motoneuron. We find that lin-12/Notch acts through ngn-1/Ngn and its regulator hlh-16/Olig to control transformation timing. lin-12 loss blocks transformation, while lin-12(gf) promotes precocious PDA formation. Early basal expression of ngn-1/Ngn and hlh-16/Olig depends on sem-4/Sall and egl-5/Hox. Later, coincident with Y cell morphological changes, ngn-1/Ngn expression is upregulated in a sem-4/Sall and egl-5/Hox-dependent but hlh-16/Olig-independent manner. Subsequently, Y cell retrograde extension forms an anchored process priming PDA axon extension. Extension requires ngn-1-dependent expression of the cytoskeleton organizers UNC-119, UNC-44/ANK, and UNC-33/CRMP, which also activate PDA terminal-gene expression. Our findings uncover cell-division-independent regulatory events leading to motoneuron generation, suggesting a conserved pathway for epithelial-to-motoneuron/motoneuron-like cell differentiation.

Rashid et al. report on a conserved epithelial-to-motoneuron transformation pathway in C. elegans requiring ngn-1/Ngn and hlh-16/Olig. lin-12/Notch regulates transformation timing through these genes, while ngn-1/Ngn and hlh-16/Olig expression levels are regulated by sem-4/Sall and egl-5/Hox. Unexpectedly, the cytoskeleton organizers UNC-119, UNC-44, and UNC-33, which are ngn-1/Ngn targets, promote motoneuron terminal identity.

Neurosensory development of the four brainstem-projecting sensory systems and their integration in the telencephalon

Somatosensory, taste, vestibular, and auditory information is first processed in the brainstem. From the brainstem, the respective information is relayed to specific regions within the cortex, where these inputs are further processed and integrated with other sensory systems to provide a comprehensive sensory experience. We provide the organization, genetics, and various neuronal connections of four sensory systems: trigeminal, taste, vestibular, and auditory systems. The development of trigeminal fibers is comparable to many sensory systems, for they project mostly contralaterally from the brainstem or spinal cord to the telencephalon. Taste bud information is primarily projected ipsilaterally through the thalamus to reach the insula. The vestibular fibers develop bilateral connections that eventually reach multiple areas of the cortex to provide a complex map. The auditory fibers project in a tonotopic contour to the auditory cortex. The spatial and tonotopic organization of trigeminal and auditory neuron projections are distinct from the taste and vestibular systems. The individual sensory projections within the cortex provide multi-sensory integration in the telencephalon that depends on context-dependent tertiary connections to integrate other cortical sensory systems across the four modalities.

Single-cell transcriptomic landscapes of the otic neuronal lineage at multiple early embryonic ages

Inner ear vestibular and spiral ganglion neurons (VGNs and SGNs) are known to play pivotal roles in balance control and sound detection. However, the molecular mechanisms underlying otic neurogenesis at early embryonic ages have remained unclear. Here, we use single-cell RNA sequencing to reveal the transcriptomes of mouse otic tissues at three embryonic ages, embryonic day 9.5 (E9.5), E11.5, and E13.5, covering proliferating and undifferentiated otic neuroblasts and differentiating VGNs and SGNs. We validate the high quality of our studies by using multiple assays, including genetic fate mapping analysis, and we uncover several genes upregulated in neuroblasts or differentiating VGNs and SGNs, such as Shox2, Myt1, Casz1, and Sall3. Notably, our findings suggest a general cascaded differentiation trajectory during early otic neurogenesis. The comprehensive understanding of early otic neurogenesis provided by our study holds critical implications for both basic and translational research.

Function of Proneural Genes Ascl1 and Asense in Neurogenesis: How Similar Are They?

Proneural genes were initially identified in Drosophila, where pioneer work on these important regulators of neural development was performed, and from which the term proneural function was coined. Subsequently, their counterparts in vertebrates were identified, and their function in neural development extensively characterized. The function of proneural transcription factors in flies and vertebrates is, however, very distinct. In flies, proneural genes play an early role in neural induction, by endowing neural competence to ectodermal cells. In contrast, vertebrate proneural genes are expressed only after neural specification, in neural stem and progenitor cells, where they play key regulatory functions in quiescence, proliferation, and neuronal differentiation. An exception to this scenario is the Drosophila proneural gene asense, which has a late onset of expression in neural stem cells of the developing embryo and larvae, similar to its vertebrate counterparts. Although the role of Asense remains poorly investigated, its expression pattern is suggestive of functions more in line with those of vertebrate proneural genes. Here, we revise our current understanding of the multiple activities of Asense and of its closest vertebrate homologue Ascl1 in neural stem/progenitor cell biology, and discuss possible parallels between the two transcription factors in neurogenesis regulation.

Early development of the breathing network

Breathing (or respiration) is a complex motor behavior that originates in the brainstem. In minimalistic terms, breathing can be divided into two phases: inspiration (uptake of oxygen, O₂) and expiration (release of carbon dioxide, CO₂). The neurons that discharge in synchrony with these phases are arranged in three major groups along the brainstem: (i) pontine, (ii) dorsal medullary, and (iii) ventral medullary. These groups are formed by diverse neuron types that coalesce into heterogeneous nuclei or complexes, among which the preBötzinger complex in the ventral medullary group contains cells that generate the respiratory rhythm (Chapter 1). The respiratory rhythm is not rigid, but instead highly adaptable to the physic demands of the organism. In order to generate the appropriate respiratory rhythm, the preBötzinger complex receives direct and indirect chemosensory information from other brainstem respiratory nuclei (Chapter 2) and peripheral organs (Chapter 3). Even though breathing is a hard-wired unconscious behavior, it can be temporarily altered at will by other higher-order brain structures (Chapter 6), and by emotional states (Chapter 7). In this chapter, we focus on the development of brainstem respiratory groups and highlight the cell lineages that contribute to central and peripheral chemoreflexes.

Cell cycle arrest determines adult neural stem cell ontogeny by an embryonic Notch-nonoscillatory Hey1 module

Quiescent neural stem cells (NSCs) in the adult mouse brain are the source of neurogenesis that regulates innate and adaptive behaviors. Adult NSCs in the subventricular zone are derived from a subpopulation of embryonic neural stem-progenitor cells (NPCs) that is characterized by a slower cell cycle relative to the more abundant rapid cycling NPCs that build the brain. Yet, how slow cell cycle can cause the establishment of adult NSCs remains largely unknown. Here, we demonstrate that Notch and an effector Hey1 form a module that is upregulated by cell cycle arrest in slowly dividing NPCs. In contrast to the oscillatory expression of the Notch effectors Hes1 and Hes5 in fast cycling progenitors, Hey1 displays a non-oscillatory stationary expression pattern and contributes to the long-term maintenance of NSCs. These findings reveal a novel division of labor in Notch effectors where cell cycle rate biases effector selection and cell fate.

Adult neural stem cells are derived from an embryonic population of slowcycling progenitor cells, though how reduced cycling speed leads to establishment of the adult population has remained elusive. Here they show that non-oscillatory Notch-Hey signaling induced by slow-cycling contributes to long term maintenance of neural stem cells.

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

Responses of Epibranchial Placodes to Disruptions of the FGF and BMP Signaling Pathways in Embryonic Mice

Placodes are ectodermal thickenings of the embryonic vertebrate head. Their descendants contribute to sensory organ development, but also give rise to sensory neurons of the cranial nerves. In mammals, the signaling pathways which regulate the morphogenesis and neurogenesis of epibranchial placodes, localized dorsocaudally to the pharyngeal clefts, are poorly understood. Therefore, we performed mouse whole embryo culture experiments to assess the impact of pan-fibroblast growth factor receptor (FGFR) inhibitors, anti-FGFR3 neutralizing antibodies or the pan-bone morphogenetic protein receptor (BMPR) inhibitor LDN193189 on epibranchial development. We demonstrate that each of the three paired epibranchial placodes is regulated by a unique combination of FGF and/or bone morphogenetic protein (BMP) signaling. Thus, neurogenesis depends on fibroblast growth factor (FGF) signals, albeit to different degrees, in all epibranchial placodes (EP), whereas only EP1 and EP3 significantly rely on neurogenic BMP signals. Furthermore, individual epibranchial placodes vary in the extent to which FGF and/or BMP signals (1) have access to certain receptor subtypes, (2) affect the production of Neurogenin (Ngn)2⁺ and/or Ngn1⁺ neuroblasts, and (3) regulate either neurogenesis alone or together with structural maintenance. In EP2 and EP3, all FGF-dependent production of Ngn2⁺ neuroblasts is mediated via FGFR3 whereas, in EP1, it depends on FGFR1 and FGFR3. Differently, production of FGF-dependent Ngn1⁺ neuroblasts almost completely depends on FGFR3 in EP1 and EP2, but not in EP3. Finally, FGF signals turned out to be responsible for the maintenance of both placodal thickening and neurogenesis in all epibranchial placodes, whereas administration of the pan-BMPR inhibitor, apart from its negative neurogenic effects in EP1 and EP3, causes only decreases in the thickness of EP3. Experimentally applied inhibitors most probably not only blocked receptors in the epibranchial placodes, but also endodermal receptors in the pharyngeal pouches, which act as epibranchial signaling centers. While high doses of pan-FGFR inhibitors impaired the development of all pharyngeal pouches, high doses of the pan-BMPR inhibitor negatively affected only the pharyngeal pouches 3 and 4. In combination with partly concordant, partly divergent findings in other vertebrate classes our observations open up new approaches for research into the complex regulation of neurogenic placode development.

Proneural genes define ground-state rules to regulate neurogenic patterning and cortical folding

Asymmetric neuronal expansion is thought to drive evolutionary transitions between lissencephalic and gyrencephalic cerebral cortices. We report that Neurog2 and Ascl1 proneural genes together sustain neurogenic continuity and lissencephaly in rodent cortices. Using transgenic reporter mice and human cerebral organoids, we found that Neurog2 and Ascl1 expression defines a continuum of four lineage-biased neural progenitor cell (NPC) pools. Double⁺ NPCs, at the hierarchical apex, are least lineage restricted due to Neurog2-Ascl1 cross-repression and display unique features of multipotency (more open chromatin, complex gene regulatory network, G₂ pausing). Strikingly, selectively eliminating double⁺ NPCs by crossing Neurog2-Ascl1 split-Cre mice with diphtheria toxin-dependent "deleter" strains locally disrupts Notch signaling, perturbs neurogenic symmetry, and triggers cortical folding. In support of our discovery that double⁺ NPCs are Notch-ligand-expressing "niche" cells that control neurogenic periodicity and cortical folding, NEUROG2, ASCL1, and HES1 transcript distribution is modular (adjacent high/low zones) in gyrencephalic macaque cortices, prefiguring future folds.

An Integrated Perspective of Evolution and Development: From Genes to Function to Ear, Lateral Line and Electroreception

Four sensory systems (vestibular, lateral line, electroreception, auditory) are unique and project exclusively to the brainstem of vertebrates. All sensory neurons depend on a common set of genes (Eya1, Sox2, Neurog1, Neurod1) that project to a dorsal nucleus and an intermediate nucleus, which differentiate into the vestibular ear, lateral line and electroreception in vertebrates. In tetrapods, a loss of two sensory systems (lateral line, electroreception) leads to the development of a unique ear and auditory system in amniotes. Lmx1a/b, Gdf7, Wnt1/3a, BMP4/7 and Atoh1 define the lateral line, electroreception and auditory nuclei. In contrast, vestibular nuclei depend on Neurog1/2, Ascl1, Ptf1a and Olig3, among others, to develop an independent origin of the vestibular nuclei. A common origin of hair cells depends on Eya1, Sox2 and Atoh1, which generate the mechanosensory cells. Several proteins define the polarity of hair cells in the ear and lateral line. A unique connection of stereocilia requires CDH23 and PCDH15 for connections and TMC1/2 proteins to perceive mechanosensory input. Electroreception has no polarity, and a different system is used to drive electroreceptors. All hair cells function by excitation via ribbons to activate neurons that innervate the distinct target areas. An integrated perspective is presented to understand the gain and loss of different sensory systems.

3D bioprinted human iPSC-derived somatosensory constructs with functional and highly purified sensory neuron networks

Engineering three-dimensional (3D) sensible tissue constructs, along with the complex microarchitecture wiring of the sensory nervous system, has been an ongoing challenge in the tissue engineering field. By combining 3D bioprinting and human pluripotent stem cell (hPSC) technologies, sensible tissue constructs could be engineered in a rapid, precise, and controllable manner to replicate 3D microarchitectures and mechanosensory functionalities of the native sensory tissue (e.g. response to external stimuli). Here, we introduce a biofabrication approach to create complex 3D microarchitecture wirings. We develop an hPSC-sensory neuron (SN) laden bioink using highly purified and functional SN populations to 3D bioprint microarchitecture wirings that demonstrate responsiveness to warm/cold sense-inducing chemicals and mechanical stress. Specifically, we tailor a conventional differentiation strategy to our purification method by utilizing p75 cell surface marker and DAPT treatment along with neuronal growth factors in order to selectively differentiate neural crest cells into SNs. To create spatial resolution in 3D architectures and grow SNs in custom patterns and directions, an induced pluripotent stem cell (iPSC)-SN-laden gelatin bioink was printed on laminin-coated substrates using extrusion-based bioprinting technique. Then the printed constructs were covered with a collagen matrix that guided SNs growing in the printed micropattern. Using a sacrificial bioprinting technique, the iPSC-SNs were seeded into the hollow microchannels created by sacrificial gelatin ink printed in the gelatin methacryloyl supporting bath, thereby demonstrating controllability over axon guidance in curved lines up to several tens of centimeters in length on 2D substrates and in straight microchannels in 3D matrices. Therefore, this biofabrication approach could be amenable to incorporate sensible SN networks into the engineered skin equivalents, regenerative skin implants, and augmented somatosensory neuro-prosthetics that have the potential to regenerate sensible functions by connecting host neuron systems in injured areas.

Dissection and surgical approaches to the mouse jugular-nodose ganglia

The jugular-nodose ganglia contain the sensory peripheral neurons of the vagus nerve, linking visceral organs to the medulla oblongata. Accessing these ganglia in smaller animals without damaging the vascular and neural structures may be challenging, as ganglionic fibers imbed deeply into the carotid sheath, and vagal parasympathetic fibers cross through the interior of the ganglia. We describe a practical protocol for locating and accessing the mouse jugular-nodose ganglia in vivo, including instructions for intraganglionic injections and postperfusion dissection.

For complete details on the use and execution of this protocol, please refer to Han et al. (2018).

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Practical approach to locate the mouse jugular-nodose ganglia

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Detailed instructions on how to perform intraganglionic injections

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Detailed description of ganglia-preserving postperfusion dissection

Vertebrate Sensory Ganglia: Common and Divergent Features of the Transcriptional Programs Generating Their Functional Specialization

Sensory fibers of the peripheral nervous system carry sensation from specific sense structures or use different tissues and organs as receptive fields, and convey this information to the central nervous system. In the head of vertebrates, each cranial sensory ganglia and associated nerves perform specific functions. Sensory ganglia are composed of different types of specialized neurons in which two broad categories can be distinguished, somatosensory neurons relaying all sensations that are felt and visceral sensory neurons sensing the internal milieu and controlling body homeostasis. While in the trunk somatosensory neurons composing the dorsal root ganglia are derived exclusively from neural crest cells, somato- and visceral sensory neurons of cranial sensory ganglia have a dual origin, with contributions from both neural crest and placodes. As most studies on sensory neurogenesis have focused on dorsal root ganglia, our understanding of the molecular mechanisms underlying the embryonic development of the different cranial sensory ganglia remains today rudimentary. However, using single-cell RNA sequencing, recent studies have made significant advances in the characterization of the neuronal diversity of most sensory ganglia. Here we summarize the general anatomy, function and neuronal diversity of cranial sensory ganglia. We then provide an overview of our current knowledge of the transcriptional networks controlling neurogenesis and neuronal diversification in the developing sensory system, focusing on cranial sensory ganglia, highlighting specific aspects of their development and comparing it to that of trunk sensory ganglia.

The Mediator Subunit, Med23 Is Required for Embryonic Survival and Regulation of Canonical WNT Signaling During Cranial Ganglia Development

Development of the vertebrate head is a complex and dynamic process, which requires integration of all three germ layers and their derivatives. Of special importance are ectoderm-derived cells that form the cranial placodes, which then differentiate into the cranial ganglia and sensory organs. Critical to a fully functioning head, defects in cranial placode and sensory organ development can result in congenital craniofacial anomalies. In a forward genetic screen aimed at identifying novel regulators of craniofacial development, we discovered an embryonically lethal mouse mutant, snouty, which exhibits malformation of the facial prominences, cranial nerves and vasculature. The snouty mutation was mapped to a single nucleotide change in a ubiquitously expressed gene, Med23, which encodes a subunit of the global transcription co-factor complex, Mediator. Phenotypic analyses revealed that the craniofacial anomalies, particularly of the cranial ganglia, were caused by a failure in the proper specification of cranial placode neuronal precursors. Molecular analyses determined that defects in cranial placode neuronal differentiation in Med23 ^sn/sn mutants were associated with elevated WNT/β-catenin signaling, which can be partially rescued through combined Lrp6 and Wise loss-of-function. Our work therefore reveals a surprisingly tissue specific role for the ubiquitously expressed mediator complex protein Med23 in placode differentiation during cranial ganglia development. This highlights the importance of coupling general transcription to the regulation of WNT signaling during embryogenesis.

Action of Actomyosin Contraction With Shh Modulation Drive Epithelial Folding in the Circumvallate Papilla

The mouse tongue possesses three types of gustatory papillae: large circumvallate papillae (CVP), foliate papillae (FOP) and fungiform papillae (FFP). Although CVP is the largest papilla and contain a high density of taste buds, little is known about CVP development. Their transition from placode to dome-shape is particularly ambiguous. Understanding this phase is crucial since dome-shaped morphology is essential for proper localization of the imminent nerve fibers and taste buds. Here, we report actomyosin-dependent apical and basal constriction of epithelial cells during dynamic epithelial folding. Furthermore, actomyosin-dependent basal constriction requires focal adhesion kinase to guide dome-shape formation. Sonic hedgehog (Shh) is closely associated with the differentiation or survival of the neurons in CVP ganglion and cytoskeletal alteration in trench epithelial cells which regulate CVP morphogenesis. Our results demonstrate the CVP morphogenesis mechanism from placode to dome-shape by actomyosin-dependent cell shape change and suggest roles that Shh may play in trench and stromal core formation during CVP development.

ngn-1/neurogenin Activates Transcription of Multiple Terminal Selector Transcription Factors in the Caenorhabditis elegans Nervous System

Proper nervous system development is required for an organism's survival and function. Defects in neurogenesis have been linked to neurodevelopmental disorders such as schizophrenia and autism. Understanding the gene regulatory networks that orchestrate neural development, specifically cascades of proneural transcription factors, can better elucidate which genes are most important during early neurogenesis. Neurogenins are a family of deeply conserved factors shown to be both necessary and sufficient for the development of neural subtypes. However, the immediate downstream targets of neurogenin are not well characterized. The objective of this study was to further elucidate the role of ngn-1/neurogenin in nervous system development and to identify its downstream transcriptional targets, using the nematode Caenorhabditis elegans as a model for this work. We found that ngn-1 is required for axon outgrowth, nerve ring architecture, and neuronal cell fate specification. We also showed that ngn-1 may have roles in neuroblast migration and epithelial integrity during embryonic development. Using RNA sequencing and comparative transcriptome analysis, we identified eight transcription factors (hlh-34/NPAS1, unc-42/PROP1, ceh-17/PHOX2A, lim-4/LHX6, fax-1/NR2E3, lin-11/LHX1, tlp-1/ZNF503, and nhr-23/RORB) whose transcription is activated, either directly or indirectly, by ngn-1 Our results show that ngn-1 has a role in transcribing known terminal regulators that establish and maintain cell fate of differentiated neural subtypes and confirms that ngn-1 functions as a proneural transcription factor in C. elegans neurogenesis.

PRDM12 Is Required for Initiation of the Nociceptive Neuron Lineage during Neurogenesis

The sensation of pain is essential for the preservation of the functional integrity of the body. However, the key molecular regulators necessary for the initiation of the development of pain-sensing neurons have remained largely unknown. Here, we report that, in mice, inactivation of the transcriptional regulator PRDM12, which is essential for pain perception in humans, results in a complete absence of the nociceptive lineage, while proprioceptive and touch-sensitive neurons remain. Mechanistically, our data reveal that PRDM12 is required for initiation of neurogenesis and activation of a cascade of downstream pro-neuronal transcription factors, including NEUROD1, BRN3A, and ISL1, in the nociceptive lineage while it represses alternative fates other than nociceptors in progenitor cells. Our results thus demonstrate that PRDM12 is necessary for the generation of the entire lineage of pain-initiating neurons.

The sensation of pain, temperature, and itch by neurons of the nociceptive lineage is essential for animal survival. Bartesaghi et al. report that the transcriptional regulator PRDM12 is indispensable in neural crest cells (NCCs) for the initiation of the sensory neuronal differentiation program that generates the entire nociceptive lineage.

Neurog2 Acts as a Classical Proneural Gene in the Ventromedial Hypothalamus and Is Required for the Early Phase of Neurogenesis

The tuberal hypothalamus is comprised of the dorsomedial, ventromedial, and arcuate nuclei, as well as parts of the lateral hypothalamic area, and it governs a wide range of physiologies. During neurogenesis, tuberal hypothalamic neurons are thought to be born in a dorsal-to-ventral and outside-in pattern, although the accuracy of this description has been questioned over the years. Moreover, the intrinsic factors that control the timing of neurogenesis in this region are poorly characterized. Proneural genes, including Achate-scute-like 1 (Ascl1) and Neurogenin 3 (Neurog3) are widely expressed in hypothalamic progenitors and contribute to lineage commitment and subtype-specific neuronal identifies, but the potential role of Neurogenin 2 (Neurog2) remains unexplored. Birthdating in male and female mice showed that tuberal hypothalamic neurogenesis begins as early as E9.5 in the lateral hypothalamic and arcuate and rapidly expands to dorsomedial and ventromedial neurons by E10.5, peaking throughout the region by E11.5. We confirmed an outside-in trend, except for neurons born at E9.5, and uncovered a rostrocaudal progression but did not confirm a dorsal-ventral patterning to tuberal hypothalamic neuronal birth. In the absence of Neurog2, neurogenesis stalls, with a significant reduction in early-born BrdU⁺ cells but no change at later time points. Further, the loss of Ascl1 yielded a similar delay in neuronal birth, suggesting that Ascl1 cannot rescue the loss of Neurog2 and that these proneural genes act independently in the tuberal hypothalamus. Together, our findings show that Neurog2 functions as a classical proneural gene to regulate the temporal progression of tuberal hypothalamic neurogenesis.SIGNIFICANCE STATEMENT Here, we investigated the general timing and pattern of neurogenesis within the tuberal hypothalamus. Our results confirmed an outside-in trend of neurogenesis and uncovered a rostrocaudal progression. We also showed that Neurog2 acts as a classical proneural gene and is responsible for regulating the birth of early-born neurons within the ventromedial hypothalamus, acting independently of Ascl1 In addition, we revealed a role for Neurog2 in cell fate specification and differentiation of ventromedial -specific neurons. Last, Neurog2 does not have cross-inhibitory effects on Neurog1, Neurog3, and Ascl1 These findings are the first to reveal a role for Neurog2 in hypothalamic development.

Suckling, Feeding, and Swallowing: Behaviors, Circuits, and Targets for Neurodevelopmental Pathology

All mammals must suckle and swallow at birth, and subsequently chew and swallow solid foods, for optimal growth and health. These initially innate behaviors depend critically upon coordinated development of the mouth, tongue, pharynx, and larynx as well as the cranial nerves that control these structures. Disrupted suckling, feeding, and swallowing from birth onward-perinatal dysphagia-is often associated with several neurodevelopmental disorders that subsequently alter complex behaviors. Apparently, a broad range of neurodevelopmental pathologic mechanisms also target oropharyngeal and cranial nerve differentiation. These aberrant mechanisms, including altered patterning, progenitor specification, and neurite growth, prefigure dysphagia and may then compromise circuits for additional behavioral capacities. Thus, perinatal dysphagia may be an early indicator of disrupted genetic and developmental programs that compromise neural circuits and yield a broad range of behavioral deficits in neurodevelopmental disorders.

Notch signalling regulates epibranchial placode patterning and segregation

Epibranchial placodes are the geniculate, petrosal and nodose placodes that generate parts of cranial nerves VII, IX and X, respectively. How the three spatially separated placodes are derived from the common posterior placodal area is poorly understood. Here, we reveal that the broad posterior placode area is first patterned into a Vgll2⁺/Irx5⁺ rostral domain and a Sox2⁺/Fgf3⁺/Etv5⁺ caudal domain relative to the first pharyngeal cleft. This initial rostral and caudal patterning is then sequentially repeated along each pharyngeal cleft for each epibranchial placode. The caudal domains give rise to the neuronal and non-neuronal cells in the placode, whereas the rostral domains are previously unrecognized structures, serving as spacers between the final placodes. Notch signalling regulates the balance between the rostral and caudal domains: high levels of Notch signalling expand the caudal domain at the expense of the rostral domain, whereas loss of Notch signalling produces the converse phenotype. Collectively, these data unravel a new patterning principle for the early phases of epibranchial placode development and a role for Notch signalling in orchestrating epibranchial placode segregation and differentiation.

Brn3/POU-IV-type POU homeobox genes-Paradigmatic regulators of neuronal identity across phylogeny

One approach to understand the construction of complex systems is to investigate whether there are simple design principles that are commonly used in building such a system. In the context of nervous system development, one may ask whether the generation of its highly diverse sets of constituents, that is, distinct neuronal cell types, relies on genetic mechanisms that share specific common features. Specifically, are there common patterns in the function of regulatory genes across different neuron types and are those regulatory mechanisms not only used in different parts of one nervous system, but are they conserved across animal phylogeny? We address these questions here by focusing on one specific, highly conserved and well-studied regulatory factor, the POU homeodomain transcription factor UNC-86. Work over the last 30 years has revealed a common and paradigmatic theme of unc-86 function throughout most of the neuron types in which Caenorhabditis elegans unc-86 is expressed. Apart from its role in preventing lineage reiterations during development, UNC-86 operates in combination with distinct partner proteins to initiate and maintain terminal differentiation programs, by coregulating a vast array of functionally distinct identity determinants of specific neuron types. Mouse orthologs of unc-86, the Brn3 genes, have been shown to fulfill a similar function in initiating and maintaining neuronal identity in specific parts of the mouse brain and similar functions appear to be carried out by the sole Drosophila ortholog, Acj6. The terminal selector function of UNC-86 in many different neuron types provides a paradigm for neuronal identity regulation across phylogeny. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Invertebrate Organogenesis > Worms Nervous System Development > Vertebrates: Regional Development.

Transcription Factors That Govern Development and Disease: An Achilles Heel in Cancer

Development requires the careful orchestration of several biological events in order to create any structure and, eventually, to build an entire organism. On the other hand, the fate transformation of terminally differentiated cells is a consequence of erroneous development, and ultimately leads to cancer. In this review, we elaborate how development and cancer share several biological processes, including molecular controls. Transcription factors (TF) are at the helm of both these processes, among many others, and are evolutionarily conserved, ranging from yeast to humans. Here, we discuss four families of TFs that play a pivotal role and have been studied extensively in both embryonic development and cancer—high mobility group box (HMG), GATA, paired box (PAX) and basic helix-loop-helix (bHLH) in the context of their role in development, cancer, and their conservation across several species. Finally, we review TFs as possible therapeutic targets for cancer and reflect on the importance of natural resistance against cancer in certain organisms, yielding knowledge regarding TF function and cancer biology.

Aryl hydrocarbon receptor modulates stroke-induced astrogliosis and neurogenesis in the adult mouse brain

BACKGROUND

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor activated by environmental agonists and dietary tryptophan metabolites for the immune response and cell cycle regulation. Emerging evidence suggests that AHR activation after acute stroke may play a role in brain ischemic injury. However, whether AHR activation alters poststroke astrogliosis and neurogenesis remains unknown.

METHODS

We adopted conditional knockout of AHR from nestin-expressing neural stem/progenitor cells (AHRcKO) and wild-type (WT) mice in the permanent middle cerebral artery occlusion (MCAO) model. WT mice were treated with either vehicle or the AHR antagonist 6,2',4'-trimethoxyflavone (TMF, 5 mg/kg/day) intraperitoneally. The animals were examined at 2 and 7 days after MCAO.

RESULTS

The AHR signaling pathway was significantly upregulated after stroke. Both TMF-treated WT and AHRcKO mice showed significantly decreased infarct volume, improved sensorimotor, and nonspatial working memory functions compared with their respective controls. AHR immunoreactivities were increased predominantly in activated microglia and astrocytes after MCAO compared with the normal WT controls. The TMF-treated WT and AHRcKO mice demonstrated significant amelioration of astrogliosis and microgliosis. Interestingly, these mice also showed augmentation of neural progenitor cell proliferation at the ipsilesional neurogenic subventricular zone (SVZ) and the hippocampal subgranular zone. At the peri-infarct cortex, the ipsilesional SVZ/striatum, and the hippocampus, both the TMF-treated and AHRcKO mice demonstrated downregulated IL-1β, IL-6, IFN-γ, CXCL1, and S100β, and concomitantly upregulated Neurogenin 2 and Neurogenin 1.

CONCLUSION

Neural cell-specific AHR activation following acute ischemic stroke increased astrogliosis and suppressed neurogenesis in adult mice. AHR inhibition in acute stroke may potentially benefit functional outcomes likely through reducing proinflammatory gliosis and preserving neurogenesis.

Taste bud formation depends on taste nerves

It has been known for more than a century that, in adult vertebrates, the maintenance of taste buds depends on their afferent nerves. However, the initial formation of taste buds is proposed to be nerve-independent in amphibians, and evidence to the contrary in mammals has been endlessly debated, mostly due to indirect and incomplete means to impede innervation during the protracted perinatal period of taste bud differentiation. Here, by genetically ablating, in mice, all somatic (i.e. touch) or visceral (i.e. taste) neurons for the oral cavity, we show that the latter but not the former are absolutely required for the proper formation of their target organs, the taste buds.

Early trigeminal ganglion afferents enter the cerebellum before the Purkinje cells are born and target the nuclear transitory zone

In the standard model for the development of climbing and mossy fiber afferent pathways to the cerebellum, the ingrowing axons target the embryonic Purkinje cell somata (around embryonic ages (E13-E16 in mice). In this report, we describe a novel earlier stage in afferent development. Immunostaining for a neurofilament-associated antigen (NAA) reveals the early axon distributions with remarkable clarity. Using a combination of DiI axon tract tracing, analysis of neurogenin1 null mice, which do not develop trigeminal ganglia, and mouse embryos maintained in vitro, we show that the first axons to innervate the cerebellar primordium as early as E9 arise from the trigeminal ganglion. Therefore, early trigeminal axons are in situ before the Purkinje cells are born. Double immunostaining for NAA and markers of the different domains in the cerebellar primordium reveal that afferents first target the nuclear transitory zone (E9-E10), and only later (E10-E11) are the axons, either collaterals from the trigeminal ganglion or a new afferent source (e.g., vestibular ganglia), seen in the Purkinje cell plate. The finding that the earliest axons to the cerebellum derive from the trigeminal ganglion and enter the cerebellar primordium before the Purkinje cells are born, where they seem to target the cerebellar nuclei, reveals a novel stage in the development of the cerebellar afferents.

Cend1, a Story with Many Tales: From Regulation of Cell Cycle Progression/Exit of Neural Stem Cells to Brain Structure and Function

Neural stem/precursor cells (NPCs) generate the large variety of neuronal phenotypes comprising the adult brain. The high diversity and complexity of this organ have its origin in embryonic life, during which NPCs undergo symmetric and asymmetric divisions and then exit the cell cycle and differentiate to acquire neuronal identities. During these processes, coordinated regulation of cell cycle progression/exit and differentiation is essential for generation of the appropriate number of neurons and formation of the correct structural and functional neuronal circuits in the adult brain. Cend1 is a neuronal lineage-specific modulator involved in synchronization of cell cycle exit and differentiation of neuronal precursors. It is expressed all along the neuronal lineage, from neural stem/progenitor cells to mature neurons, and is associated with the dynamics of neuron-generating divisions. Functional studies showed that Cend1 has a critical role during neurogenesis in promoting cell cycle exit and neuronal differentiation. Mechanistically, Cend1 acts via the p53-dependent/Cyclin D1/pRb signaling pathway as well as via a p53-independent route involving a tripartite interaction with RanBPM and Dyrk1B. Upon Cend1 function, Notch1 signaling is suppressed and proneural genes such as Mash1 and Neurogenins 1/2 are induced. Due to its neurogenic activity, Cend1 is a promising candidate therapeutic gene for brain repair, while the Cend1 minimal promoter is a valuable tool for neuron-specific gene delivery in the CNS. Mice with Cend1 genetic ablation display increased NPC proliferation, decreased migration, and higher levels of apoptosis during development. As a result, they show in the adult brain deficits in a range of motor and nonmotor behaviors arising from irregularities in cerebellar cortex lamination and impaired Purkinje cell differentiation as well as a paucity in GABAergic interneurons of the cerebral cortex, hippocampus, and amygdala. Taken together, these studies highlight the necessity for Cend1 expression in the formation of a structurally and functionally normal brain.

Primary sensory map formations reflect unique needs and molecular cues specific to each sensory system

Interaction with the world around us requires extracting meaningful signals to guide behavior. Each of the six mammalian senses (olfaction, vision, somatosensation, hearing, balance, and taste) has a unique primary map that extracts sense-specific information. Sensory systems in the periphery and their target neurons in the central nervous system develop independently and must develop specific connections for proper sensory processing. In addition, the regulation of sensory map formation is independent of and prior to central target neuronal development in several maps. This review provides an overview of the current level of understanding of primary map formation of the six mammalian senses. Cell cycle exit, combined with incompletely understood molecules and their regulation, provides chemoaffinity-mediated primary maps that are further refined by activity. The interplay between cell cycle exit, molecular guidance, and activity-mediated refinement is the basis of dominance stripes after redundant organ transplantations in the visual and balance system. A more advanced level of understanding of primary map formation could benefit ongoing restoration attempts of impaired senses by guiding proper functional connection formations of restored sensory organs with their central nervous system targets.

New Neuronal Subtypes With a "Pre-Pancreatic" Signature in the Sea Urchin Stongylocentrotus purpuratus

Neurons and pancreatic endocrine cells have a common physiology and express a similar toolkit of transcription factors during development. To explain these common features, it has been hypothesized that pancreatic cells most likely co-opted a pre-existing gene regulatory program from ancestral neurons. To test this idea, we looked for neurons with a "pre-pancreatic" program in an early-branched deuterostome, the sea urchin. Only vertebrates have a proper pancreas, however, our lab previously found that cells with a pancreatic-like signature are localized within the sea urchin embryonic gut. We also found that the pancreatic transcription factors Xlox/Pdx1 and Brn1/2/4 co-localize in a sub-population of ectodermal cells. Here, we find that the ectodermal SpLox+ SpBrn1/2/4 cells are specified as SpSoxC and SpPtf1a neuronal precursors that become the lateral ganglion and the apical organ neurons. Two of the SpLox+ SpBrn1/2/4 cells also express another pancreatic transcription factor, the LIM-homeodomain gene islet-1. Moreover, we find that SpLox neurons produce the neuropeptide SpANP2, and that SpLox regulates SpANP2 expression. Taken together, our data reveal that there is a subset of sea urchin larval neurons with a gene program that predated pancreatic cells. These findings suggest that pancreatic endocrine cells co-opted a regulatory signature from an ancestral neuron that was already present in an early-branched deuterostome.

Requirements for Neurogenin2 during mouse postnatal retinal neurogenesis

During embryonic retinal development, the bHLH factor Neurog2 regulates the temporal progression of neurogenesis, but no role has been assigned for this gene in the postnatal retina. Using Neurog2 conditional mutants, we found that Neurog2 is necessary for the development of an early, embryonic cohort of rod photoreceptors, but also required by both a subset of cone bipolar subtypes, and rod bipolars. Using transcriptomics, we identified a subset of downregulated genes in P2 Neurog2 mutants, which act during rod differentiation, outer segment morphogenesis or visual processing. We also uncovered defects in neuronal cell culling, which suggests that the rod and bipolar cell phenotypes may arise via more complex mechanisms rather than a simple cell fate shift. However, given an overall phenotypic resemblance between Neurog2 and Blimp1 mutants, we explored the relationship between these two factors. We found that Blimp1 is downregulated between E12-birth in Neurog2 mutants, which probably reflects a dependence on Neurog2 in embryonic progenitor cells. Overall, we conclude that the Neurog2 gene is expressed and active prior to birth, but also exerts an influence on postnatal retinal neuron differentiation.

A non-canonical role for the proneural gene Neurog1 as a negative regulator of neocortical neurogenesis

Neural progenitors undergo temporal identity transitions to sequentially generate the neuronal and glial cells that make up the mature brain. Proneural genes have well-characterised roles in promoting neural cell differentiation and subtype specification, but they also regulate the timing of identity transitions through poorly understood mechanisms. Here, we investigated how the highly related proneural genes Neurog1 and Neurog2 interact to control the timing of neocortical neurogenesis. We found that Neurog1 acts in an atypical fashion as it is required to suppress rather than promote neuronal differentiation in early corticogenesis. In Neurog1-/- neocortices, early born neurons differentiate in excess, whereas, in vitro, Neurog1-/- progenitors have a decreased propensity to proliferate and form neurospheres. Instead, Neurog1-/- progenitors preferentially generate neurons, a phenotype restricted to the Neurog2+ progenitor pool. Mechanistically, Neurog1 and Neurog2 heterodimerise, and while Neurog1 and Neurog2 individually promote neurogenesis, misexpression together blocks this effect. Finally, Neurog1 is also required to induce the expression of neurogenic factors (Dll1 and Hes5) and to repress the expression of neuronal differentiation genes (Fezf2 and Neurod6). Neurog1 thus employs different mechanisms to temper the pace of early neocortical neurogenesis.

Integral bHLH factor regulation of cell cycle exit and RGC differentiation

BACKGROUND

In the developing mouse embryo, the bHLH transcription factor Neurog2 is transiently expressed by retinal progenitor cells and required for the initial wave of neurogenesis. Remarkably, another bHLH factor, Ascl1, normally not present in the embryonic Neurog2 retinal lineage, can rescue the temporal phenotypes of Neurog2 mutants.

RESULTS

Here we show that Neurog2 simultaneously promotes terminal cell cycle exit and retinal ganglion cell differentiation, using mitotic window labeling and integrating these results with retinal marker quantifications. We also analyzed the transcriptomes of E12.5 GFP-expressing cells from Neurog2GFP/+ , Neurog2GFP/GFP , and Neurog2Ascl1KI/GFP eyes, and validated the most significantly affected genes using qPCR assays.

CONCLUSIONS

Our data support the hypothesis that Neurog2 acts at the top of a retinal bHLH transcription factor hierarchy. The combined expression levels of these downstream factors are sufficiently induced by ectopic Ascl1 to restore RGC genesis, highlighting the robustness of this gene network during retinal ganglion cell neurogenesis. Developmental Dynamics 247:965-975, 2018. © 2018 Wiley Periodicals, Inc.

Rapid Chromatin Switch in the Direct Reprogramming of Fibroblasts to Neurons

How transcription factors (TFs) reprogram one cell lineage to another remains unclear. Here, we define chromatin accessibility changes induced by the proneural TF Ascl1 throughout conversion of fibroblasts into induced neuronal (iN) cells. Thousands of genomic loci are affected as early as 12 hr after Ascl1 induction. Surprisingly, over 80% of the accessibility changes occur between days 2 and 5 of the 3-week reprogramming process. This chromatin switch coincides with robust activation of endogenous neuronal TFs and nucleosome phasing of neuronal promoters and enhancers. Subsequent morphological and functional maturation of iN cells is accomplished with relatively little chromatin reconfiguration. By integrating chromatin accessibility and transcriptome changes, we built a network model of dynamic TF regulation during iN cell reprogramming and identified Zfp238, Sox8, and Dlx3 as key TFs downstream of Ascl1. These results reveal a singular, coordinated epigenomic switch during direct reprogramming, in contrast to stepwise cell fate transitions in development.

Lateral line placodes of aquatic vertebrates are evolutionarily conserved in mammals

Placodes are focal thickenings of the surface ectoderm which, together with neural crest, generate the peripheral nervous system of the vertebrate head. Here we examine how, in embryonic mice, apoptosis contributes to the remodelling of the primordial posterior placodal area (PPA) into physically separated otic and epibranchial placodes. Using pharmacological inhibition of apoptosis-associated caspases, we find evidence that apoptosis eliminates hitherto undiscovered rudiments of the lateral line sensory system which, in fish and aquatic amphibia, serves to detect movements, pressure changes or electric fields in the surrounding water. Our results refute the evolutionary theory, valid for more than a century that the whole lateral line was completely lost in amniotes. Instead, those parts of the PPA which, under experimental conditions, escape apoptosis have retained the developmental potential to produce lateral line placodes and the primordia of neuromasts that represent the major functional units of the mechanosensory lateral line system.

Summary: Inhibition of apoptosis in mouse embryos reveals rudiments of the lateral line system, a sensory system common to fish and aquatic amphibia, but hypothesized to be completely lost in amniotes.

Mib1 prevents Notch Cis-inhibition to defer differentiation and preserve neuroepithelial integrity during neural delamination

The vertebrate neuroepithelium is composed of elongated progenitors whose reciprocal attachments ensure the continuity of the ventricular wall. As progenitors commit to differentiation, they translocate their nucleus basally and eventually withdraw their apical endfoot from the ventricular surface. However, the mechanisms allowing this delamination process to take place while preserving the integrity of the neuroepithelial tissue are still unclear. Here, we show that Notch signaling, which is classically associated with an undifferentiated state, remains active in prospective neurons until they delaminate. During this transition period, prospective neurons rapidly reduce their apical surface and only later down-regulate N-Cadherin levels. Upon Notch blockade, nascent neurons disassemble their junctions but fail to reduce their apical surface. This disrupted sequence weakens the junctional network and eventually leads to breaches in the ventricular wall. We also provide evidence that the Notch ligand Delta-like 1 (Dll1) promotes differentiation by reducing Notch signaling through a Cis-inhibition mechanism. However, during the delamination process, the ubiquitin ligase Mindbomb1 (Mib1) transiently blocks this Cis-inhibition and sustains Notch activity to defer differentiation. We propose that the fine-tuned balance between Notch Trans-activation and Cis-inhibition allows neuroepithelial cells to seamlessly delaminate from the ventricular wall as they commit to differentiation.

bHLH transcription factors in neural development, disease, and reprogramming

The formation of functional neural circuits in the vertebrate central nervous system (CNS) requires that appropriate numbers of the correct types of neuronal and glial cells are generated in their proper places and times during development. In the embryonic CNS, multipotent progenitor cells first acquire regional identities, and then undergo precisely choreographed temporal identity transitions (i.e. time-dependent changes in their identity) that determine how many neuronal and glial cells of each type they will generate. Transcription factors of the basic-helix-loop-helix (bHLH) family have emerged as key determinants of neural cell fate specification and differentiation, ensuring that appropriate numbers of specific neuronal and glial cell types are produced. Recent studies have further revealed that the functions of these bHLH factors are strictly regulated. Given their essential developmental roles, it is not surprising that bHLH mutations and de-regulated expression are associated with various neurological diseases and cancers. Moreover, the powerful ability of bHLH factors to direct neuronal and glial cell fate specification and differentiation has been exploited in the relatively new field of cellular reprogramming, in which pluripotent stem cells or somatic stem cells are converted to neural lineages, often with a transcription factor-based lineage conversion strategy that includes one or more of the bHLH genes. These concepts are reviewed herein.

Wilhelm His' lasting insights into hindbrain and cranial ganglia development and evolution

Wilhelm His (1831-1904) provided lasting insights into the development of the central and peripheral nervous system using innovative technologies such as the microtome, which he invented. 150 years after his resurrection of the classical germ layer theory of Wolff, von Baer and Remak, his description of the developmental origin of cranial and spinal ganglia from a distinct cell population, now known as the neural crest, has stood the test of time and more recently sparked tremendous advances regarding the molecular development of these important cells. In addition to his 1868 treatise on 'Zwischenstrang' (now neural crest), his work on the development of the human hindbrain published in 1890 provided novel ideas that more than 100 years later form the basis for penetrating molecular investigations of the regionalization of the hindbrain neural tube and of the migration and differentiation of its constituent neuron populations. In the first part of this review we briefly summarize the major discoveries of Wilhelm His and his impact on the field of embryology. In the second part we relate His' observations to current knowledge about the molecular underpinnings of hindbrain development and evolution. We conclude with the proposition, present already in rudimentary form in the writings of His, that a primordial spinal cord-like organization has been molecularly supplemented to generate hindbrain 'neomorphs' such as the cerebellum and the auditory and vestibular nuclei and their associated afferents and sensory organs.

Biphasic functions for the GDNF-Ret signaling pathway in chemosensory neuron development and diversification

While knowledge of signaling mechanisms orchestrating the development and diversification of peripheral somatosensory neurons is extensive, our understanding of the mechanisms controlling chemosensory neuron specification remains rudimentary. Lingually projecting sensory neurons of the geniculate ganglion are receptive to the five taste qualities, as well as temperature and tactile stimuli, but the mechanisms responsible for the diversification of the unique subpopulations that respond to one, or several, of these stimuli remain unknown. Here we demonstrate that the GDNF-Ret signaling pathway exerts a unique, dual function in peripheral taste system development and postnatal function. Ret acts embryonically to regulate the expression of the chemosensory master regulator Phox2b, thus inducing chemosensory differentiation, while postnatally acting to specify a molecularly unique subpopulation of lingual mechanoreceptors.

The development of the taste system relies on the coordinated regulation of cues that direct the simultaneous development of both peripheral taste organs and innervating sensory ganglia, but the underlying mechanisms remain poorly understood. In this study, we describe a novel, biphasic function for glial cell line-derived neurotrophic factor (GDNF) in the development and subsequent diversification of chemosensory neurons within the geniculate ganglion (GG). GDNF, acting through the receptor tyrosine kinase Ret, regulates the expression of the chemosensory fate determinant Phox2b early in GG development. Ret^−/− mice, but not Ret^fx/fx; Phox2b-Cre mice, display a profound loss of Phox2b expression with subsequent chemosensory innervation deficits, indicating that Ret is required for the initial amplification of Phox2b expression but not its maintenance. Ret expression is extinguished perinatally but reemerges postnatally in a subpopulation of large-diameter GG neurons expressing the mechanoreceptor marker NF200 and the GDNF coreceptor GFRα1. Intriguingly, we observed that ablation of these neurons in adult Ret-Cre/ER^T2; Rosa26^LSL-DTA mice caused a specific loss of tactile, but not chemical or thermal, electrophysiological responses. Overall, the GDNF-Ret pathway exerts two critical and distinct functions in the peripheral taste system: embryonic chemosensory cell fate determination and the specification of lingual mechanoreceptors.

An Eya1-Notch axis specifies bipotential epibranchial differentiation in mammalian craniofacial morphogenesis

Craniofacial morphogenesis requires proper development of pharyngeal arches and epibranchial placodes. We show that the epibranchial placodes, in addition to giving rise to cranial sensory neurons, generate a novel lineage-related non-neuronal cell population for mouse pharyngeal arch development. Eya1 is essential for the development of epibranchial placodes and proximal pharyngeal arches. We identify an Eya1-Notch regulatory axis that specifies both the neuronal and non-neuronal commitment of the epibranchial placode, where Notch acts downstream of Eya1 and promotes the non-neuronal cell fate. Notch is regulated by the threonine phosphatase activity of Eya1. Eya1 dephosphorylates p-threonine-2122 of the Notch1 intracellular domain (Notch1 ICD), which increases the stability of Notch1 ICD and maintains Notch signaling activity in the non-neuronal epibranchial placodal cells. Our data unveil a more complex differentiation program in epibranchial placodes and an important role for the Eya1-Notch axis in craniofacial morphogenesis.

Organizing activity of Fgf8 on the anterior telencephalon

The anterior part of the embryonic telencephalon gives rise to several brain regions that are important for animal behavior, including the frontal cortex (FC) and the olfactory bulb. The FC plays an important role in decision-making behaviors, such as social and cognitive behavior, and the olfactory bulb is involved in olfaction. Here, we show the organizing activity of fibroblast growth factor 8 (Fgf8) in the regionalization of the anterior telencephalon, specifically the FC and the olfactory bulb. Misexpression of Fgf8 in the most anterior part of the mouse telencephalon at embryonic day 11.5 (E11.5) by ex utero electroporation resulted in a lateral shift of dorsal FC subdivision markers and a lateral expansion of the dorsomedial part of the FC, the future anterior cingulate and prelimbic cortex. Fgf8-transfected brains had lacked ventral FC, including the future orbital cortex, which was replaced by the expanded olfactory bulb. The olfactory region occupied a larger area of the FC when transfection efficiency of Fgf8 was higher. These results suggest that Fgf8 regulates the proportions of the FC and olfactory bulb in the anterior telencephalon and has a medializing effect on the formation of FC subdivisions.

One-Step piggyBac Transposon-Based CRISPR/Cas9 Activation of Multiple Genes

Neural cell fate is determined by a tightly controlled transcription regulatory network during development. The ability to manipulate the expression of multiple transcription factors simultaneously is required to delineate the complex picture of neural cell development. Because of the limited carrying capacity of the commonly used viral vectors, such as lentiviral or retroviral vectors, it is often challenging to perform perturbation experiments on multiple transcription factors. Here we have developed a piggyBac (PB) transposon-based CRISPR activation (CRISPRa) all-in-one system, which allows for simultaneous and stable endogenous transactivation of multiple transcription factors and long non-coding RNAs. As a proof of principle, we showed that the PB-CRISPRa system could accelerate the differentiation of human induced pluripotent stem cells into neurons and astrocytes by triggering endogenous expression of different sets of transcription factors. The PB-CRISPRa system has the potential to become a convenient and robust tool in neuroscience, which can meet the needs of a variety of in vitro and in vivo gain-of-function applications.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Characterization of chromosomal abnormalities in pregnancy losses reveals critical genes and loci for human early development

Detailed characterization of chromosomal abnormalities, a common cause for congenital abnormalities and pregnancy loss, is critical for elucidating genes for human fetal development. Here, 2,186 product-of-conception samples were tested for copy-number variations (CNVs) at two clinical diagnostic centers using whole-genome sequencing and high-resolution chromosomal microarray analysis. We developed a new gene discovery approach to predict potential developmental genes and identified 275 candidate genes from CNVs detected from both datasets. Based on Mouse Genome Informatics (MGI) and Zebrafish model organism database (ZFIN), 75% of identified genes could lead to developmental defects when mutated. Genes involved in embryonic development, gene transcription, and regulation of biological processes were significantly enriched. Especially, transcription factors and gene families sharing specific protein domains predominated, which included known developmental genes such as HOX, NKX homeodomain genes, and helix-loop-helix containing HAND2, NEUROG2, and NEUROD1 as well as potential novel developmental genes. We observed that developmental genes were denser in certain chromosomal regions, enabling identification of 31 potential genomic loci with clustered genes associated with development.