Reprogramming of avian neural crest axial identity and cell fate

Tlx3 mediates neuronal differentiation and proper condensation of the developing trigeminal ganglion

The trigeminal ganglion, the largest of the vertebrate cranial ganglia, is comprised of sensory neurons that relay sensations of pain, touch, and temperature to the brain. These neurons are derived from two embryonic cell types, the neural crest and ectodermal placodes, whose interactions are critical for proper ganglion formation. While the T-cell leukemia homeobox 3 (Tlx3) gene is known to be expressed in placodally-derived sensory neurons and necessary for their differentiation, little was known about Tlx3 expression and/or function in the neural crest-derived component of the developing trigeminal ganglion. By combining lineage labeling with in situ hybridization in the chick embryo, we show that neural crest-derived cells that contribute to the cranial trigeminal ganglion express Tlx3 at a time point that coincides with the onset of ganglion condensation. Importantly, loss of Tlx3 function in vivo diminishes the overall size and abundance of neurons within the trigeminal ganglion. Conversely, ectopic expression of Tlx3 in migrating cranial neural crest results in their premature neuronal differentiation. Taken together, our results demonstrate a critical role for Tlx3 in neural crest-derived cells during chick trigeminal gangliogenesis.

In vitro induction of patterned branchial arch-like aggregate from human pluripotent stem cells

Early patterning of neural crest cells (NCCs) in the craniofacial primordium is important for subsequent development of proper craniofacial structures. However, because of the complexity of the environment of developing tissues, surveying the early specification and patterning of NCCs is difficult. In this study, we develop a simplified in vitro 3D model using human pluripotent stem cells to analyze the early stages of facial development. In this model, cranial NCC-like cells spontaneously differentiate from neural plate border-like cells into maxillary arch-like mesenchyme after a long-term culture. Upon the addition of EDN1 and BMP4, these aggregates are converted into a mandibular arch-like state. Furthermore, temporary treatment with EDN1 and BMP4 induces the formation of spatially separated domains expressing mandibular and maxillary arch markers within a single aggregate. These results suggest that this in vitro model is useful for determining the mechanisms underlying cell fate specification and patterning during early facial development.

TFAP2 paralogs regulate midfacial development in part through a conserved ALX genetic pathway

Cranial neural crest development is governed by positional gene regulatory networks (GRNs). Fine-tuning of the GRN components underlies facial shape variation, yet how those networks in the midface are connected and activated remain poorly understood. Here, we show that concerted inactivation of Tfap2a and Tfap2b in the murine neural crest, even during the late migratory phase, results in a midfacial cleft and skeletal abnormalities. Bulk and single-cell RNA-seq profiling reveal that loss of both TFAP2 family members dysregulates numerous midface GRN components involved in midface morphogenesis, patterning and differentiation. Notably, Alx1, Alx3 and Alx4 (ALX) transcript levels are reduced, whereas ChIP-seq analyses suggest TFAP2 family members directly and positively regulate ALX gene expression. Tfap2a, Tfap2b and ALX co-expression in midfacial neural crest cells of both mouse and zebrafish implies conservation of this regulatory axis across vertebrates. Consistent with this notion, tfap2a zebrafish mutants present with abnormal alx3 expression patterns, Tfap2a binds ALX loci and tfap2a-alx3 genetic interactions are observed. Together, these data demonstrate TFAP2 paralogs regulate vertebrate midfacial development in part by activating expression of ALX transcription factor genes.

Cooperation of TGF-β and FGF signalling pathways in skin development

The skin is a multi-layered structure composed of the epidermis, dermis and hypodermis. The epidermis originates entirely from the ectoderm, whereas the dermis originates from various germ layers depending on its anatomical location; thus, there are different developmental patterns of the skin. Although the regulatory mechanisms of epidermal formation are well understood, mechanisms regulating dermis development are not clear owing to the complex origin. It has been shown that several morphogenetic pathways regulate dermis development. Of these, transforming growth factor-β (TGF-β) and fibroblast growth factor (FGF) signalling pathways are the main modulators regulating skin cell induction, fate decision, migration and differentiation. Recently, the successful generation of human skin by modulating TGF-β and FGF signals further demonstrated the irreplaceable roles of these pathways in skin regeneration. This review provides evidence of the role of TGF-β and FGF signalling pathways in the development of different skin layers, especially the disparate dermis of different body regions. This review also provides new perspectives on the distinct developmental patterns of skin and explores new ideas for clinical applications in the future.

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.

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.

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

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.

Ancient vertebrate dermal armor evolved from trunk neural crest

Bone is an evolutionary novelty of vertebrates, likely to have first emerged as part of ancestral dermal armor that consisted of osteogenic and odontogenic components. Whether these early vertebrate structures arose from mesoderm or neural crest cells has been a matter of considerable debate. To examine the developmental origin of the bony part of the dermal armor, we have performed in vivo lineage tracing in the sterlet sturgeon, a representative of nonteleost ray-finned fish that has retained an extensive postcranial dermal skeleton. The results definitively show that sterlet trunk neural crest cells give rise to osteoblasts of the scutes. Transcriptional profiling further reveals neural crest gene signature in sterlet scutes as well as bichir scales. Finally, histological and microCT analyses of ray-finned fish dermal armor show that their scales and scutes are formed by bone, dentin, and hypermineralized covering tissues, in various combinations, that resemble those of the first armored vertebrates. Taken together, our results support a primitive skeletogenic role for the neural crest along the entire body axis, that was later progressively restricted to the cranial region during vertebrate evolution. Thus, the neural crest was a crucial evolutionary innovation driving the origin and diversification of dermal armor along the entire body axis.

TFAP2 paralogs regulate midfacial development in part through a conserved ALX genetic pathway

Cranial neural crest development is governed by positional gene regulatory networks (GRNs). Fine-tuning of the GRN components underly facial shape variation, yet how those in the midface are connected and activated remain poorly understood. Here, we show that concerted inactivation of Tfap2a and Tfap2b in the murine neural crest even during the late migratory phase results in a midfacial cleft and skeletal abnormalities. Bulk and single-cell RNA-seq profiling reveal that loss of both Tfap2 members dysregulated numerous midface GRN components involved in midface fusion, patterning, and differentiation. Notably, Alx1/3/4 (Alx) transcript levels are reduced, while ChIP-seq analyses suggest TFAP2 directly and positively regulates Alx gene expression. TFAP2 and ALX co-expression in midfacial neural crest cells of both mouse and zebrafish further implies conservation of this regulatory axis across vertebrates. Consistent with this notion, tfap2a mutant zebrafish present abnormal alx3 expression patterns, and the two genes display a genetic interaction in this species. Together, these data demonstrate a critical role for TFAP2 in regulating vertebrate midfacial development in part through ALX transcription factor gene expression.

Reprogramming of trunk neural crest to a cranial crest-like identity alters their transcriptome and developmental potential

Neural crest cells along the body axis of avian embryos differ in their developmental potential, such that the cranial neural crest forms cartilage and bone whereas the trunk neural crest is unable to do so. Previous studies have identified a cranial crest-specific subcircuit that can imbue the trunk neural crest with the ability to form cartilage after grafting to the head. Here, we examine transcriptional and cell fate changes that accompany this reprogramming. First, we examined whether reprogrammed trunk neural crest maintain the ability to form cartilage in their endogenous environment in the absence of cues from the head. The results show that some reprogrammed cells contribute to normal trunk neural crest derivatives, whereas others migrate ectopically to the forming vertebrae and express cartilage markers, thus mimicking heterotypically transplanted cranial crest cells. We find that reprogrammed trunk neural crest upregulated more than 3000 genes in common with cranial neural crest, including numerous transcriptional regulators. In contrast, many trunk neural crest genes are downregulated. Together, our findings show that reprogramming trunk neural crest with cranial crest subcircuit genes alters their gene regulatory program and developmental potential to be more cranial crest-like.

vwa1 Knockout in Zebrafish Causes Abnormal Craniofacial Chondrogenesis by Regulating FGF Pathway

Hemifacial microsomia (HFM), a rare disorder of first- and second-pharyngeal arch development, has been linked to a point mutation in VWA1 (von Willebrand factor A domain containing 1), encoding the protein WARP in a five-generation pedigree. However, how the VWA1 mutation relates to the pathogenesis of HFM is largely unknown. Here, we sought to elucidate the effects of the VWA1 mutation at the molecular level by generating a vwa1-knockout zebrafish line using CRISPR/Cas9. Mutants and crispants showed cartilage dysmorphologies, including hypoplastic Meckel’s cartilage and palatoquadrate cartilage, malformed ceratohyal with widened angle, and deformed or absent ceratobranchial cartilages. Chondrocytes exhibited a smaller size and aspect ratio and were aligned irregularly. In situ hybridization and RT-qPCR showed a decrease in barx1 and col2a1a expression, indicating abnormal cranial neural crest cell (CNCC) condensation and differentiation. CNCC proliferation and survival were also impaired in the mutants. Expression of FGF pathway components, including fgf8a, fgfr1, fgfr2, fgfr3, fgfr4, and runx2a, was decreased, implying a role for VWA1 in regulating FGF signaling. Our results demonstrate that VWA1 is essential for zebrafish chondrogenesis through effects on condensation, differentiation, proliferation, and apoptosis of CNCCs, and likely impacts chondrogenesis through regulation of the FGF pathway.

Making a head: Neural crest and ectodermal placodes in cranial sensory development

During development of the vertebrate sensory system, many important components like the sense organs and cranial sensory ganglia arise within the head and neck. Two progenitor populations, the neural crest, and cranial ectodermal placodes, contribute to these developing vertebrate peripheral sensory structures. The interactions and contributions of these cell populations to the development of the lens, olfactory, otic, pituitary gland, and cranial ganglia are vital for appropriate peripheral nervous system development. Here, we review the origins of both neural crest and placode cells at the neural plate border of the early vertebrate embryo and investigate the molecular and environmental signals that influence specification of different sensory regions. Finally, we discuss the underlying molecular pathways contributing to the complex vertebrate sensory system from an evolutionary perspective, from basal vertebrates to amniotes.

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.

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.

The cardiac neural crest gene MafB ectopically directs CXCR4 expression in the trunk neural crest

The cardiac neural crest is a subpopulation of cells arising from the caudal hindbrain. The delaminated cardiac neural crest cells migrate to the heart using the CXCR/SDF1 chemokine signaling system. These cells contribute to the formation of the cardiovascular system, including the septation of the outflow tract, which is unique to these cells. Here, we investigated the effect of ectopic expression of the cardiac neural crest gene MafB on trunk neural crest cells. First, we found that MafB has the potential to activate its own cis-regulatory element in enteric and trunk neural crest cells but not in cranial neural crest cells. Forced expression of two cardiac neural crest genes, Ets1 and Sox8, together with or without MafB, induced ectopic Sox10E2 enhancer activity in the trunk region. Finally, we uncovered that the expression of MafB, Ets1 and Sox8 can induce ectopic CXCR4 expression in the trunk neural crest cells, resulting in acquisition of responsiveness to the SDF1 signal. These results demonstrate that MafB, Ets1 and Sox8 are critical components for generation of the identity of the cardiac neural crest, especially the cell migration property.

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.

Control of cranial ectomesenchyme fate by Nr2f nuclear receptors

Certain cranial neural crest cells are uniquely endowed with the ability to make skeletal cell types otherwise only derived from mesoderm. As these cells migrate into the pharyngeal arches, they downregulate neural crest specifier genes and upregulate so-called ectomesenchyme genes that are characteristic of skeletal progenitors. Although both external and intrinsic factors have been proposed as triggers of this transition, the details remain obscure. Here, we report the Nr2f nuclear receptors as intrinsic activators of the ectomesenchyme program: zebrafish nr2f5 single and nr2f2;nr2f5 double mutants show marked delays in upregulation of ectomesenchyme genes, such as dlx2a, prrx1a, prrx1b, sox9a, twist1a and fli1a, and in downregulation of sox10, which is normally restricted to early neural crest and non-ectomesenchyme lineages. Mutation of sox10 fully rescued skeletal development in nr2f5 single but not nr2f2;nr2f5 double mutants, but the initial ectomesenchyme delay persisted in both. Sox10 perdurance thus antagonizes the recovery but does not explain the impaired ectomesenchyme transition. Unraveling the mechanisms of Nr2f function will help solve the enduring puzzle of how cranial neural crest cells transition to the skeletal progenitor state.

Gene-environment interactions in the pathogenesis of common craniofacial anomalies

Craniofacial anomalies often exhibit phenotype variability and non-mendelian inheritance due to their multifactorial origin, involving both genetic and environmental factors. A combination of epidemiologic studies, genome-wide association, and analysis of animal models have provided insight into the effects of gene-environment interactions on craniofacial and brain development and the pathogenesis of congenital disorders. In this chapter, we briefly summarize the etiology and pathogenesis of common craniofacial anomalies, focusing on orofacial clefts, hemifacial microsomia, and microcephaly. We then discuss how environmental risk factors interact with genes to modulate the incidence and phenotype severity of craniofacial anomalies. Identifying environmental risk factors and dissecting their interaction with different genes and modifiers is central to improved strategies for preventing craniofacial anomalies.

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.

Spatiotemporal single-cell regulatory atlas reveals neural crest lineage diversification and cellular function during tooth morphogenesis

Cranial neural crest cells are an evolutionary innovation of vertebrates for craniofacial development and function, yet the mechanisms that govern the cell fate decisions of postmigratory cranial neural crest cells remain largely unknown. Using the mouse molar as a model, we perform single-cell transcriptome profiling to interrogate the cell fate diversification of postmigratory cranial neural crest cells. We reveal the landscape of transcriptional heterogeneity and define the specific cellular domains during the progression of cranial neural crest cell-derived dental lineage diversification, and find that each domain makes a specific contribution to distinct molar mesenchymal tissues. Furthermore, IGF signaling-mediated cell-cell interaction between the cellular domains highlights the pivotal role of autonomous regulation of the dental mesenchyme. Importantly, we reveal cell-type-specific gene regulatory networks in the dental mesenchyme and show that Foxp4 is indispensable for the differentiation of periodontal ligament. Our single-cell atlas provides comprehensive mechanistic insight into the cell fate diversification process of the cranial neural crest cell-derived odontogenic populations.

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.

Time to go: neural crest cell epithelial-to-mesenchymal transition

Neural crest cells (NCCs) are a dynamic, multipotent, vertebrate-specific population of embryonic stem cells. These ectodermally-derived cells contribute to diverse tissue types in developing embryos including craniofacial bone and cartilage, the peripheral and enteric nervous systems and pigment cells, among a host of other cell types. Due to their contribution to a significant number of adult tissue types, the mechanisms that drive their formation, migration and differentiation are highly studied. NCCs have a unique ability to transition from tightly adherent epithelial cells to mesenchymal and migratory cells by altering their polarity, expression of cell-cell adhesion molecules and gaining invasive abilities. In this Review, we discuss classical and emerging factors driving NCC epithelial-to-mesenchymal transition and migration, highlighting the role of signaling and transcription factors, as well as novel modifying factors including chromatin remodelers, small RNAs and post-translational regulators, which control the availability and longevity of major NCC players.

Vertebrate Cell Differentiation, Evolution, and Diseases: The Vertebrate-Specific Developmental Potential Guardians VENTX/NANOG and POU5/OCT4 Enter the Stage

During vertebrate development, embryonic cells pass through a continuum of transitory pluripotent states that precede multi-lineage commitment and morphogenesis. Such states are referred to as “refractory/naïve” and “competent/formative” pluripotency. The molecular mechanisms maintaining refractory pluripotency or driving the transition to competent pluripotency, as well as the cues regulating multi-lineage commitment, are evolutionarily conserved. Vertebrate-specific “Developmental Potential Guardians” (vsDPGs; i.e., VENTX/NANOG, POU5/OCT4), together with MEK1 (MAP2K1), coordinate the pluripotency continuum, competence for multi-lineage commitment and morphogenesis in vivo. During neurulation, vsDPGs empower ectodermal cells of the neuro-epithelial border (NEB) with multipotency and ectomesenchyme potential through an “endogenous reprogramming” process, giving rise to the neural crest cells (NCCs). Furthermore, vsDPGs are expressed in undifferentiated-bipotent neuro-mesodermal progenitor cells (NMPs), which participate in posterior axis elongation and growth. Finally, vsDPGs are involved in carcinogenesis, whereby they confer selective advantage to cancer stem cells (CSCs) and therapeutic resistance. Intriguingly, the heterogenous distribution of vsDPGs in these cell types impact on cellular potential and features. Here, we summarize the findings about the role of vsDPGs during vertebrate development and their selective advantage in evolution. Our aim to present a holistic view regarding vsDPGs as facilitators of both cell plasticity/adaptability and morphological innovation/variation. Moreover, vsDPGs may also be at the heart of carcinogenesis by allowing malignant cells to escape from physiological constraints and surveillance mechanisms.

Retroviral lineage analysis reveals dual contribution from ectodermal placodes and neural crest cells to avian olfactory sensory and GnRH neurons

The origin of the neurons and glia in the olfactory system of vertebrates has been controversial, with different cell types attributed to being of ectodermal placode versus neural crest lineage, depending upon the species. Here, we use replication incompetent avian (RIA) retroviruses to perform prospective cell lineage analysis of either presumptive olfactory placode or neural crest cells during early development of the chick embryo. Surprisingly, the results reveal a dual contribution from both the olfactory placode and neural crest cells to sensory neurons in the nose and Gonadotropin Releasing Hormone (GnRH) neurons migrating to the olfactory bulb. We also confirm that olfactory ensheathing glia are solely derived from the neural crest. Finally, our results show that neural crest cells and olfactory placode cells contribute to p63 positive cells, likely to be basal stem cells of the olfactory epithelium. Taken together, these finding provide evidence for previously unknown contributions of neural crest cells to some cell types in the chick olfactory system and help resolve previous discrepancies in the literature.

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.

The effect of BMP4, FGF8 and WNT3a on mouse iPS cells differentiating to odontoblast-like cells

We investigated whether BMP4, FGF8, and/or WNT3a on neural crest-like cells (NCLC) derived from mouse induced pluripotent stem (miPS) cells will promote differentiation of odontoblasts-like cells. After the miPS cells matured into embryonic body (EB) cells, they were cultured in a neural induction medium to produce NCLC. As the differentiation of NCLC were confirmed by RT-qPCR, they were then disassociated and cultured with a medium containing, BMP4, FGF8, and/or WNT3a for 7 and 14 days. The effect of these stimuli on NCLC were assessed by RT-qPCR, ALP staining, and immunocytochemistry. The cultured EB cells presented a significant increase of Snai1, Slug, and Sox 10 substantiating the differentiation of NCLC. NCLC stimulated with more than two stimuli significantly increased the odontoblast markers Dmp-1, Dspp, Nestin, Alp, and Runx2 expression compared to control with no stimulus. The expression of Dmp-1 and Dspp upregulated more when FGF8 was combined with WNT3a. ALP staining was positive in groups containing BMP4 and fluorescence was observed in immunocytochemistry of the common significant groups between Dmp-1 and Dspp. After stimulation, the cell morphology demonstrated a spindle-shaped cells with long projections resembling odontoblasts. Simultaneous BMP4, FGF8, and WNT3a stimuli significantly differentiated NCLC into odontoblast-like cells.

The amazing and anomalous axolotls as scientific models

Ambystoma mexicanum (axolotl) embryos and juveniles have been used as model organisms for developmental and regenerative research for many years. This neotenic aquatic species maintains the unique capability to regenerate most, if not all, of its tissues well into adulthood. With large externally developing embryos, axolotls were one of the original model species for developmental biology. However, increased access to, and use of, organisms with sequenced and annotated genomes, such as Xenopus laevis and tropicalis and Danio rerio, reduced the prevalence of axolotls as models in embryogenesis studies. Recent sequencing of the large axolotl genome opens up new possibilities for defining the recipes that drive the formation and regeneration of tissues like the limbs and spinal cord. However, to decode the large Ambystoma mexicanum genome will take a herculean effort, community resources, and the development of novel techniques. Here, we provide an updated axolotl-staging chart ranging from 1-cell stage to immature adult paired with a perspective on both historical and current axolotl research that spans from their use in early studies of development to the recent cutting-edge research, employment of transgenesis, high resolution imaging, and study of mechanisms deployed in regeneration. This article is protected by copyright. All rights reserved.

Elevated Hoxb5b Expands Vagal Neural Crest Pool and Blocks Enteric Neuronal Development in Zebrafish

Neural crest cells (NCCs) are a migratory, transient, and multipotent stem cell population essential to vertebrate embryonic development, contributing to numerous cell lineages in the adult organism. While great strides have been made in elucidating molecular and cellular events that drive NCC specification, comprehensive knowledge of the genetic factors that orchestrate NCC developmental programs is still far from complete. We discovered that elevated Hoxb5b levels promoted an expansion of zebrafish NCCs, which persisted throughout multiple stages of development. Correspondingly, elevated Hoxb5b also specifically expanded expression domains of the vagal NCC markers foxd3 and phox2bb. Increases in NCCs were most apparent after pulsed ectopic Hoxb5b expression at early developmental stages, rather than later during differentiation stages, as determined using a novel transgenic zebrafish line. The increase in vagal NCCs early in development led to supernumerary Phox2b+ enteric neural progenitors, while leaving many other NCC-derived tissues without an overt phenotype. Surprisingly, these NCC-derived enteric progenitors failed to expand properly into sufficient quantities of enterically fated neurons and stalled in the gut tissue. These results suggest that while Hoxb5b participates in vagal NCC development as a driver of progenitor expansion, the supernumerary, ectopically localized NCC fail to initiate expansion programs in timely fashion in the gut. All together, these data point to a model in which Hoxb5b regulates NCCs both in a tissue specific and temporally restricted manner.

Lifelong single-cell profiling of cranial neural crest diversification in zebrafish

The cranial neural crest generates a huge diversity of derivatives, including the bulk of connective and skeletal tissues of the vertebrate head. How neural crest cells acquire such extraordinary lineage potential remains unresolved. By integrating single-cell transcriptome and chromatin accessibility profiles of cranial neural crest-derived cells across the zebrafish lifetime, we observe progressive and region-specific establishment of enhancer accessibility for distinct fates. Neural crest-derived cells rapidly diversify into specialized progenitors, including multipotent skeletal progenitors, stromal cells with a regenerative signature, fibroblasts with a unique metabolic signature linked to skeletal integrity, and gill-specific progenitors generating cell types for respiration. By retrogradely mapping the emergence of lineage-specific chromatin accessibility, we identify a wealth of candidate lineage-priming factors, including a Gata3 regulatory circuit for respiratory cell fates. Rather than multilineage potential being established during cranial neural crest specification, our findings support progressive and region-specific chromatin remodeling underlying acquisition of diverse potential.

Single-cell reconstruction with spatial context of migrating neural crest cells and their microenvironments during vertebrate head and neck formation

The dynamics of multipotent neural crest cell differentiation and invasion as cells travel throughout the vertebrate embryo remain unclear. Here, we preserve spatial information to derive the transcriptional states of migrating neural crest cells and the cellular landscape of the first four chick cranial to cardiac branchial arches (BA1-4) using label-free, unsorted single-cell RNA sequencing. The faithful capture of branchial arch-specific genes led to identification of novel markers of migrating neural crest cells and 266 invasion genes common to all BA1-4 streams. Perturbation analysis of a small subset of invasion genes and time-lapse imaging identified their functional role to regulate neural crest cell behaviors. Comparison of the neural crest invasion signature to other cell invasion phenomena revealed a shared set of 45 genes, a subset of which showed direct relevance to human neuroblastoma cell lines analyzed after exposure to the in vivo chick embryonic neural crest microenvironment. Our data define an important spatio-temporal reference resource to address patterning of the vertebrate head and neck, and previously unidentified cell invasion genes with the potential for broad impact.

Draxin inhibits chick trunk neural crest delamination and migration by increasing cell adhesion

The neural crest is a multipotent cell population that migrates extensively to play important roles during embryonic development. After acquiring motility, trunk neural crest cells delaminate from the spinal cord and migrate to various regions of the body. Several cellular adhesion molecules, such as vinculin, are involved in the regulation of neural crest delamination and migration. In the present study, we found that draxin could inhibit delamination and migration of neural crest cells from the chick spinal cord and abnormal aggregation of the migrating neural crest cells. In the presence of draxin, the resuspended neural crest regained its adhesive ability such that it was significantly increased. Overexpression of draxin caused increased vinculin expression in vivo. Our data indicate that draxin might control delamination and migration of chick trunk neural crest by increasing cell adhesion.

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.

The lysine methyltransferase SETD2 is a dynamically expressed regulator of early neural crest development

SETD2 is a histone H3 lysine 36 (H3K36) tri-methylase that is upregulated in response to neural crest induction. Because the H3K36 di-methylase NSD3 and cytoplasmic non-histone protein methylation are necessary for neural crest development, we investigated the expression and requirement for SETD2 in the neural crest. SetD2 is expressed throughout the chick blastoderm beginning at gastrulation. Subsequently, SetD2 mRNA becomes restricted to the neural plate, where it is strongly and dynamically expressed as neural tissue is regionalized and cell fate decisions are made. This includes expression in premigratory neural crest cells, which is downregulated prior to migration. Likely due to the early onset of its expression, SETD2 morpholino knockdown does not significantly alter premigratory Sox10 expression or neural crest migration; however, both are disrupted by a methyltransferase mutant SETD2 construct. These results suggest that SETD2 activity is essential for early neural crest development, further demonstrating that lysine methylation is an important mechanism regulating the neural crest.

Riding the crest to get a head: neural crest evolution in vertebrates

In their seminal 1983 paper, Gans and Northcutt proposed that evolution of the vertebrate 'new head' was made possible by the advent of the neural crest and cranial placodes. The neural crest is a stem cell population that arises adjacent to the forming CNS and contributes to important cell types, including components of the peripheral nervous system and craniofacial skeleton and elements of the cardiovascular system. In the past few years, the new head hypothesis has been challenged by the discovery in invertebrate chordates of cells with some, but not all, characteristics of vertebrate neural crest cells. Here, we discuss recent findings regarding how neural crest cells may have evolved during the course of deuterostome evolution. The results suggest that there was progressive addition of cell types to the repertoire of neural crest derivatives throughout vertebrate evolution. Novel genomic tools have enabled higher resolution insight into neural crest evolution, from both a cellular and a gene regulatory perspective. Together, these data provide clues regarding the ancestral neural crest state and how the neural crest continues to evolve to contribute to the success of vertebrates as efficient predators.

Essential function and targets of BMP signaling during midbrain neural crest delamination

BMP signaling plays iterative roles during vertebrate neural crest development from induction through craniofacial morphogenesis. However, far less is known about the role of BMP activity in cranial neural crest epithelial-to-mesenchymal transition and delamination. By measuring canonical BMP signaling activity as a function of time from specification through early migration of avian midbrain neural crest cells, we found elevated BMP signaling during delamination stages. Moreover, inhibition of canonical BMP activity via a dominant negative mutant Type I BMP receptor showed that BMP signaling is required for neural crest migration from the midbrain, independent from an effect on EMT and delamination. Transcriptome profiling on control compared to BMP-inhibited cranial neural crest cells identified novel BMP targets during neural crest delamination and early migration including targets of the Notch pathway that are upregulated following BMP inhibition. These results suggest potential crosstalk between the BMP and Notch pathways in early migrating cranial neural crest and provide novel insight into mechanisms regulated by BMP signaling during early craniofacial development.

Zebrafish Cdx4 regulates neural crest cell specification and migratory behaviors in the posterior body

The neural crest (NC) is a transient multipotent cell population that migrates extensively to produce a remarkable array of vertebrate cell types. NC cell specification progresses in an anterior to posterior fashion, resulting in distinct, axial-restricted subpopulations. The anterior-most, cranial, population of NC is specified as gastrulation concludes and neurulation begins, while more posterior populations become specified as the body elongates. The mechanisms that govern development of the more posterior NC cells remain incompletely understood. Here, we report a key role for zebrafish Cdx4, a homeodomain transcription factor, in the development of posterior NC cells. We demonstrate that cdx4 is expressed in trunk NC cell progenitors, directly binds NC cell-specific enhancers in the NC GRN, and regulates expression of the key NC development gene foxd3 in the posterior body. Moreover, cdx4 mutants show disruptions to the segmental pattern of trunk NC cell migration due to loss of normal leader/follower cell dynamics. Finally, using cell transplantation to generate chimeric specimens, we show that Cdx4 does not function in the paraxial mesoderm-the environment adjacent to which crest migrates-to influence migratory behaviors. We conclude that cdx4 plays a critical, and likely tissue autonomous, role in the establishment of trunk NC migratory behaviors. Together, our results indicate that cdx4 functions as an early NC specifier gene in the posterior body of zebrafish embryos.

Kindlin2 regulates neural crest specification via integrin-independent regulation of the FGF signaling pathway

The focal adhesion protein Kindlin2 is essential for integrin activation, a process that is fundamental to cell-extracellular matrix adhesion. Kindlin 2 (Fermt2) is widely expressed in mouse embryos, and its absence causes lethality at the peri-implantation stage due to the failure to trigger integrin activation. The function of kindlin2 during embryogenesis has not yet been fully elucidated as a result of this early embryonic lethality. Here, we showed that kindlin2 is essential for neural crest (NC) formation in Xenopus embryos. Loss-of-function assays performed with kindlin2-specific morpholino antisense oligos (MOs) or with CRISPR/Cas9 techniques in Xenopus embryos severely inhibit the specification of the NC. Moreover, integrin-binding-deficient mutants of Kindlin2 rescued the phenotype caused by loss of kindlin2, suggesting that the function of kindlin2 during NC specification is independent of integrins. Mechanistically, we found that Kindlin2 regulates the fibroblast growth factor (FGF) pathway, and promotes the stability of FGF receptor 1. Our study reveals a novel function of Kindlin2 in regulating the FGF signaling pathway and provides mechanistic insights into the function of Kindlin2 during NC specification.

A single-plasmid approach for genome editing coupled with long-term lineage analysis in chick embryos

An important strategy for establishing mechanisms of gene function during development is through mutation of individual genes and analysis of subsequent effects on cell behavior. Here, we present a single-plasmid approach for genome editing in chick embryos to study experimentally perturbed cells in an otherwise normal embryonic environment. To achieve this, we have engineered a plasmid that encodes Cas9 protein, gene-specific guide RNA (gRNA), and a fluorescent marker within the same construct. Using transfection- and electroporation-based approaches, we show that this construct can be used to perturb gene function in early embryos as well as human cell lines. Importantly, insertion of this cistronic construct into replication-incompetent avian retroviruses allowed us to couple gene knockouts with long-term lineage analysis. We demonstrate the application of our newly engineered constructs and viruses by perturbing β-catenin in vitro and Sox10, Pax6 and Pax7 in the neural crest, retina, and neural tube and segmental plate in vivo, respectively. Together, this approach enables genes of interest to be knocked out in identifiable cells in living embryos and can be broadly applied to numerous genes in different embryonic tissues.

Generation of Vascular Smooth Muscle Cells From Induced Pluripotent Stem Cells: Methods, Applications, and Considerations

The developmental origin of vascular smooth muscle cells (VSMCs) has been increasingly recognized as a major determinant for regional susceptibility or resistance to vascular diseases. As a human material-based complement to animal models and human primary cultures, patient induced pluripotent stem cell iPSC-derived VSMCs have been leveraged to conduct basic research and develop therapeutic applications in vascular diseases. However, iPSC-VSMCs (induced pluripotent stem cell VSMCs) derived by most existing induction protocols are heterogeneous in developmental origins. In this review, we summarize signaling networks that govern in vivo cell fate decisions and in vitro derivation of distinct VSMC progenitors, as well as key regulators that terminally specify lineage-specific VSMCs. We then highlight the significance of leveraging patient-derived iPSC-VSMCs for vascular disease modeling, drug discovery, and vascular tissue engineering and discuss several obstacles that need to be circumvented to fully unleash the potential of induced pluripotent stem cells for precision vascular medicine.

Modelling Cell Invasion: A Review of What JD Murray and the Embryo Can Teach Us

Cell invasion and cell plasticity are critical to human development but are also striking features of cancer metastasis. By distributing a multipotent cell type from a place of birth to distal locations, the vertebrate embryo builds organs. In comparison, metastatic tumor cells often acquire a de-differentiated phenotype and migrate away from a primary site to inhabit new microenvironments, disrupting normal organ function. Countless observations of both embryonic cell migration and tumor metastasis have demonstrated complex cell signaling and interactive behaviors that have long confounded scientist and clinician alike. James D. Murray realized the important role of mathematics in biology and developed a unique strategy to address complex biological questions such as these. His work offers a practical template for constructing clear, logical, direct and verifiable models that help to explain complex cell behaviors and direct new experiments. His pioneering work at the interface of development and cancer made significant contributions to glioblastoma cancer and embryonic pattern formation using often simple models with tremendous predictive potential. Here, we provide a brief overview of advances in cell invasion and cell plasticity using the embryonic neural crest and its ancestral relationship to aggressive cancers that put into current context the timeless aspects of his work.

An atlas of neural crest lineages along the posterior developing zebrafish at single-cell resolution

Neural crest cells (NCCs) are vertebrate stem cells that give rise to various cell types throughout the developing body in early life. Here, we utilized single-cell transcriptomic analyses to delineate NCC-derivatives along the posterior developing vertebrate, zebrafish, during the late embryonic to early larval stage, a period when NCCs are actively differentiating into distinct cellular lineages. We identified several major NCC/NCC-derived cell-types including mesenchyme, neural crest, neural, neuronal, glial, and pigment, from which we resolved over three dozen cellular subtypes. We dissected gene expression signatures of pigment progenitors delineating into chromatophore lineages, mesenchyme cells, and enteric NCCs transforming into enteric neurons. Global analysis of NCC derivatives revealed they were demarcated by combinatorial hox gene codes, with distinct profiles within neuronal cells. From these analyses, we present a comprehensive cell-type atlas that can be utilized as a valuable resource for further mechanistic and evolutionary investigations of NCC differentiation.

Craniofacial transitions: the role of EMT and MET during head development

Within the developing head, tissues undergo cell-fate transitions to shape the forming structures. This starts with the neural crest, which undergoes epithelial-to-mesenchymal transition (EMT) to form, amongst other tissues, many of the skeletal tissues of the head. In the eye and ear, these neural crest cells then transform back into an epithelium, via mesenchymal-to-epithelial transition (MET), highlighting the flexibility of this population. Elsewhere in the head, the epithelium loses its integrity and transforms into mesenchyme. Here, we review these craniofacial transitions, looking at why they happen, the factors that trigger them, and the cell and molecular changes they involve. We also discuss the consequences of aberrant EMT and MET in the head.

Neural crest-like stem cells for tissue regeneration

Neural crest stem cells (NCSCs) are a transient population of cells that arise during early vertebrate development and harbor stem cell properties, such as self‐renewal and multipotency. These cells form at the interface of non‐neuronal ectoderm and neural tube and undergo extensive migration whereupon they contribute to a diverse array of cell and tissue derivatives, ranging from craniofacial tissues to cells of the peripheral nervous system. Neural crest‐like stem cells (NCLSCs) can be derived from pluripotent stem cells, placental tissues, adult tissues, and somatic cell reprogramming. NCLSCs have a differentiation capability similar to NCSCs, and possess great potential for regenerative medicine applications. In this review, we present recent developments on the various approaches to derive NCLSCs and the therapeutic application of these cells for tissue regeneration.

A regulatory sub-circuit downstream of Wnt signaling controls developmental transitions in neural crest formation

The process of cell fate commitment involves sequential changes in the gene expression profiles of embryonic progenitors. This is exemplified in the development of the neural crest, a migratory stem cell population derived from the ectoderm of vertebrate embryos. During neural crest formation, cells transition through distinct transcriptional states in a stepwise manner. The mechanisms underpinning these shifts in cell identity are still poorly understood. Here we employ enhancer analysis to identify a genetic sub-circuit that controls developmental transitions in the nascent neural crest. This sub-circuit links Wnt target genes in an incoherent feedforward loop that controls the sequential activation of genes in the neural crest lineage. By examining the cis-regulatory apparatus of Wnt effector gene AXUD1, we found that multipotency factor SP5 directly promotes neural plate border identity, while inhibiting premature expression of specification genes. Our results highlight the importance of repressive interactions in the neural crest gene regulatory network and illustrate how genes activated by the same upstream signal become temporally segregated during progressive fate restriction.

Augmented BMP signaling commits cranial neural crest cells to a chondrogenic fate by suppressing autophagic β-catenin degradation

Cranial neural crest cells (CNCCs) are a population of multipotent stem cells that give rise to craniofacial bone and cartilage during development. Bone morphogenetic protein (BMP) signaling and autophagy have been individually implicated in stem cell homeostasis. Mutations that cause constitutive activation of the BMP type I receptor ACVR1 cause the congenital disorder fibrodysplasia ossificans progressiva (FOP), which is characterized by ectopic cartilage and bone in connective tissues in the trunk and sometimes includes ectopic craniofacial bones. Here, we showed that enhanced BMP signaling through the constitutively activated ACVR1 (ca-ACVR1) in CNCCs in mice induced ectopic cartilage formation in the craniofacial region through an autophagy-dependent mechanism. Enhanced BMP signaling suppressed autophagy by activating mTORC1, thus blocking the autophagic degradation of β-catenin, which, in turn, caused CNCCs to adopt a chondrogenic identity. Transient blockade of mTORC1, reactivation of autophagy, or suppression of Wnt-β-catenin signaling reduced ectopic cartilages in ca-Acvr1 mutants. Our results suggest that BMP signaling and autophagy coordinately regulate β-catenin activity to direct the fate of CNCCs during craniofacial development. These findings may also explain why some patients with FOP develop ectopic bones through endochondral ossification in craniofacial regions.