NEURODEVELOPMENT. Shared regulatory programs suggest retention of blastula-stage potential in neural crest cells

Shared features of blastula and neural crest stem cells evolved at the base of vertebrates

The neural crest is a vertebrate-specific stem cell population that helped drive the origin and evolution of vertebrates. A distinguishing feature of these cells is their multi-germ layer potential, which has parallels to another stem cell population-pluripotent stem cells of the vertebrate blastula. Here, we investigate the evolutionary origins of neural crest potential by comparing neural crest and pluripotency gene regulatory networks of a jawed vertebrate, Xenopus, and a jawless vertebrate, lamprey. We reveal an ancient evolutionary origin of shared regulatory factors in these gene regulatory networks that dates to the last common ancestor of extant vertebrates. Focusing on the key pluripotency factor pou5, we show that a lamprey pou5 orthologue is expressed in animal pole cells but is absent from neural crest. Both lamprey and Xenopus pou5 promote neural crest formation, suggesting that pou5 activity was lost from the neural crest of jawless vertebrates or acquired along the jawed vertebrate stem. Finally, we provide evidence that pou5 acquired novel, neural crest-enhancing activity after evolving from an ancestral pou3-like clade. This work provides evidence that both the neural crest and blastula pluripotency networks arose at the base of the vertebrates and that this may be linked to functional evolution of pou5.

SoxB1 transcription factors are essential for initiating and maintaining neural plate border gene expression

SoxB1 transcription factors (Sox2/3) are well known for their role in early neural fate specification in the embryo, but little is known about functional roles for SoxB1 factors in non-neural ectodermal cell types, such as the neural plate border (NPB). Using Xenopus laevis, we set out to determine whether SoxB1 transcription factors have a regulatory function in NPB formation. Here, we show that SoxB1 factors are necessary for NPB formation, and that prolonged SoxB1 factor activity blocks the transition from a NPB to a neural crest state. Using ChIP-seq, we demonstrate that Sox3 is enriched upstream of NPB genes in early NPB cells and in blastula stem cells. Depletion of SoxB1 factors in blastula stem cells results in downregulation of NPB genes. Finally, we identify Pou5f3 factors as potential Sox3 partners in regulating the formation of the NPB and show that their combined activity is needed for normal NPB gene expression. Together, these data identify a role for SoxB1 factors in the establishment and maintenance of the NPB, in part through partnership with Pou5f3 factors.

Exploring the origins of neurodevelopmental proteasomopathies associated with cardiac malformations: are neural crest cells central to certain pathological mechanisms?

Neurodevelopmental proteasomopathies constitute a recently defined class of rare Mendelian disorders, arising from genomic alterations in proteasome-related genes. These alterations result in the dysfunction of proteasomes, which are multi-subunit protein complexes essential for maintaining cellular protein homeostasis. The clinical phenotype of these diseases manifests as a syndromic association involving impaired neural development and multisystem abnormalities, notably craniofacial anomalies and malformations of the cardiac outflow tract (OFT). These observations suggest that proteasome loss-of-function variants primarily affect specific embryonic cell types which serve as origins for both craniofacial structures and the conotruncal portion of the heart. In this hypothesis article, we propose that neural crest cells (NCCs), a highly multipotent cell population, which generates craniofacial skeleton, mesenchyme as well as the OFT of the heart, in addition to many other derivatives, would exhibit a distinctive vulnerability to protein homeostasis perturbations. Herein, we introduce the diverse cellular compensatory pathways activated in response to protein homeostasis disruption and explore their potential implications for NCC physiology. Altogether, the paper advocates for investigating proteasome biology within NCCs and their early cranial and cardiac derivatives, offering a rationale for future exploration and laying the initial groundwork for therapeutic considerations.

BET activity plays an essential role in control of stem cell attributes in Xenopus

Neural crest cells are a stem cell population unique to vertebrate embryos that retains broad multi-germ layer developmental potential through neurulation. Much remains to be learned about the genetic and epigenetic mechanisms that control the potency of neural crest cells. Here, we examine the role that epigenetic readers of the BET (bromodomain and extra terminal) family play in controlling the potential of pluripotent blastula and neural crest cells. We find that inhibiting BET activity leads to loss of pluripotency at blastula stages and a loss of neural crest at neurula stages. We compare the effects of HDAC (an eraser of acetylation marks) and BET (a reader of acetylation) inhibition and find that they lead to similar cellular outcomes through distinct effects on the transcriptome. Interestingly, loss of BET activity in cells undergoing lineage restriction is coupled to increased expression of genes linked to pluripotency and prolongs the competence of initially pluripotent cells to transit to a neural progenitor state. Together these findings advance our understanding of the epigenetic control of pluripotency and the formation of the vertebrate neural crest.

Ascidian embryonic cells with properties of neural-crest cells and neuromesodermal progenitors of vertebrates

Neural-crest cells and neuromesodermal progenitors (NMPs) are multipotent cells that are important for development of vertebrate embryos. In embryos of ascidians, which are the closest invertebrate relatives of vertebrates, several cells located at the border between the neural plate and the epidermal region have neural-crest-like properties; hence, the last common ancestor of ascidians and vertebrates may have had ancestral cells similar to neural-crest cells. However, these ascidian neural-crest-like cells do not produce cells that are commonly of mesodermal origin. Here we showed that a cell population located in the lateral region of the neural plate has properties resembling those of vertebrate neural-crest cells and NMPs. Among them, cells with Tbx6-related expression contribute to muscle near the tip of the tail region and cells with Sox1/2/3 expression give rise to the nerve cord. These observations and cross-species transcriptome comparisons indicate that these cells have properties similar to those of NMPs. Meanwhile, transcription factor genes Dlx.b, Zic-r.b and Snai, which are reminiscent of a gene circuit in vertebrate neural-crest cells, are involved in activation of Tbx6-related.b. Thus, the last common ancestor of ascidians and vertebrates may have had cells with properties of neural-crest cells and NMPs and such ancestral cells may have produced cells commonly of ectodermal and mesodermal origins.

Epithelial-mesenchymal transition in tissue repair and degeneration

Epithelial-mesenchymal transitions (EMTs) are the epitome of cell plasticity in embryonic development and cancer; during EMT, epithelial cells undergo dramatic phenotypic changes and become able to migrate to form different tissues or give rise to metastases, respectively. The importance of EMTs in other contexts, such as tissue repair and fibrosis in the adult, has become increasingly recognized and studied. In this Review, we discuss the function of EMT in the adult after tissue damage and compare features of embryonic and adult EMT. Whereas sustained EMT leads to adult tissue degeneration, fibrosis and organ failure, its transient activation, which confers phenotypic and functional plasticity on somatic cells, promotes tissue repair after damage. Understanding the mechanisms and temporal regulation of different EMTs provides insight into how some tissues heal and has the potential to open new therapeutic avenues to promote repair or regeneration of tissue damage that is currently irreversible. We also discuss therapeutic strategies that modulate EMT that hold clinical promise in ameliorating fibrosis, and how precise EMT activation could be harnessed to enhance tissue repair.

Small molecule-mediated reprogramming of Xenopus blastula stem cells to a neural crest state

Neural crest cells are a stem cell population unique to vertebrates that give rise to a diverse array of derivatives, including much of the peripheral nervous system, pigment cells, cartilage, mesenchyme, and bone. Acquisition of these cells drove the evolution of vertebrates and defects in their development underlies a broad set of neurocristopathies. Moreover, studies of neural crest can inform differentiation protocols for pluripotent stem cells and regenerative medicine applications. Xenopus embryos are an important system for studies of the neural crest and have provided numerous insights into the signals and transcription factors that control the formation and later lineage diversification of these stem cells. Pluripotent animal pole explants are a particularly powerful tool in this system as they can be cultured in simple salt solution and instructed to give rise to any cell type including the neural crest. Here we report a protocol for small molecule-mediated induction of the neural crest state from blastula stem cells and validate it using transcriptome analysis and grafting experiments. This is an powerful new tool for generating this important cell type that will facilitate future studies of neural crest development and mutations and variants linked to neurocristopathies.

Shared features of blastula and neural crest stem cells evolved at the base of vertebrates

The neural crest is vertebrate-specific stem cell population that helped drive the origin and evolution of the vertebrate clade. A distinguishing feature of these stem cells is their multi-germ layer potential, which has drawn developmental and evolutionary parallels to another stem cell population-pluripotent embryonic stem cells (animal pole cells or ES cells) of the vertebrate blastula. Here, we investigate the evolutionary origins of neural crest potential by comparing neural crest and pluripotency gene regulatory networks (GRNs) in both jawed ( Xenopus ) and jawless (lamprey) vertebrates. Through comparative gene expression analysis and transcriptomics, we reveal an ancient evolutionary origin of shared regulatory factors between neural crest and pluripotency GRNs that dates back to the last common ancestor of extant vertebrates. Focusing on the key pluripotency factor pou5 (formerly oct4), we show that the lamprey genome encodes a pou5 ortholog that is expressed in animal pole cells, as in jawed vertebrates, but is absent from the neural crest. However, gain-of-function experiments show that both lamprey and Xenopus pou5 enhance neural crest formation, suggesting that pou5 was lost from the neural crest of jawless vertebrates. Finally, we show that pou5 is required for neural crest specification in jawed vertebrates and that it acquired novel neural crest-enhancing activity after evolving from an ancestral pou3 -like clade that lacks this functionality. We propose that a pluripotency-neural crest GRN was assembled in stem vertebrates and that the multi-germ layer potential of the neural crest evolved by deploying this regulatory program.

Cranial Neural Crest Cells Contribution to Craniofacial Bone Development and Regeneration

PURPOSE OF REVIEW

This review aims to summarize (i) the latest evidence on cranial neural crest cells (CNCC) contribution to craniofacial development and ossification; (ii) the recent discoveries on the mechanisms responsible for their plasticity; and (iii) the newest procedures to ameliorate maxillofacial tissue repair.

RECENT FINDINGS

CNCC display a remarkable differentiation potential that exceeds the capacity of their germ layer of origin. The mechanisms by which they expand their plasticity was recently described. Their ability to participate to craniofacial bone development and regeneration open new perspectives for treatments of traumatic craniofacial injuries or congenital syndromes. These conditions can be life-threatening, require invasive maxillofacial surgery and can leave deep sequels on our health or quality of life. With accumulating evidence showing how CNCC-derived stem cells potential can ameliorate craniofacial reconstruction and tissue repair, we believe a deeper understanding of the mechanisms regulating CNCC plasticity is essential to ameliorate endogenous regeneration and improve tissue repair therapies.

The Foxi3 transcription factor is necessary for the fate restriction of placodal lineages at the neural plate border

The Foxi3 transcription factor, expressed in the neural plate border at the end of gastrulation, is necessary for the formation of posterior placodes and is thus important for ectodermal patterning. We have created two knock-in mouse lines expressing GFP or a tamoxifen-inducible Cre recombinase to show that Foxi3 is one of the earliest genes to label the border between the neural tube and epidermis, and that Foxi3-expressing neural plate border progenitors contribute primarily to cranial placodes and epidermis from the onset of expression, but not to the neural crest or neural tube lineages. By simultaneously knocking out Foxi3 in neural plate border cells and following their fates, we show that neural plate border cells lacking Foxi3 contribute to all four lineages of the ectoderm - placodes, epidermis, crest and neural tube. We contrast Foxi3 with another neural plate border transcription factor, Zic5, the progenitors of which initially contribute broadly to all germ layers until gastrulation and gradually become restricted to the neural crest lineage and dorsal neural tube cells. Our study demonstrates that Foxi3 uniquely acts early at the neural plate border to restrict progenitors to a placodal and epidermal fate.

SoxB1 transcription factors are essential for initiating and maintaining the neural plate border gene expression

SoxB1 transcription factors (Sox2/3) are well known for their role in early neural fate specification in the embryo, but little is known about functional roles for SoxB1 factors in non-neural ectodermal cell types, such as the neural plate border (NPB). Using Xenopus laevis , we set out to determine if SoxB1 transcription factors have a regulatory function in NPB formation. Herein, we show that SoxB1 factors are necessary for NPB formation, and that prolonged SoxB1 factor activity blocks the transition from a NPB to a neural crest state. Using ChIP-seq we demonstrate that Sox3 is enriched upstream of NPB genes in early NPB cells and, surprisingly, in blastula stem cells. Depletion of SoxB1 factors in blastula stem cells results in downregulation of NPB genes. Finally, we identify Pou5f3 factors as a potential SoxB1 partners in regulating the formation of the NPB and show their combined activity is needed to maintain NPB gene expression. Together, these data identify a novel role for SoxB1 factors in the establishment and maintenance of the NPB, in part through partnership with Pou5f3 factors.

Maintenance of pluripotency-like signature in the entire ectoderm leads to neural crest stem cell potential

The ability of the pluripotent epiblast to contribute progeny to all three germ layers is thought to be lost after gastrulation. The later-forming neural crest (NC) rises from ectoderm and it remains poorly understood how its exceptionally high stem-cell potential to generate mesodermal- and endodermal-like derivatives is obtained. Here, we monitor transcriptional changes from gastrulation to neurulation using single-cell-Multiplex-Spatial-Transcriptomics (scMST) complemented with RNA-sequencing. We show maintenance of pluripotency-like signature (Nanog, Oct4/PouV, Klf4-positive) in undecided pan-ectodermal stem-cells spanning the entire ectoderm late during neurulation with ectodermal patterning completed only at the end of neurulation when the pluripotency-like signature becomes restricted to NC, challenging our understanding of gastrulation. Furthermore, broad ectodermal pluripotency-like signature is found at multiple axial levels unrelated to the NC lineage the cells later commit to, suggesting a general role in stemness enhancement and proposing a mechanism by which the NC acquires its ability to form derivatives beyond "ectodermal-capacity" in chick and mouse embryos.

Shaping faces: genetic and epigenetic control of craniofacial morphogenesis

Major differences in facial morphology distinguish vertebrate species. Variation of facial traits underlies the uniqueness of human individuals, and abnormal craniofacial morphogenesis during development leads to birth defects that significantly affect quality of life. Studies during the past 40 years have advanced our understanding of the molecular mechanisms that establish facial form during development, highlighting the crucial roles in this process of a multipotent cell type known as the cranial neural crest cell. In this Review, we discuss recent advances in multi-omics and single-cell technologies that enable genes, transcriptional regulatory networks and epigenetic landscapes to be closely linked to the establishment of facial patterning and its variation, with an emphasis on normal and abnormal craniofacial morphogenesis. Advancing our knowledge of these processes will support important developments in tissue engineering, as well as the repair and reconstruction of the abnormal craniofacial complex.

scRNA-sequencing in chick suggests a probabilistic model for cell fate allocation at the neural plate border

The vertebrate 'neural plate border' is a transient territory located at the edge of the neural plate containing precursors for all ectodermal derivatives: the neural plate, neural crest, placodes and epidermis. Elegant functional experiments in a range of vertebrate models have provided an in-depth understanding of gene regulatory interactions within the ectoderm. However, these experiments conducted at tissue level raise seemingly contradictory models for fate allocation of individual cells. Here, we carry out single cell RNA sequencing of chick ectoderm from primitive streak to neurulation stage, to explore cell state diversity and heterogeneity. We characterise the dynamics of gene modules, allowing us to model the order of molecular events which take place as ectodermal fates segregate. Furthermore, we find that genes previously classified as neural plate border 'specifiers' typically exhibit dynamic expression patterns and are enriched in either neural, neural crest or placodal fates, revealing that the neural plate border should be seen as a heterogeneous ectodermal territory and not a discrete transitional transcriptional state. Analysis of neural, neural crest and placodal markers reveals that individual NPB cells co-express competing transcriptional programmes suggesting that their ultimate identify is not yet fixed. This population of 'border located undecided progenitors' (BLUPs) gradually diminishes as cell fate decisions take place. Considering our findings, we propose a probabilistic model for cell fate choice at the neural plate border. Our data suggest that the probability of a progenitor's daughters to contribute to a given ectodermal derivative is related to the balance of competing transcriptional programmes, which in turn are regulated by the spatiotemporal position of a progenitor.

Neural induction drives body axis formation during embryogenesis, but a neural induction-like process drives tumorigenesis in postnatal animals

Characterization of cancer cells and neural stem cells indicates that tumorigenicity and pluripotency are coupled cell properties determined by neural stemness, and tumorigenesis represents a process of progressive loss of original cell identity and gain of neural stemness. This reminds of a most fundamental process required for the development of the nervous system and body axis during embryogenesis, i.e., embryonic neural induction. Neural induction is that, in response to extracellular signals that are secreted by the Spemann-Mangold organizer in amphibians or the node in mammals and inhibit epidermal fate in ectoderm, the ectodermal cells lose their epidermal fate and assume the neural default fate and consequently, turn into neuroectodermal cells. They further differentiate into the nervous system and also some non-neural cells via interaction with adjacent tissues. Failure in neural induction leads to failure of embryogenesis, and ectopic neural induction due to ectopic organizer or node activity or activation of embryonic neural genes causes a formation of secondary body axis or a conjoined twin. During tumorigenesis, cells progressively lose their original cell identity and gain of neural stemness, and consequently, gain of tumorigenicity and pluripotency, due to various intra-/extracellular insults in cells of a postnatal animal. Tumorigenic cells can be induced to differentiation into normal cells and integrate into normal embryonic development within an embryo. However, they form tumors and cannot integrate into animal tissues/organs in a postnatal animal because of lack of embryonic inducing signals. Combination of studies of developmental and cancer biology indicates that neural induction drives embryogenesis in gastrulating embryos but a similar process drives tumorigenesis in a postnatal animal. Tumorigenicity is by nature the manifestation of aberrant occurrence of pluripotent state in a postnatal animal. Pluripotency and tumorigenicity are both but different manifestations of neural stemness in pre- and postnatal stages of animal life, respectively. Based on these findings, I discuss about some confusion in cancer research, propose to distinguish the causality and associations and discriminate causal and supporting factors involved in tumorigenesis, and suggest revisiting the focus of cancer research.

Reassessing the embryonic origin and potential of craniofacial ectomesenchyme

Of all the cell types arising from the neural crest, ectomesenchyme is likely the most unusual. In contrast to the neuroglial cells generated by neural crest throughout the embryo, consistent with its ectodermal origin, cranial neural crest-derived cells (CNCCs) generate many connective tissue and skeletal cell types in common with mesoderm. Whether this ectoderm-derived mesenchyme (ectomesenchyme) potential reflects a distinct developmental origin from other CNCC lineages, and/or epigenetic reprogramming of the ectoderm, remains debated. Whereas decades of lineage tracing studies have defined the potential of CNCC ectomesenchyme, these are being revisited by modern genetic techniques. Recent work is also shedding light on the extent to which intrinsic and extrinsic cues determine ectomesenchyme potential, and whether maintenance or reacquisition of CNCC multipotency influences craniofacial repair.

Nuclear receptor Nr5a2 promotes diverse connective tissue fates in the jaw

Organ development involves the sustained production of diverse cell types with spatiotemporal precision. In the vertebrate jaw, neural-crest-derived progenitors produce not only skeletal tissues but also later-forming tendons and salivary glands. Here we identify the pluripotency factor Nr5a2 as essential for cell-fate decisions in the jaw. In zebrafish and mice, we observe transient expression of Nr5a2 in a subset of mandibular postmigratory neural-crest-derived cells. In zebrafish nr5a2 mutants, nr5a2-expressing cells that would normally form tendons generate excess jaw cartilage. In mice, neural-crest-specific Nr5a2 loss results in analogous skeletal and tendon defects in the jaw and middle ear, as well as salivary gland loss. Single-cell profiling shows that Nr5a2, distinct from its roles in pluripotency, promotes jaw-specific chromatin accessibility and gene expression that is essential for tendon and gland fates. Thus, repurposing of Nr5a2 promotes connective tissue fates to generate the full repertoire of derivatives required for jaw and middle ear function.

Post-transcriptional regulation in cranial neural crest cells expands developmental potential

Developmental potential is progressively restricted after germ layer specification during gastrulation. However, cranial neural crest cells challenge this paradigm, as they develop from anterior ectoderm, yet give rise to both ectodermal derivatives of the peripheral nervous system and ectomesenchymal bone and cartilage. How cranial neural crest cells differentiate into multiple lineages is poorly understood. Here, we demonstrate that cranial neural crest cells possess a transient state of increased chromatin accessibility. We profile the spatiotemporal emergence of premigratory neural crest and find evidence of lineage bias toward either a neuronal or ectomesenchymal fate, with each expressing distinct factors from earlier stages of development. We identify the miR-302 miRNA family to be highly expressed in cranial neural crest cells and genetic deletion leads to precocious specification of the ectomesenchymal lineage. Loss of mir-302 results in reduced chromatin accessibility in the neuronal progenitor lineage of neural crest and a reduction in peripheral neuron differentiation. Mechanistically, we find that mir-302 directly targets Sox9 to slow the timing of ectomesenchymal neural crest specification and represses multiple genes involved in chromatin condensation to promote accessibility required for neuronal differentiation. Our findings reveal a posttranscriptional mechanism governed by miRNAs to expand developmental potential of cranial neural crest.

Maintenance of pluripotency in the entire ectoderm enables neural crest formation

The ability of the pluripotent epiblast to contribute progeny to all three germ layers is thought to be lost after gastrulation. The later-forming neural crest (NC) rises from ectoderm and it remains poorly understood how its exceptionally high stem-cell potential to generate mesodermal- and endodermal-like cells is obtained. We monitored transcriptional changes from gastrulation to neurulation using single-cell-Multiplex-Spatial-Transcriptomics (scMST) complemented with RNA-sequencing. Unexpectedly, we find maintenance of undecided Nanog/Oct4-PouV/Klf4-positive pluripotent-like pan-ectodermal stem-cells spanning the entire ectoderm late in the neurulation process with ectodermal patterning completed only at the end of neurulation when pluripotency becomes restricted to NC, challenging our understanding of gastrulation. Furthermore, broad ectodermal pluripotency is found at all axial levels unrelated to the NC lineage the cells later commit to, suggesting a general role in stemness enhancement and proposing a mechanism by which the NC acquires its ability to form derivatives beyond "ectodermal-capacity" in chick and mouse embryos.

Membrane potential drives the exit from pluripotency and cell fate commitment via calcium and mTOR

Transitioning from pluripotency to differentiated cell fates is fundamental to both embryonic development and adult tissue homeostasis. Improving our understanding of this transition would facilitate our ability to manipulate pluripotent cells into tissues for therapeutic use. Here, we show that membrane voltage (Vm) regulates the exit from pluripotency and the onset of germ layer differentiation in the embryo, a process that affects both gastrulation and left-right patterning. By examining candidate genes of congenital heart disease and heterotaxy, we identify KCNH6, a member of the ether-a-go-go class of potassium channels that hyperpolarizes the Vm and thus limits the activation of voltage gated calcium channels, lowering intracellular calcium. In pluripotent embryonic cells, depletion of kcnh6 leads to membrane depolarization, elevation of intracellular calcium levels, and the maintenance of a pluripotent state at the expense of differentiation into ectodermal and myogenic lineages. Using high-resolution temporal transcriptome analysis, we identify the gene regulatory networks downstream of membrane depolarization and calcium signaling and discover that inhibition of the mTOR pathway transitions the pluripotent cell to a differentiated fate. By manipulating Vm using a suite of tools, we establish a bioelectric pathway that regulates pluripotency in vertebrates, including human embryonic stem cells.

Pluripotency factors are repurposed to shape the epigenomic landscape of neural crest cells

Yamanaka factors are essential for establishing pluripotency in embryonic stem cells, but their function in multipotent stem cell populations is poorly understood. Here, we show that OCT4 and SOX2 cooperate with tissue-specific transcription factors to promote neural crest formation. By assessing avian and human neural crest cells at distinct developmental stages, we characterized the epigenomic changes that occur during their specification, migration, and early differentiation. This analysis determined that the OCT4-SOX2 dimer is required to establish a neural crest epigenomic signature that is lost upon cell fate commitment. The OCT4-SOX2 genomic targets in the neural crest differ from those of embryonic stem cells, indicating the dimer displays context-specific functions. Binding of OCT4-SOX2 to neural crest enhancers requires pioneer factor TFAP2A, which physically interacts with the dimer to modify its genomic targets. Our results demonstrate how Yamanaka factors are repurposed in multipotent cells to control chromatin organization and define their developmental potential.

Regulation of Oct4 in stem cells and neural crest cells

During embryonic development, cells gradually restrict their developmental potential as they exit pluripotency and differentiate into various cell types. The POU transcription factor Oct4 (encoded by Pou5f1) lies at the center of the pluripotency machinery that regulates stemness and differentiation in stem cells, and is required for reprogramming of somatic cells into induced pluripotent stem cells (iPSCs). Several studies have revealed that Oct4 and other stemness genes are also expressed in multipotent cell populations such as neural crest cells (NCCs), and are required to expand the NCC developmental potential. Transcriptional regulation of Oct4 has been studied extensively in stem cells during early embryonic development and reprogramming, but not in NCCs. Here, we review how Oct4 is regulated in pluripotent stem cells, and address some of the gaps in knowledge about regulation of the pluripotency network in NCCs.

Tissue Rotation of the Xenopus Anterior-Posterior Neural Axis Reveals Profound but Transient Plasticity at the Mid-Gastrula Stage

The establishment of anterior-posterior (AP) regional identity is an essential step in the appropriate development of the vertebrate central nervous system. An important aspect of AP neural axis formation is the inherent plasticity that allows developing cells to respond to and recover from the various perturbations that embryos continually face during the course of development. While the mechanisms governing the regionalization of the nervous system have been extensively studied, relatively less is known about the nature and limits of early neural plasticity of the anterior-posterior neural axis. This study aims to characterize the degree of neural axis plasticity in Xenopus laevis by investigating the response of embryos to a 180-degree rotation of their AP neural axis during gastrula stages by assessing the expression of regional marker genes using in situ hybridization. Our results reveal the presence of a narrow window of time between the mid- and late gastrula stage, during which embryos are able undergo significant recovery following a 180-degree rotation of their neural axis and eventually express appropriate regional marker genes including Otx, Engrailed, and Krox. By the late gastrula stage, embryos show misregulation of regional marker genes following neural axis rotation, suggesting that this profound axial plasticity is a transient phenomenon that is lost by late gastrula stages.

The homeodomain transcription factor Ventx2 regulates respiratory progenitor cell number and differentiation timing during Xenopus lung development

Ventx2 is an Antennapedia superfamily/NK-like subclass homeodomain transcription factor best known for its roles in the regulation of early dorsoventral patterning during Xenopus gastrulation and in the maintenance of neural crest multipotency. In this work we characterize the spatiotemporal expression pattern of ventx2 in progenitor cells of the Xenopus respiratory system epithelium. We find that ventx2 is directly induced by BMP signaling in the ventral foregut prior to nkx2-1, the earliest epithelial marker of the respiratory lineage. Functional studies demonstrate that Ventx2 regulates the number of Nkx2-1/Sox9+ respiratory progenitor cells induced during foregut development, the timing and level of surfactant protein gene expression, and proper tracheal-esophageal separation. Our data suggest that Ventx2 regulates the balance of respiratory progenitor cell expansion and differentiation. While the ventx gene family has been lost from the mouse genome during evolution, humans have retained a ventx2-like gene (VENTX). Finally, we discuss how our findings might suggest a possible function of VENTX in human respiratory progenitor cells.

Multi-layered transcriptional control of cranial neural crest development

The neural crest (NC) is an emblematic population of embryonic stem-like cells with remarkable migratory ability. These distinctive attributes have inspired the curiosity of developmental biologists for over 150 years, however only recently the regulatory mechanisms controlling the complex features of the NC have started to become elucidated at genomic scales. Regulatory control of NC development is achieved through combinatorial transcription factor binding and recruitment of associated transcriptional complexes to distal cis-regulatory elements. Together, they regulate when, where and to what extent transcriptional programmes are actively deployed, ultimately shaping ontogenetic processes. Here, we discuss how transcriptional networks control NC ontogeny, with a special emphasis on the molecular mechanisms underlying specification of the cephalic NC. We also cover emerging properties of transcriptional regulation revealed in diverse developmental systems, such as the role of three-dimensional conformation of chromatin, and how they are involved in the regulation of NC ontogeny. Finally, we highlight how advances in deciphering the NC transcriptional network have afforded new insights into the molecular basis of human diseases.

Pluripotency factors regulate the onset of Hox cluster activation in the early embryo

Pluripotent cells are a transient population of the mammalian embryo dependent on transcription factors, such as OCT4 and NANOG, which maintain pluripotency while suppressing lineage specification. However, these factors are also expressed during early phases of differentiation, and their role in the transition from pluripotency to lineage specification is largely unknown. We found that pluripotency factors play a dual role in regulating key lineage specifiers, initially repressing their expression and later being required for their proper activation. We show that Oct4 is necessary for activation of HoxB genes during differentiation of embryonic stem cells and in the embryo. In addition, we show that the HoxB cluster is coordinately regulated by OCT4 binding sites located at the 3' end of the cluster. Our results show that core pluripotency factors are not limited to maintaining the precommitted epiblast but are also necessary for the proper deployment of subsequent developmental programs.

Folate Carrier Deficiency Drives Differential Methylation and Enhanced Cellular Potency in the Neural Plate Border

The neural plate border (NPB) of vertebrate embryos segregates from the neural and epidermal regions, and it is comprised of an intermingled group of multipotent progenitor cells. Folate is the precursor of S-adenosylmethionine, the main methyl donor for DNA methylation, and it is critical for embryonic development, including the specification of progenitors which reside in the NPB. Despite the fact that several intersecting signals involved in the specification and territorial restriction of NPB cells are known, the role of epigenetics, particularly DNA methylation, has been a matter of debate. Here, we examined the temporal and spatial distribution of the methyl source and analyzed the abundance of 5mC/5 hmC and their epigenetic writers throughout the segregation of the neural and NPB territories. Reduced representation bisulfite sequencing (RRBS) on Reduced Folate Carrier 1 (RFC1)-deficient embryos leads to the identification of differentially methylated regions (DMRs). In the RFC1-deficient embryos, we identified several DMRs in the Notch1 locus, and the spatiotemporal expression of Notch1 and its downstream target gene Bmp4 were expanded in the NPB. Cell fate analysis on folate deficient embryos revealed a significant increase in the number of cells coexpressing both neural (SOX2) and NPB (PAX7) markers, which may represent an enhancing effect in the cellular potential of those progenitors. Taken together, our findings propose a model where the RFC1 deficiency drives methylation changes in specific genomic regions that are correlated with a dysregulation of pathways involved in early development such as Notch1 and BMP4 signaling. These changes affect the potency of the progenitors residing in the juncture of the neural plate and NPB territories, thus driving them to a primed state.

On the evolutionary origins and regionalization of the neural crest

The neural crest is a vertebrate-specific embryonic stem cell population that gives rise to a vast array of cell types throughout the animal body plan. These cells are first born at the edges of the central nervous system, from which they migrate extensively and differentiate into multiple cellular derivatives. Given the unique set of structures these cells comprise, the origin of the neural crest is thought to have important implications for the evolution and diversification of the vertebrate clade. In jawed vertebrates, neural crest cells exist as distinct subpopulations along the anterior-posterior axis. These subpopulations differ in terms of their respective differentiation potential and cellular derivatives. Thus, the modern neural crest is characterized as multipotent, migratory, and regionally segregated throughout the embryo. Here, we retrace the evolutionary origins of the neural crest, from the appearance of conserved regulatory circuitry in basal chordates to the emergence of neural crest subpopulations in higher vertebrates. Finally, we discuss a stepwise trajectory by which these cells may have arisen and diversified throughout vertebrate evolution.

Cellular plasticity in the neural crest and cancer

In vertebrates, neural crest cells (NCCs) are a multipotent embryonic population generating both neural/neuronal and mesenchymal derivatives, and thus the neural crest (NC) is often referred to as the fourth germ layer. NC development is a dynamic process, where NCCs possess substantial plasticity in transcriptional and epigenomic profiles. Recent technical advances in single-cell and low-input sequencing have empowered fine-resolution characterisation of NC development. In this review, we summarise the latest models underlying NC-plasticity acquirement and cell-fate restriction, outline the connections between NC plasticity and NC-derived cancer and envision the new opportunities in studying NC plasticity and its link to cancer.

Neural stemness unifies cell tumorigenicity and pluripotent differentiation potential

Neural stemness is suggested to be the ground state of tumorigenicity and pluripotent differentiation potential. However, the relationship between these cell properties is unclear. Here, by disrupting the neural regulatory network in neural stem and cancer cells and by serial transplantation of cancer cells, we show that tumorigenicity and pluripotent differentiation potential are coupled cell properties unified by neural stemness. We show that loss of neural stemness via inhibition of SETDB1, an oncoprotein with enriched expression in embryonic neural cells during vertebrate embryogenesis, led to neuronal differentiation with reduced tumorigenicity and pluripotent differentiation potential in neural stem and cancer cells, whereas enhancement of neural stemness by SETDB1 overexpression caused the opposite effects. SETDB1 maintains a regulatory network comprising proteins involved in developmental programs and basic cellular functional machineries, including epigenetic modifications (EZH2), ribosome biogenesis (RPS3), translation initiation (EIF4G), and spliceosome assembly (SF3B1); all of these proteins are enriched in embryonic neural cells and play active roles in cancers. In addition, SETDB1 represses the transcription of genes promoting differentiation and cell cycle and growth arrest. Serial transplantation of cancer cells showed that neural stemness, tumorigenicity, and pluripotent differentiation potential were simultaneously enhanced; these effects were accompanied by increased expression of proteins involved in developmental programs and basic machineries, including SETDB1 and the abovementioned proteins, as well as by increased alternative splicing events. These results indicate that basic machineries work together to define a highly proliferative state with pluripotent differentiation potential and also suggest that neural stemness unifies tumorigenicity and differentiation potential.

Intramuscular delivery of neural crest stem cell spheroids enhances neuromuscular regeneration after denervation injury

BACKGROUND

Muscle denervation from trauma and motor neuron disease causes disabling morbidities. A limiting step in functional recovery is the regeneration of neuromuscular junctions (NMJs) for reinnervation. Stem cells have the potential to promote these regenerative processes, but current approaches have limited success, and the optimal types of stem cells remain to be determined. Neural crest stem cells (NCSCs), as the developmental precursors of the peripheral nervous system, are uniquely advantageous, but the role of NCSCs in neuromuscular regeneration is not clear. Furthermore, a cell delivery approach that can maintain NCSC survival upon transplantation is critical.

METHODS

We established a streamlined protocol to derive, isolate, and characterize functional p75+ NCSCs from human iPSCs without genome integration of reprogramming factors. To enhance survival rate upon delivery in vivo, NCSCs were centrifuged in microwell plates to form spheroids of desirable size by controlling suspension cell density. Human bone marrow mesenchymal stem cells (MSCs) were also studied for comparison. NCSC or MSC spheroids were injected into the gastrocnemius muscle with denervation injury, and the effects on NMJ formation and functional recovery were investigated. The spheroids were also co-cultured with engineered neuromuscular tissue to assess effects on NMJ formation in vitro.

RESULTS

NCSCs cultured in spheroids displayed enhanced secretion of soluble factors involved in neuromuscular regeneration. Intramuscular transplantation of spheroids enabled long-term survival and retention of NCSCs, in contrast to the transplantation of single-cell suspensions. Furthermore, NCSC spheroids significantly improved functional recovery after four weeks as shown by gait analysis, electrophysiology, and the rate of NMJ innervation. MSC spheroids, on the other hand, had insignificant effect. In vitro co-culture of NCSC or MSC spheroids with engineered myotubes and motor neurons further evidenced improved innervated NMJ formation with NCSC spheroids.

CONCLUSIONS

We demonstrate that stem cell type is critical for neuromuscular regeneration and that NCSCs have a distinct advantage and therapeutic potential to promote reinnervation following peripheral nerve injury. Biophysical effects of spheroidal culture, in particular, enable long-term NCSC survival following in vivo delivery. Furthermore, synthetic neuromuscular tissue, or "tissues-on-a-chip," may offer a platform to evaluate stem cells for neuromuscular regeneration.

The developmental and evolutionary origins of cellular pluripotency in the vertebrate neural crest

Neural crest cells are central to vertebrate development and evolution, endowing vertebrates with a "new head" that resulted in morphological, physiological, and behavioral features that allowed vertebrates to become active predators. One remarkable feature of neural crest cells is their multi-germ layer potential that allows for the formation of both ectodermal (pigmentation, peripheral glia, sensory neurons) and mesenchymal (connective tissue, cartilage/bone, dermis) cell types. Understanding the cellular and evolutionary origins of this broad cellular potential in the neural crest has been a long-standing focus for developmental biologists. Here, we review recent work that has demonstrated that neural crest cells share key features with pluripotent blastula stem cells, including expression of the Yamanaka stem cell factors (Oct3/4, Klf4, Sox2, c-Myc). These shared features suggest that pluripotency is either retained in the neural crest from blastula stages or subsequently reactivated as the neural crest forms. We highlight the cellular and molecular parallels between blastula stem cells and neural crest cells and discuss the work that has led to current models for the cellular origins of broad potential in the crest. Finally, we explore how these themes can provide new insights into how and when neural crest cells and pluripotency evolved in vertebrates and the evolutionary relationship between these populations.

Expression atlas of avian neural crest proteins: Neurulation to migration

Neural crest (NC) cells are a dynamic population of embryonic stem cells that create various adult tissues in vertebrate species including craniofacial bone and cartilage and the peripheral and enteric nervous systems. NC development is thought to be a conserved and complex process that is controlled by a tightly-regulated gene regulatory network (GRN) of morphogens, transcription factors, and cell adhesion proteins. While multiple studies have characterized the expression of several GRN factors in single species, a comprehensive protein analysis that directly compares expression across development is lacking. To address this lack in information, we used three closely related avian models, Gallus gallus (chicken), Coturnix japonica (Japanese quail), and Pavo cristatus (Indian peafowl), to compare the localization and timing of four GRN transcription factors, PAX7, SNAI2, SOX9, and SOX10, from the onset of neurulation to migration. While the spatial expression of these factors is largely conserved, we find that quail NC cells express SNAI2, SOX9, and SOX10 proteins at the equivalent of earlier developmental stages than chick and peafowl. In addition, quail NC cells migrate farther and more rapidly than the larger organisms. These data suggest that despite a conservation of NC GRN players, differences in the timing of NC development between species remain a significant frontier to be explored with functional studies.

Proliferation maintains the undifferentiated status of stem cells: The role of the planarian cell cycle regulator Cdh1

The coincidence of cell cycle exit and differentiation has been described in a wide variety of stem cells and organisms for decades, but the causal relationship is still unclear due to the complicated regulation of the cell cycle. Here, we used the planarian Dugesia japonica since they may possess a simple cell cycle regulation in which Cdh1 is one of the factors responsible for exiting the cell cycle. When cdh1 was functionally inhibited, the planarians could not maintain their tissue homeostasis and could not regenerate their missing body parts. While the knockdown of cdh1 caused pronounced accumulation of the stem cells, the progenitor and differentiated cells were decreased. Further analyses indicated that the stem cells with cdh1 knockdown did not undergo differentiation even though they received ERK signaling activation as an induction signal. These results suggested that stem cells could not acquire differentiation competence without cell cycle exit. Thus, we propose that cell cycle regulation determines the differentiation competence and that cell cycle exit to G0 enables stem cells to undergo differentiation.

Cyclical fate restriction: a new view of neural crest cell fate specification

Neural crest cells are crucial in development, not least because of their remarkable multipotency. Early findings stimulated two hypotheses for how fate specification and commitment from fully multipotent neural crest cells might occur, progressive fate restriction (PFR) and direct fate restriction, differing in whether partially restricted intermediates were involved. Initially hotly debated, they remain unreconciled, although PFR has become favoured. However, testing of a PFR hypothesis of zebrafish pigment cell development refutes this view. We propose a novel 'cyclical fate restriction' hypothesis, based upon a more dynamic view of transcriptional states, reconciling the experimental evidence underpinning the traditional hypotheses.

Neural is Fundamental: Neural Stemness as the Ground State of Cell Tumorigenicity and Differentiation Potential

Tumorigenic cells are similar to neural stem cells or embryonic neural cells in regulatory networks, tumorigenicity and pluripotent differentiation potential. By integrating the evidence from developmental biology, tumor biology and evolution, I will make a detailed discussion on the observations and propose that neural stemness underlies two coupled cell properties, tumorigenicity and pluripotent differentiation potential. Neural stemness property of tumorigenic cells can hopefully integrate different observations/concepts underlying tumorigenesis. Neural stem cells and tumorigenic cells share regulatory networks; both exhibit neural stemness, tumorigenicity and pluripotent differentiation potential; both depend on expression or activation of ancestral genes; both rely primarily on aerobic glycolytic metabolism; both can differentiate into various cells/tissues that are derived from three germ layers, leading to tumor formation resembling severely disorganized or more degenerated process of embryonic tissue differentiation; both are enriched in long genes with more splice variants that provide more plastic scaffolds for cell differentiation, etc. Neural regulatory networks, which include higher levels of basic machineries of cell physiological functions and developmental programs, work concertedly to define a basic state with fast cell cycle and proliferation. This is predestined by the evolutionary advantage of neural state, the ground or initial state for multicellularity with adaptation to an ancient environment. Tumorigenesis might represent a process of restoration of neural ground state, thereby restoring a state with fast proliferation and pluripotent differentiation potential in somatic cells. Tumorigenesis and pluripotent differentiation potential might be better understood from understanding neural stemness, and cancer therapy should benefit more from targeting neural stemness.

Seq Your Destiny: Neural Crest Fate Determination in the Genomic Era

Neural crest stem/progenitor cells arise early during vertebrate embryogenesis at the border of the forming central nervous system. They subsequently migrate throughout the body, eventually differentiating into diverse cell types ranging from neurons and glia of the peripheral nervous system to bones of the face, portions of the heart, and pigmentation of the skin. Along the body axis, the neural crest is heterogeneous, with different subpopulations arising in the head, neck, trunk, and tail regions, each characterized by distinct migratory patterns and developmental potential. Modern genomic approaches like single-cell RNA- and ATAC-sequencing (seq) have greatly enhanced our understanding of cell lineage trajectories and gene regulatory circuitry underlying the developmental progression of neural crest cells. Here, we discuss how genomic approaches have provided new insights into old questions in neural crest biology by elucidating transcriptional and posttranscriptional mechanisms that govern neural crest formation and the establishment of axial level identity. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Neural crest multipotency and specification: power and limits of single cell transcriptomic approaches

The neural crest is a unique population of multipotent cells forming in vertebrate embryos. Their vast cell fate potential enables the generation of a diverse array of differentiated cell types in vivo. These include, among others, connective tissue, cartilage and bone of the face and skull, neurons and glia of the peripheral nervous system (including enteric nervous system), and melanocytes. Following migration, these derivatives extensively populate multiple germ layers. Within the competent neural border ectoderm, an area located at the junction between the neural and non-neural ectoderm during embryonic development, neural crest cells form in response to a series of inductive secreted cues including BMP, Wnt, and FGF signals. As cells become progressively specified, they express transcriptional modules conducive with their stage of fate determination or cell state. Those sequential states include the neural border state, the premigratory neural crest state, the epithelium-to-mesenchyme transitional state, and the migratory state to end with post-migratory and differentiation states. However, despite the extensive knowledge accumulated over 150 years of neural crest biology, many key questions remain open, in particular the timing of neural crest lineage determination, the control of potency during early developmental stages, and the lineage relationships between different subpopulations of neural crest cells. In this review, we discuss the recent advances in understanding early neural crest formation using cutting-edge high-throughput single cell sequencing approaches. We will discuss how this new transcriptomic data, from 2017 to 2021, has advanced our knowledge of the steps in neural crest cell lineage commitment and specification, the mechanisms driving multipotency, and diversification. We will then discuss the questions that remain to be resolved and how these approaches may continue to unveil the biology of these fascinating cells.

Craniofacial Development: Neural Crest in Molecular Embryology

Craniofacial development, one of the most complex sequences of developmental events in embryology, features a uniquely transient, pluripotent stem cell-like population known as the neural crest (NC). Neural crest cells (NCCs) originate from the dorsal aspect of the neural tube and migrate along pre-determined routes into the developing branchial arches and frontonasal plate. The exceptional rates of proliferation and migration of NCCs enable their diverse contribution to a wide variety of craniofacial structures. Subsequent differentiation of these cells gives rise to cartilage, bones, and a number of mesenchymally-derived tissues. Deficiencies in any stage of differentiation can result in facial clefts and abnormalities associated with craniofacial syndromes. A small number of conserved signaling pathways are involved in controlling NC differentiation and craniofacial development. They are used in a reiterated fashion to help define precise temporospatial cell and tissue formation. Although many aspects of their cellular and molecular control have yet to be described, it is clear that together they form intricately integrated signaling networks required for spatial orientation and developmental stability and plasticity, which are hallmarks of craniofacial development. Mutations that affect the functions of these signaling pathways are often directly or indirectly identified in congenital syndromes. Clinical applications of NC-derived mesenchymal stem/progenitor cells, persistent into adulthood, hold great promise for tissue repair and regeneration. Realization of NCC potential for regenerative therapies motivates understanding of the intricacies of cell communication and differentiation that underlie the complexities of NC-derived tissues.

Reactivation of the pluripotency program precedes formation of the cranial neural crest

During development, cells progress from a pluripotent state to a more restricted fate within a particular germ layer. However, cranial neural crest cells (CNCCs), a transient cell population that generates most of the craniofacial skeleton, have much broader differentiation potential than their ectodermal lineage of origin. Here, we identify a neuroepithelial precursor population characterized by expression of canonical pluripotency transcription factors that gives rise to CNCCs and is essential for craniofacial development. Pluripotency factor Oct4 is transiently reactivated in CNCCs and is required for the subsequent formation of ectomesenchyme. Furthermore, open chromatin landscapes of Oct4⁺ CNCC precursors resemble those of epiblast stem cells, with additional features suggestive of priming for mesenchymal programs. We propose that CNCCs expand their developmental potential through a transient reacquisition of molecular signatures of pluripotency.

From the Basis of Epimorphic Regeneration to Enhanced Regenerative Therapies

Current cell-based therapies to treat degenerative diseases such as osteoarthritis (OA) fail to offer long-term beneficial effects. The therapeutic effects provided by mesenchymal stem cell (MSC) injection, characterized by reduced pain and an improved functional activity in patients with knee OA, are reported at short-term follow-up since the improved outcomes plateau or, even worse, decline several months after MSC administration. This review tackles the limitations of MSC-based therapy for degenerative diseases and highlights the lessons learned from regenerative species to comprehend the coordination of molecular and cellular events critical for complex regeneration processes. We discuss how MSC injection generates a positive cascade of events resulting in a long-lasting systemic immune regulation with limited beneficial effects on tissue regeneration while in regenerative species fine-tuned inflammation is required for progenitor cell proliferation, differentiation, and regeneration. Finally, we stress the direct or indirect involvement of neural crest derived cells (NCC) in most if not all adult regenerative models studied so far. This review underlines the regenerative potential of NCC and the limitations of MSC-based therapy to open new avenues for the treatment of degenerative diseases such as OA.

The RNA helicase DDX3 induces neural crest by promoting AKT activity

Mutations in the RNA helicase DDX3 have emerged as a frequent cause of intellectual disability in humans. Because many individuals carrying DDX3 mutations have additional defects in craniofacial structures and other tissues containing neural crest (NC)-derived cells, we hypothesized that DDX3 is also important for NC development. Using Xenopus tropicalis as a model, we show that DDX3 is required for normal NC induction and craniofacial morphogenesis by regulating AKT kinase activity. Depletion of DDX3 decreases AKT activity and AKT-dependent inhibitory phosphorylation of GSK3β, leading to reduced levels of β-catenin and Snai1: two GSK3β substrates that are crucial for NC induction. DDX3 function in regulating these downstream signaling events during NC induction is likely mediated by RAC1, a small GTPase whose translation depends on the RNA helicase activity of DDX3. These results suggest an evolutionarily conserved role of DDX3 in NC development by promoting AKT activity, and provide a potential mechanism for the NC-related birth defects displayed by individuals harboring mutations in DDX3 and its downstream effectors in this signaling cascade.

Sorting Sox: Diverse Roles for Sox Transcription Factors During Neural Crest and Craniofacial Development

Sox transcription factors play many diverse roles during development, including regulating stem cell states, directing differentiation, and influencing the local chromatin landscape. Of the twenty vertebrate Sox factors, several play critical roles in the development the neural crest, a key vertebrate innovation, and the subsequent formation of neural crest-derived structures, including the craniofacial complex. Herein, we review the specific roles for individual Sox factors during neural crest cell formation and discuss how some factors may have been essential for the evolution of the neural crest. Additionally, we describe how Sox factors direct neural crest cell differentiation into diverse lineages such as melanocytes, glia, and cartilage and detail their involvement in the development of specific craniofacial structures. Finally, we highlight several SOXopathies associated with craniofacial phenotypes.

Building the Border: Development of the Chordate Neural Plate Border Region and Its Derivatives

The paired cranial sensory organs and peripheral nervous system of vertebrates arise from a thin strip of cells immediately adjacent to the developing neural plate. The neural plate border region comprises progenitors for four key populations of cells: neural plate cells, neural crest cells, the cranial placodes, and epidermis. Putative homologues of these neural plate border derivatives can be found in protochordates such as amphioxus and tunicates. In this review, we summarize key signaling pathways and transcription factors that regulate the inductive and patterning events at the neural plate border region that give rise to the neural crest and placodal lineages. Gene regulatory networks driven by signals from WNT, fibroblast growth factor (FGF), and bone morphogenetic protein (BMP) signaling primarily dictate the formation of the crest and placodal lineages. We review these studies and discuss the potential of recent advances in spatio-temporal transcriptomic and epigenomic analyses that would allow a mechanistic understanding of how these signaling pathways and their downstream transcriptional cascades regulate the formation of the neural plate border region.

Distinct molecular profile and restricted stem cell potential defines the prospective human cranial neural crest from embryonic stem cell state

Neural crest cells are an embryonic multipotent stem cell population. Recent studies in model organisms have suggested that neural crest cells are specified earlier than previously thought, at blastula stages. However, the molecular dynamics of early neural crest specification, and functional changes from pluripotent precursors to early specified NC, remain to be elucidated. In this report, we utilized a robust human model of cranial neural crest formation to address the distinct molecular character of the earliest stages of neural crest specification and assess the functional differences from its embryonic stem cell precursor. Our human neural crest model reveals a rapid change in the epigenetic state of neural crest and pluripotency genes, accompanied by changes in gene expression upon Wnt-based induction from embryonic stem cells. These changes in gene expression are directly regulated by the transcriptional activity of β-catenin. Furthermore, prospective cranial neural crest cells are characterized by restricted stem cell potential compared to embryonic stem cells. Our results suggest that human neural crest induced by Wnt/β-catenin signaling from human embryonic stem cells rapidly acquire a prospective neural crest cell state defined by a unique molecular signature and endowed with limited potential compared to pluripotent stem cells.

Insights Into the Early Gene Regulatory Network Controlling Neural Crest and Placode Fate Choices at the Neural Border

The neural crest (NC) cells and cranial placodes are two ectoderm-derived innovations in vertebrates that led to the acquisition of a complex head structure required for a predatory lifestyle. They both originate from the neural border (NB), a portion of the ectoderm located between the neural plate (NP), and the lateral non-neural ectoderm. The NC gives rise to a vast array of tissues and cell types such as peripheral neurons and glial cells, melanocytes, secretory cells, and cranial skeletal and connective cells. Together with cells derived from the cranial placodes, which contribute to sensory organs in the head, the NC also forms the cranial sensory ganglia. Multiple in vivo studies in different model systems have uncovered the signaling pathways and genetic factors that govern the positioning, development, and differentiation of these tissues. In this literature review, we give an overview of NC and placode development, focusing on the early gene regulatory network that controls the formation of the NB during early embryonic stages, and later dictates the choice between the NC and placode progenitor fates.

On the road again: Establishment and maintenance of stemness in the neural crest from embryo to adulthood

Unique to vertebrates, the neural crest (NC) is an embryonic stem cell population that contributes to a greatly expanding list of derivatives ranging from neurons and glia of the peripheral nervous system, facial cartilage and bone, pigment cells of the skin to secretory cells of the endocrine system. Here, we focus on what is specifically known about establishment and maintenance of NC stemness and ultimate fate commitment mechanisms, which could help explain its exceptionally high stem cell potential that exceeds the "rules set during gastrulation." In fact, recent discoveries have shed light on the existence of NC cells that coexpress commonly accepted pluripotency factors like Nanog, Oct4/PouV, and Klf4. The coexpression of pluripotency factors together with the exceptional array of diverse NC derivatives encouraged us to propose a new term "pleistopotent" (Greek for abundant, a substantial amount) to be used to reflect the uniqueness of the NC as compared to other post-gastrulation stem cell populations in the vertebrate body, and to differentiate them from multipotent lineage restricted stem cells. We also discuss studies related to the maintenance of NC stemness within the challenging context of being a transient and thus a constantly changing population of stem cells without a permanent niche. The discovery of the stem cell potential of Schwann cell precursors as well as multiple adult NC-derived stem cell reservoirs during the past decade has greatly increased our understanding of how NC cells contribute to tissues formed after its initial migration stage in young embryos.

Development of extrinsic innervation in the abdominal intestines of human embryos

Compared to the intrinsic enteric nervous system (ENS), development of the extrinsic ENS is poorly documented, even though its presence is easily detectable with histological techniques. We visualised its development in human embryos and foetuses of 4–9.5 weeks post‐fertilisation using Amira 3D‐reconstruction and Cinema 4D‐remodelling software. The extrinsic ENS originated from small, basophilic neural crest cells (NCCs) that migrated to the para‐aortic region and then continued ventrally to the pre‐aortic region, where they formed autonomic pre‐aortic plexuses. From here, nerve fibres extended along the ventral abdominal arteries and finally connected to the intrinsic system. Schwann cell precursors (SCPs), a subgroup of NCCs that migrate on nerve fibres, showed region‐specific differences in differentiation. SCPs developed into scattered chromaffin cells of the adrenal medulla dorsolateral to the coeliac artery (CA) and into more tightly packed chromaffin cells of the para‐aortic bodies ventrolateral to the inferior mesenteric artery (IMA), with reciprocal topographic gradients between both fates. The extrinsic ENS first extended along the CA and then along the superior mesenteric artery (SMA) and IMA 5 days later. Apart from the branch to the caecum, extrinsic nerves did not extend along SMA branches in the herniated parts of the midgut until the gut loops had returned in the abdominal cavity, suggesting a permissive role of the intraperitoneal environment. Accordingly, extrinsic innervation had not yet reached the distal (colonic) loop of the midgut at 9.5 weeks development. Based on intrinsic ENS‐dependent architectural remodelling of the gut layers, extrinsic innervation followed intrinsic innervation 3–4 Carnegie stages later.

Natural size variation among embryos leads to the corresponding scaling in gene expression

Xenopus laevis frogs from laboratory stocks normally lay eggs exhibiting extensive size variability. We find that these initial size differences subsequently affect the size of the embryos prior to the onset of growth, and the size of tadpoles during the growth period. Even though these tadpoles differ in size, their tissues, organs, and structures always seem to be properly proportioned, i.e. they display static allometry. Initial axial patterning events in Xenopus occur in a spherical embryo, allowing easy documentation of their size-dependent features. We examined the size distribution of early Xenopus laevis embryos and measured diameters that differed by about 38% with a median of about 1.43 ​mm. This range of embryo sizes corresponds to about a 1.9-fold difference in surface area and a 2.6-fold difference in volume. We examined the relationship between embryo size and gene expression and observed a significant correlation between diameter and RNA content during gastrula stages. In addition, we investigated the expression levels of genes that pattern the mesoderm, induce the nervous system and mediate the progression of ectodermal cells to neural precursors in large and small embryos. We found that most of these factors were expressed at levels that scaled with the different embryo sizes and total embryo RNA content. In agreement with the changes in transcript levels, the expression domains in larger embryos increased proportionally with the increase in surface area, maintaining their relative expression domain size in relation to the total size of the embryo. Thus, our study identified a mechanism for adapting gene expression domains to embryo size by adjusting the transcript levels of the genes regulating mesoderm induction and patterning. In the neural plate, besides the scaling of the expression domains, we observed similar cell sizes and cell densities in small and large embryos suggesting that additional cell divisions took place in large embryos to compensate for the increased size. Our results show in detail the size variability among Xenopus laevis embryos and the transcriptional adaptation to scale gene expression with size. The observations further support the involvement of BMP/ADMP signaling in the scaling process.