Neural crest induction at the neural plate border in vertebrates

Region-specific gene expression profiling of early mouse mandible uncovered SATB2 as a key molecule for teeth patterning

Mammalian dentition exhibits distinct heterodonty, with more simple teeth located in the anterior area of the jaw and more complex teeth situated posteriorly. While some region-specific differences in signalling have been described previously, here we performed a comprehensive analysis of gene expression at the early stages of odontogenesis to obtain complete knowledge of the signalling pathways involved in early jaw patterning. Gene expression was analysed separately on anterior and posterior areas of the lower jaw at two early stages (E11.5 and E12.5) of odontogenesis. Gene expression profiling revealed distinct region-specific expression patterns in mouse mandibles, including several known BMP and FGF signalling members and we also identified several new molecules exhibiting significant differences in expression along the anterior-posterior axis, which potentially can play the role during incisor and molar specification. Next, we followed one of the anterior molecules, SATB2, which was expressed not only in the anterior mesenchyme where incisor germs are initiated, however, we uncovered a distinct SATB2-positive region in the mesenchyme closely surrounding molars. Satb2-deficient animals demonstrated defective incisor development confirming a crucial role of SATB2 in formation of anterior teeth. On the other hand, ectopic tooth germs were observed in the molar area indicating differential effect of Satb2-deficiency in individual jaw regions. In conclusion, our data provide a rich source of fundamental information, which can be used to determine molecular regulation driving early embryonic jaw patterning and serve for a deeper understanding of molecular signalling directed towards incisor and molar development.

Rat nasal mucosa-derived ectodermal mesenchymal stem cells: A new therapeutic option for chronic rhinosinusitis

OBJECTIVE

To investigate the effect of nasal mucosa-derived ectodermal mesenchymal stem cells (NM-EMSCs) on the inflammatory state of rats with chronic rhinosinusitis (CRS) and the underlying therapeutic mechanism.

METHODS

NM-EMSCs were isolated and extracted to construct a rat model of CRS. Fifteen Sprague‒Dawley (SD) rats were randomly divided into three groups: CK + NS group rats were injected locally with saline in the nasal mucosa; CRS + NS group rats were injected locally with saline in the nasal mucosa; and CRS + EMSCs group rats were injected locally with NM-EMSCs in the nasal mucosa. One rat from the CRS + EMSCs group was randomly euthanized at 2, 4, and 6 days after injection, and the nasal mucosa tissues were collected for HE staining, Masson's trichrome staining, and periodic acid-Schiff staining.

RESULTS

NM-EMSCs specifically expressing CD73, CD105, and CD90 were successfully isolated from the nasal mucosa of rats and were able to differentiate into adipocytes, osteoblasts, and chondrocytes. After saline and NM-EMSC injection, compared with those in the blank control CK + NS group, the nasal mucosa in the CRS + NS and CRS + EMSC groups exhibited obvious thickening, a large amount of inflammatory cell infiltration, and increased collagen and mucin distribution. Four days post-NM-EMSC injection, the thickening of the nasal mucosa in the CRS group was gradually alleviated, the inflammatory cell infiltration gradually decreased, and the distribution of collagen and mucin and the collagen-positive area gradually decreased. Moreover, only a small number of inflammatory cells were visible, and the distribution of mucins was limited to 6 days post-NM-EMSC injection.

CONCLUSION

NM-EMSCs effectively attenuated inflammation in the nasal mucosa of CRS model rats.

Xenopus as a model system for studying pigmentation and pigmentary disorders

Human pigmentary disorders encompass a broad spectrum of phenotypic changes arising from disruptions in various stages of melanocyte formation, the melanogenesis process, or the transfer of pigment from melanocytes to keratinocytes. A large number of pigmentation genes associated with pigmentary disorders have been identified, many of them awaiting in vivo confirmation. A more comprehensive understanding of the molecular basis of pigmentary disorders requires a vertebrate animal model where changes in pigmentation are easily observable in vivo and can be combined to genomic modifications and gain/loss-of-function tools. Here we present the amphibian Xenopus with its unique features that fulfill these requirements. Changes in pigmentation are particularly easy to score in Xenopus embryos, allowing whole-organism based phenotypic screening. The development and behavior of Xenopus melanocytes closely mimic those observed in mammals. Interestingly, both Xenopus and mammalian skins exhibit comparable reactions to ultraviolet radiation. This review highlights how Xenopus constitutes an alternative and complementary model to the more commonly used mouse and zebrafish, contributing to the advancement of knowledge in melanocyte cell biology and related diseases.

The genomic landscape of mammal domestication might be orchestrated by selected transcription factors regulating brain and craniofacial development

Domestication transforms once wild animals into tamed animals that can be then exploited by humans. The process entails modifications in the body, cognition, and behavior that are essentially driven by differences in gene expression patterns. Although genetic and epigenetic mechanisms were shown to underlie such differences, less is known about the role exerted by trans-regulatory molecules, notably transcription factors (TFs) in domestication. In this paper, we conducted extensive in silico analyses aimed to clarify the TF landscape of mammal domestication. We first searched the literature, so as to establish a large list of genes selected with domestication in mammals. From this list, we selected genes experimentally demonstrated to exhibit TF functions. We also considered TFs displaying a statistically significant number of targets among the entire list of (domestication) selected genes. This workflow allowed us to identify 5 candidate TFs (SOX2, KLF4, MITF, NR3C1, NR3C2) that were further assessed in terms of biochemical and functional properties. We found that such TFs-of-interest related to mammal domestication are all significantly involved in the development of the brain and the craniofacial region, as well as the immune response and lipid metabolism. A ranking strategy, essentially based on a survey of protein-protein interactions datasets, allowed us to identify SOX2 as the main candidate TF involved in domestication-associated evolutionary changes. These findings should help to clarify the molecular mechanics of domestication and are of interest for future studies aimed to understand the behavioral and cognitive changes associated to domestication.

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.

Cleft palate and minor metabolic disturbances in a mouse global Arl15 gene knockout

ARL15, a small GTPase protein, was linked to metabolic traits in association studies. We aimed to test the Arl15 gene as a functional candidate for metabolic traits in the mouse. CRISPR/Cas9 germline knockout (KO) of Arl15 showed that homozygotes were postnatal lethal and exhibited a complete cleft palate (CP). Also, decreased cell migration was observed from Arl15 KO mouse embryonic fibroblasts (MEFs). Metabolic phenotyping of heterozygotes showed that females had reduced fat mass on a chow diet from 14 weeks of age. Mild body composition phenotypes were also observed in heterozygous mice on a high-fat diet (HFD)/low-fat diet (LFD). Females on a HFD showed reduced body weight, gonadal fat depot weight and brown adipose tissue (BAT) weight. In contrast, in the LFD group, females showed increased bone mineral density (BMD), while males showed a trend toward reduced BMD. Clinical biochemistry analysis of plasma on HFD showed transient lower adiponectin at 20 weeks of age in females. Urinary and plasma Mg2+ concentrations were not significantly different. Our phenotyping data showed that Arl15 is essential for craniofacial development. Adult metabolic phenotyping revealed potential roles in brown adipose tissue and bone development.

Tissue interplay during morphogenesis

The process by which biological systems such as cells, tissues and organisms acquire shape has been named as morphogenesis and it is central to a plethora of biological contexts including embryo development, wound healing, or even cancer. Morphogenesis relies in both self-organising properties of the system and in environmental inputs (biochemical and biophysical). The classical view of morphogenesis is based on the study of external biochemical molecules, such as morphogens. However, recent studies are establishing that the mechanical environment is also used by cells to communicate within tissues, suggesting that this mechanical crosstalk is essential to synchronise morphogenetic transitions and self-organisation. In this article we discuss how tissue interaction drive robust morphogenesis, starting from a classical biochemical view, to finalise with more recent advances on how the biophysical properties of a tissue feedback with their surroundings to allow form acquisition. We also comment on how in silico models aid to integrate and predict changes in cell and tissue behaviour. Finally, considering recent advances from the developmental biomechanics field showing that mechanical inputs work as cues that promote morphogenesis, we invite to revisit the concept of morphogen.

RNA-binding protein Elavl1/HuR is required for maintenance of cranial neural crest specification

While neural crest development is known to be transcriptionally controlled via sequential activation of gene regulatory networks (GRNs), recent evidence increasingly implicates a role for post-transcriptional regulation in modulating the output of these regulatory circuits. Using available single-cell RNA-sequencing datasets from avian embryos to identify potential post-transcriptional regulators, we found that Elavl1, which encodes for an RNA-binding protein with roles in transcript stability, was enriched in the premigratory cranial neural crest. Perturbation of Elavl1 resulted in premature neural crest delamination from the neural tube as well as significant reduction in transcripts associated with the neural crest specification GRN, phenotypes that are also observed with downregulation of the canonical Wnt inhibitor Draxin. That Draxin is the primary target for stabilization by Elavl1 during cranial neural crest specification was shown by RNA-sequencing, RNA immunoprecipitation, RNA decay measurement, and proximity ligation assays, further supporting the idea that the downregulation of neural crest specifier expression upon Elavl1 knockdown was largely due to loss of Draxin. Importantly, exogenous Draxin rescued cranial neural crest specification defects observed with Elavl1 knockdown. Thus, Elavl1 plays a critical a role in the maintenance of cranial neural crest specification via Draxin mRNA stabilization. Together, these data highlight an important intersection of post-transcriptional regulation with modulation of the neural crest specification GRN.

Pax3/7 regulates neural tube closure and patterning in a non-vertebrate chordate

Pax3/7 factors play numerous roles in the development of the dorsal nervous system of vertebrates. From specifying neural crest at the neural plate borders, to regulating neural tube closure and patterning of the resulting neural tube. However, it is unclear which of these roles are conserved in non-vertebrate chordates. Here we investigate the expression and function of Pax3/7 in the model tunicate Ciona. Pax3/7 is expressed in neural plate border cells during neurulation, and in central nervous system progenitors shortly after neural tube closure. We find that separate cis-regulatory elements control the expression in these two distinct lineages. Using CRISPR/Cas9-mediated mutagenesis, we knocked out Pax3/7 in F0 embryos specifically in these two separate territories. Pax3/7 knockout in the neural plate borders resulted in neural tube closure defects, suggesting an ancient role for Pax3/7 in this chordate-specific process. Furthermore, knocking out Pax3/7 in the neural impaired Motor Ganglion neuron specification, confirming a conserved role for this gene in patterning the neural tube as well. Taken together, these results suggests that key functions of Pax3/7 in neural tube development are evolutionarily ancient, dating back at least to the last common ancestor of vertebrates and tunicates.

Feedback Regulation of Signaling Pathways for Precise Pre-Placodal Ectoderm Formation in Vertebrate Embryos

Intracellular signaling pathways are essential to establish embryonic patterning, including embryonic axis formation. Ectodermal patterning is also governed by a series of morphogens. Four ectodermal regions are thought to be controlled by morphogen gradients, but some perturbations are expected to occur during dynamic morphogenetic movement. Therefore, a mechanism to define areas precisely and reproducibly in embryos, including feedback regulation of signaling pathways, is necessary. In this review, we outline ectoderm pattern formation and signaling pathways involved in the establishment of the pre-placodal ectoderm (PPE). We also provide an example of feedback regulation of signaling pathways for robust formation of the PPE, showing the importance of this regulation.

The Emergence of Embryonic Myosin Heavy Chain during Branchiomeric Muscle Development

A prerequisite for discovering the properties and therapeutic potential of branchiomeric muscles is an understanding of their fate determination, pattering and differentiation. Although the expression of differentiation markers such as myosin heavy chain (MyHC) during trunk myogenesis has been more intensively studied, little is known about its expression in the developing branchiomeric muscle anlagen. To shed light on this, we traced the onset of MyHC expression in the facial and neck muscle anlagen by using the whole-mount in situ hybridization between embryonic days E9.5 and E15.5 in the mouse. Unlike trunk muscle, the facial and neck muscle anlagen express MyHC at late stages. Within the branchiomeric muscles, our results showed variation in the emergence of MyHC expression. MyHC was first detected in the first arch-derived muscle anlagen, while its expression in the second arch-derived muscle and non-somitic neck muscle began at a later time point. Additionally, we show that non-ectomesenchymal neural crest invasion of the second branchial arch is delayed compared with that of the first brachial arch in chicken embryos. Thus, our findings reflect the timing underlying branchiomeric muscle differentiation.

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.

Shaping axial identity during human pluripotent stem cell differentiation to neural crest cells

The neural crest (NC) is a multipotent cell population which can give rise to a vast array of derivatives including neurons and glia of the peripheral nervous system, cartilage, cardiac smooth muscle, melanocytes and sympathoadrenal cells. An attractive strategy to model human NC development and associated birth defects as well as produce clinically relevant cell populations for regenerative medicine applications involves the in vitro generation of NC from human pluripotent stem cells (hPSCs). However, in vivo, the potential of NC cells to generate distinct cell types is determined by their position along the anteroposterior (A-P) axis and, therefore the axial identity of hPSC-derived NC cells is an important aspect to consider. Recent advances in understanding the developmental origins of NC and the signalling pathways involved in its specification have aided the in vitro generation of human NC cells which are representative of various A-P positions. Here, we explore recent advances in methodologies of in vitro NC specification and axis patterning using hPSCs.

Repressive Interactions Between Transcription Factors Separate Different Embryonic Ectodermal Domains

The embryonic ectoderm is composed of four domains: neural plate, neural crest, pre-placodal region (PPR) and epidermis. Their formation is initiated during early gastrulation by dorsal-ventral and anterior-posterior gradients of signaling factors that first divide the embryonic ectoderm into neural and non-neural domains. Next, the neural crest and PPR domains arise, either via differential competence of the neural and non-neural ectoderm (binary competence model) or via interactions between the neural and non-neural ectoderm tissues to produce an intermediate neural border zone (NB) (border state model) that subsequently separates into neural crest and PPR. Many previous gain- and loss-of-function experiments demonstrate that numerous TFs are expressed in initially overlapping zones that gradually resolve into patterns that by late neurula stages are characteristic of each of the four domains. Several of these studies suggested that this is accomplished by a combination of repressive TF interactions and competence to respond to local signals. In this study, we ectopically expressed TFs that at neural plate stages are characteristic of one domain in a different domain to test whether they act cell autonomously as repressors. We found that almost all tested TFs caused reduced expression of the other TFs. At gastrulation these effects were strictly within the lineage-labeled cells, indicating that the effects were cell autonomous, i.e., due to TF interactions within individual cells. Analysis of previously published single cell RNAseq datasets showed that at the end of gastrulation, and continuing to neural tube closure stages, many ectodermal cells express TFs characteristic of more than one neural plate stage domain, indicating that different TFs have the opportunity to interact within the same cell. At neurula stages repression was observed both in the lineage-labeled cells and in adjacent cells not bearing detectable lineage label, suggesting that cell-to-cell signaling has begun to contribute to the separation of the domains. Together, these observations directly demonstrate previous suggestions in the literature that the segregation of embryonic ectodermal domains initially involves cell autonomous, repressive TF interactions within an individual cell followed by the subsequent advent of non-cell autonomous signaling to neighbors.

Human pluripotent stem cell-derived ectomesenchymal stromal cells promote more robust functional recovery than umbilical cord-derived mesenchymal stromal cells after hypoxic-ischaemic brain damage

Aims: Hypoxic-ischaemic encephalopathy (HIE) is one of the most serious complications in neonates and infants. Mesenchymal stromal cell (MSC)-based therapy is emerging as a promising treatment avenue for HIE. However, despite its enormous potential, the clinical application of MSCs is limited by cell heterogeneity, low isolation efficiency and unpredictable effectiveness. In this study, we examined the therapeutic effects and underlying mechanisms of human pluripotent stem cell-derived ectomesenchymal stromal cells (hPSC-EMSCs) in a rat model of HIE.

Methods: hPSC-EMSCs were induced from either human embryonic stem cells or induced pluripotent stem cells. Stem cells or the conditioned medium (CM) derived from stem cells were delivered intracranially or intranasally to neonatal rats with HIE. Human umbilical cord-derived MSCs (hUC-MSCs) were used as the therapeutic comparison control and phosphate-buffered saline (PBS) was used as a negative control. Lesion size, apoptosis, neurogenesis, astrogliosis and microgliosis were evaluated. The rotarod test and Morris water maze were used to determine brain functional recovery. The PC-12 cell line, rat primary cortical neurons and neural progenitor cells were used to evaluate neurite outgrowth and the neuroprotective and neurogenesis effects of hPSC-EMSCs/hUC-MSCs. RNA-seq and enzyme-linked immunosorbent assays were used to determine the secretory factors that were differentially expressed between hPSC-EMSCs and hUC-MSCs. The activation and suppression of extracellular signal-regulated kinase (ERK) and cAMP response element-binding protein (CREB) were characterised using western blotting and immunofluorescent staining.

Results: hPSC-EMSCs showed a higher neuroprotective potential than hUC-MSCs, as demonstrated by a more significant reduction in lesion size and apoptosis in the rat brain following hypoxia-ischaemia (HI). Compared with PBS treatment, hPSC-EMSCs promoted endogenous neurogenesis and alleviated astrogliosis and microgliosis. hPSC-EMSCs were more effective than hUC-MSCs. hPSC-EMSCs achieved a greater recovery of brain function than hUC-MSCs and PBS in rats with HIE. CM derived from hPSC-EMSCs had neuroprotective and neurorestorative effects in vitro through anti-apoptotic and neurite outgrowth- and neurogenesis-promoting effects. Direct comparisons between hPSC-EMSCs and hUC-MSCs revealed the significant enrichment of a group of secretory factors in hPSC-EMSCs, including nerve growth factor (NGF), platelet-derived growth factor-AA and transforming growth factor-β₂, which are involved in neurogenesis, synaptic transmission and neurotransmitter transport, respectively. Mechanistically, the CM derived from hPSC-EMSCs was found to potentiate NGF-induced neurite outgrowth and the neuronal differentiation of NPCs via the ERK/CREB pathway. Suppression of ERK or CREB abolished CM-potentiated neuritogenesis and neuronal differentiation. Finally, intranasal delivery of the CM derived from hPSC-EMSCs significantly reduced brain lesion size, promoted endogenous neurogenesis, mitigated inflammatory responses and improved functional recovery in rats with HIE.

Conclusion: hPSC-EMSCs promote functional recovery after HI through multifaceted neuromodulatory activities via paracrine/trophic mechanisms. We propose the use of hPSC-EMSCs for the treatment of HIE, as they offer an excellent unlimited cellular source of MSCs.

Early development of the breathing network

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

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.

How Neural Crest Transcription Factors Contribute to Melanoma Heterogeneity, Cellular Plasticity, and Treatment Resistance

Cutaneous melanoma represents one of the deadliest types of skin cancer. The prognosis strongly depends on the disease stage, thus early detection is crucial. New therapies, including BRAF and MEK inhibitors and immunotherapies, have significantly improved the survival of patients in the last decade. However, intrinsic and acquired resistance is still a challenge. In this review, we discuss two major aspects that contribute to the aggressiveness of melanoma, namely, the embryonic origin of melanocytes and melanoma cells and cellular plasticity. First, we summarize the physiological function of epidermal melanocytes and their development from precursor cells that originate from the neural crest (NC). Next, we discuss the concepts of intratumoral heterogeneity, cellular plasticity, and phenotype switching that enable melanoma to adapt to changes in the tumor microenvironment and promote disease progression and drug resistance. Finally, we further dissect the connection of these two aspects by focusing on the transcriptional regulators MSX1, MITF, SOX10, PAX3, and FOXD3. These factors play a key role in NC initiation, NC cell migration, and melanocyte formation, and we discuss how they contribute to cellular plasticity and drug resistance in melanoma.

Nerve growth factor (NGF) and NGF receptors in mesenchymal stem/stromal cells: Impact on potential therapies

Mesenchymal stem/stromal cells (MSCs) are promising for the treatment of degenerative diseases and traumatic injuries. However, MSC engraftment is not always successful and requires a strong comprehension of the cytokines and their receptors that mediate the biological behaviors of MSCs. The effects of nerve growth factor (NGF) and its two receptors, TrkA and p75NTR, on neural cells are well studied. Increasing evidence shows that NGF, TrkA, and p75NTR are also involved in various aspects of MSC function, including their survival, growth, differentiation, and angiogenesis. The regulatory effect of NGF on MSCs is thought to be achieved mainly through its binding to TrkA. p75NTR, another receptor of NGF, is regarded as a novel surface marker of MSCs. This review provides an overview of advances in understanding the roles of NGF and its receptors in MSCs as well as the effects of MSC‐derived NGF on other cell types, which will provide new insight for the optimization of MSC‐based therapy.

Evolution of new cell types at the lateral neural border

During the course of evolution, animals have become increasingly complex by the addition of novel cell types and regulatory mechanisms. A prime example is represented by the lateral neural border, known as the neural plate border in vertebrates, a region of the developing ectoderm where presumptive neural and non-neural tissue meet. This region has been intensively studied as the source of two important embryonic cell types unique to vertebrates-the neural crest and the ectodermal placodes-which contribute to diverse differentiated cell types including the peripheral nervous system, pigment cells, bone, and cartilage. How did these multipotent progenitors originate in animal evolution? What triggered the elaboration of the border during the course of chordate evolution? How is the lateral neural border patterned in various bilaterians and what is its fate? Here, we review and compare the development and fate of the lateral neural border in vertebrates and invertebrates and we speculate about its evolutionary origin. Taken together, the data suggest that the lateral neural border existed in bilaterian ancestors prior to the origin of vertebrates and became a developmental source of exquisite evolutionary change that frequently enabled the acquisition of new cell types.

Transcriptomic Identification of Draxin-Responsive Targets During Cranial Neural Crest EMT

Canonical Wnt signaling plays an essential role in proper craniofacial morphogenesis, at least partially due to regulation of various aspects of cranial neural crest development. In an effort to gain insight into the etiology of craniofacial abnormalities resulting from Wnt signaling and/or cranial neural crest dysfunction, we sought to identify Wnt-responsive targets during chick cranial neural crest development. To this end, we leveraged overexpression of a canonical Wnt antagonist, Draxin, in conjunction with RNA-sequencing of cranial neural crest cells that have just activated their epithelial-mesenchymal transition (EMT) program. Through differential expression analysis, gene list functional annotation, hybridization chain reaction (HCR), and quantitative reverse transcription polymerase chain reaction (RT-qPCR), we validated a novel downstream target of canonical Wnt signaling in cranial neural crest - RHOB - and identified possible signaling pathway crosstalk underlying cranial neural crest migration. The results reveal novel putative targets of canonical Wnt signaling during cranial neural crest EMT and highlight important intersections across signaling pathways involved in craniofacial development.

Using an aquatic model, Xenopus laevis, to uncover the role of chromodomain 1 in craniofacial disorders

The chromodomain family member chromodomain 1 (CHD1) has been shown to have numerous critical molecular functions including transcriptional regulation, splicing, and DNA repair. Complete loss of function of this gene is not compatible with life. On the other hand, missense and copy number variants of CHD1 can result in intellectual disabilities and craniofacial malformations in human patients including cleft palate and Pilarowski-Bjornsson Syndrome. We have used the aquatic developmental model organism Xenopus laevis, to determine a specific role for Chd1 in such cranioafcial disorders. Protein and gene knockdown techniques in Xenopus, including antisense oligos and mosaic Crispr/Cas9-mediated mutagenesis, recapitulated the craniofacial defects observed in humans. Further analysis indicated that embryos deficient in Chd1 had defects in cranial neural crest development and jaw cartilage morphology. Additionally, flow cytometry and immunohistochemistry revealed that decreased Chd1 resulted in increased in apoptosis in the developing head. Together, these experiments demonstrate that Chd1 is critical for fundamental processes and cell survival in craniofacial development. We also presented evidence that Chd1 is regulated by retinoic acid signaling during craniofacial development. Expression levels of chd1 mRNA, specifically in the head, were increased by RAR agonist exposure and decreased upon antagonist treatment. Subphenotypic levels of an RAR antagonist and Chd1 morpholinos synergized to result in orofacial defects. Further, RAR DNA binding sequences (RAREs) were detected in chd1 regulatory regions by bioinformatic analysis. In summary, by combining human genetics and experiments in an aquatic model we now have a better understanding of the role of CHD1 in craniofacial disorders.

Using Xenopus to analyze neurocristopathies like Kabuki syndrome

Neurocristopathies are human congenital syndromes that arise from defects in neural crest (NC) development and are typically associated with malformations of the craniofacial skeleton. Genetic analyses have been very successful in identifying pathogenic mutations, however, model organisms are required to characterize how these mutations affect embryonic development thereby leading to complex clinical conditions. The African clawed frog Xenopus laevis provides a broad range of in vivo and in vitro tools allowing for a detailed characterization of NC development. Due to the conserved nature of craniofacial morphogenesis in vertebrates, Xenopus is an efficient and versatile system to dissect the morphological and cellular phenotypes as well as the signaling events leading to NC defects. Here, we review a set of techniques and resources how Xenopus can be used as a disease model to investigate the pathogenesis of Kabuki syndrome and neurocristopathies in a wider sense.

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.

Genetics Underlying the Interactions between Neural Crest Cells and Eye Development

The neural crest is a unique, transient stem cell population that is critical for craniofacial and ocular development. Understanding the genetics underlying the steps of neural crest development is essential for gaining insight into the pathogenesis of congenital eye diseases. The neural crest cells play an under-appreciated key role in patterning the neural epithelial-derived optic cup. These interactions between neural crest cells within the periocular mesenchyme and the optic cup, while not well-studied, are critical for optic cup morphogenesis and ocular fissure closure. As a result, microphthalmia and coloboma are common phenotypes in human disease and animal models in which neural crest cell specification and early migration are disrupted. In addition, neural crest cells directly contribute to numerous ocular structures including the cornea, iris, sclera, ciliary body, trabecular meshwork, and aqueous outflow tracts. Defects in later neural crest cell migration and differentiation cause a constellation of well-recognized ocular anterior segment anomalies such as Axenfeld–Rieger Syndrome and Peters Anomaly. This review will focus on the genetics of the neural crest cells within the context of how these complex processes specifically affect overall ocular development and can lead to congenital eye diseases.

Bimodal function of chromatin remodeler Hmga1 in neural crest induction and Wnt-dependent emigration

During gastrulation, neural crest cells are specified at the neural plate border, as characterized by Pax7 expression. Using single-cell RNA sequencing coupled with high-resolution in situ hybridization to identify novel transcriptional regulators, we show that chromatin remodeler Hmga1 is highly expressed prior to specification and maintained in migrating chick neural crest cells. Temporally controlled CRISPR-Cas9-mediated knockouts uncovered two distinct functions of Hmga1 in neural crest development. At the neural plate border, Hmga1 regulates Pax7-dependent neural crest lineage specification. At premigratory stages, a second role manifests where Hmga1 loss reduces cranial crest emigration from the dorsal neural tube independent of Pax7. Interestingly, this is rescued by stabilized ß-catenin, thus implicating Hmga1 as a canonical Wnt activator. Together, our results show that Hmga1 functions in a bimodal manner during neural crest development to regulate specification at the neural plate border, and subsequent emigration from the neural tube via canonical Wnt signaling.

The neural plate is a structure that serves as the basis for the brain and central nervous system during the development of animals with a backbone. In particular, the tissues at the border of the neural plate become the neural crest, a group of highly mobile cells that can specialize to form nerves and parts of the face. The exact molecular mechanisms that allow the crest to emerge are still unknown.

The protein Hmga1 alters how genes are packaged and organized inside cells, which in turn influences how genes are switched on and off. Here, Gandhi et al. studied how Hmga1 helps to shape the neural crest in developing chicken embryos. To do so, they harnessed a genetic tool called CRISPR-Cas9, and deleted the gene that encodes Hmga1 at specific developmental stages. This manipulation highlighted two periods where Hmga1 is active. First, Hmga1 helped to define neural crest cells at the neural plate border by activating a gene called pax7. Then, at a later stage, Hmga1 allowed these cells to move to other parts of the body by triggering the Wnt communication system.

Failure for the neural crest to develop properly causes birth defects and cancers such as melanoma and childhood neuroblastoma, highlighting the need to better understand how this structure is formed. In addition, a better grasp of the roles of Hmga1 in healthy development could help to appreciate how it participates in a range of adult cancers.

Hedgehog Signaling in Skeletal Development: Roles of Indian Hedgehog and the Mode of Its Action

Hedgehog (Hh) signaling is highly conserved among species and plays indispensable roles in various developmental processes. There are three Hh members in mammals; one of them, Indian hedgehog (Ihh), is expressed in prehypertrophic and hypertrophic chondrocytes during endochondral ossification. Based on mouse genetic studies, three major functions of Ihh have been proposed: (1) Regulation of chondrocyte differentiation via a negative feedback loop formed together with parathyroid hormone-related protein (PTHrP), (2) promotion of chondrocyte proliferation, and (3) specification of bone-forming osteoblasts. Gli transcription factors mediate the major aspect of Hh signaling in this context. Gli3 has dominant roles in the growth plate chondrocytes, whereas Gli1, Gli2, and Gli3 collectively mediate biological functions of Hh signaling in osteoblast specification. Recent studies have also highlighted postnatal roles of the signaling in maintenance and repair of skeletal tissues.

Evolutionary and Developmental Associations of Neural Crest and Placodes in the Vertebrate Head: Insights From Jawless Vertebrates

Neural crest and placodes are key innovations of the vertebrate clade. These cells arise within the dorsal ectoderm of all vertebrate embryos and have the developmental potential to form many of the morphological novelties within the vertebrate head. Each cell population has its own distinct developmental features and generates unique cell types. However, it is essential that neural crest and placodes associate together throughout embryonic development to coordinate the emergence of several features in the head, including almost all of the cranial peripheral sensory nervous system and organs of special sense. Despite the significance of this developmental feat, its evolutionary origins have remained unclear, owing largely to the fact that there has been little comparative (evolutionary) work done on this topic between the jawed vertebrates and cyclostomes—the jawless lampreys and hagfishes. In this review, we briefly summarize the developmental mechanisms and genetics of neural crest and placodes in both jawed and jawless vertebrates. We then discuss recent studies on the role of neural crest and placodes—and their developmental association—in the head of lamprey embryos, and how comparisons with jawed vertebrates can provide insights into the causes and consequences of this event in early vertebrate evolution.

Single Amino Acid Change Underlies Distinct Roles of H2A.Z Subtypes in Human Syndrome

The developmental disorder Floating-Harbor syndrome (FHS) is caused by heterozygous truncating mutations in SRCAP, a gene encoding a chromatin remodeler mediating incorporation of histone variant H2A.Z. Here, we demonstrate that FHS-associated mutations result in loss of SRCAP nuclear localization, alter neural crest gene programs in human in vitro models and Xenopus embryos, and cause craniofacial defects. These defects are mediated by one of two H2A.Z subtypes, H2A.Z.2, whose knockdown mimics and whose overexpression rescues the FHS phenotype. Selective rescue by H2A.Z.2 is conferred by one of the three amino acid differences between the H2A.Z subtypes, S38/T38. We further show that H2A.Z.1 and H2A.Z.2 genomic occupancy patterns are qualitatively similar, but quantitatively distinct, and H2A.Z.2 incorporation at AT-rich enhancers and expression of their associated genes are both sensitized to SRCAP truncations. Altogether, our results illuminate the mechanism underlying a human syndrome and uncover selective functions of H2A.Z subtypes during development.

Generation and validation of a PITX2-EGFP reporter line of human induced pluripotent stem cells enables isolation of periocular mesenchymal cells

PITX2 (Paired-like homeodomain transcription factor 2) plays important roles in asymmetric development of the internal organs and symmetric development of eye tissues. During eye development, cranial neural crest cells migrate from the neural tube and form the periocular mesenchyme (POM). POM cells differentiate into several ocular cell types, such as corneal endothelial cells, keratocytes, and some ocular mesenchymal cells. In this study, we used transcription activator-like effector nuclease technology to establish a human induced pluripotent stem cell (hiPSC) line expressing a fluorescent reporter gene from the PITX2 promoter. Using homologous recombination, we heterozygously inserted a PITX2-IRES2-EGFP sequence downstream of the stop codon in exon 8 of PITX2 Cellular pluripotency was monitored with alkaline phosphatase and immunofluorescence staining of pluripotency markers, and the hiPSC line formed normal self-formed ectodermal autonomous multizones. Using a combination of previously reported methods, we induced PITX2 in the hiPSC line and observed simultaneous EGFP and PITX2 expression, as indicated by immunoblotting and immunofluorescence staining. PITX2 mRNA levels were increased in EGFP-positive cells, which were collected by cell sorting, and marker gene expression analysis of EGFP-positive cells induced in self-formed ectodermal autonomous multizones revealed that they were genuine POM cells. Moreover, after 2 days of culture, EGFP-positive cells expressed the PITX2 protein, which co-localized with forkhead box C1 (FOXC1) protein in the nucleus. We anticipate that the PITX2-EGFP hiPSC reporter cell line established and validated here can be utilized to isolate POM cells and to analyze PITX2 expression during POM cell induction.

Comparison of differentiation gene batteries for migratory mechanosensory neurons across bilaterians

In embryos of distantly related bilaterian phyla, their lateral neural borders give rise to the peripheral nervous system elements, including various mechanosensory cells derived from migratory precursors, such as hair cells and dorsal root ganglion (DRG) neurons in vertebrates, bipolar tail neuron (BTN) in Ciona, chordotonal organ in Drosophila, and AVM/PVM in Caenorhabditis elegans. Developmental genetics studies had revealed a couple of transcription factors (TFs) regulating differentiation of mechanosensory cells shared by vertebrates and arthropods. However, unbiased systematic profiling of regulators is needed to demonstrate conservation of differentiation gene batteries for mechanosensory cells across bilaterians. At first, we observed that in both C. elegans Q neuroblasts and Drosophila lateral neuroectoderm, conserved NPB specifier Msx/vab-15 regulates Atoh1/lin-32, supporting the homology of mechanosensory neuron development in lateral neural border lineage of Ecdysozia. So we used C. elegans as a protostomia model. Single-cell resolution expression profiling of TFs and genetic analysis revealed a differentiation gene battery (Atonh1/lin-32, Drg11/alr-1, Gfi1/pag-3, Lhx5/mec-3, and Pou4/unc-86) for AVM/PVM mechanosensory neurons. The worm-gene battery significantly overlaps with both that of placode-derived Atonh1/lin-32-dependent hair cells and that of NPB-derived Neurogenin-dependent DRG neurons in vertebrates, supporting the homology of molecular mechanisms underlying the differentiation of neural border-derived mechanosensory cells between protostome and deuterostome. At last, Ciona BTN, the homolog of vertebrate DRG, also expresses Atonh1/lin-32, further supporting the homology notion and indicating a common origin of hair cells and DRG in vertebrate lineage.

Modeling of human neurulation using bioengineered pluripotent stem cell culture

Leveraging the developmental potential and self-organizing property of human pluripotent stem (hPS) cells, researchers have developed tractable models of human embryonic development. Owing to their compatibility to live imaging, genome editing, mechanical perturbation and measurement, these models offer promising quantitative experimental platforms to advance human embryology and regenerative medicine. Herein, we provide a review of recent progress in using hPS cells to generate models of early human neural development or neurulation, including neural induction and regional patterning of the neural tube. These models, even in their nascent developmental stages, have already revealed intricate cell-cell signaling and mechanoregulation mechanisms likely involved in tissue patterning during early neural development. We also discuss future opportunities in modeling early neural development by incorporating bioengineering tools to control precisely neural tissue morphology and architecture, morphogen dynamics, intracellular signaling events, and cell-cell interactions to further the development of this emerging field and expand its applications.

The origin and evolution of vertebrate neural crest cells

The neural crest is a vertebrate-specific migratory stem cell population that generates a remarkably diverse set of cell types and structures. Because many of the morphological, physiological and behavioural novelties of vertebrates are derived from neural crest cells, it is thought that the origin of this cell population was an important milestone in early vertebrate history. An outstanding question in the field of vertebrate evolutionary-developmental biology (evo-devo) is how this cell type evolved in ancestral vertebrates. In this review, we briefly summarize neural crest developmental genetics in vertebrates, focusing in particular on the gene regulatory interactions instructing their early formation within and migration from the dorsal neural tube. We then discuss how studies searching for homologues of neural crest cells in invertebrate chordates led to the discovery of neural crest-like cells in tunicates and the potential implications this has for tracing the pre-vertebrate origins of the neural crest population. Finally, we synthesize this information to propose a model to explain the origin of neural crest cells. We suggest that at least some of the regulatory components of early stages of neural crest development long pre-date vertebrate origins, perhaps dating back to the last common bilaterian ancestor. These components, originally directing neuroectodermal patterning and cell migration, served as a gene regulatory 'scaffold' upon which neural crest-like cells with limited migration and potency evolved in the last common ancestor of tunicates and vertebrates. Finally, the acquisition of regulatory programmes controlling multipotency and long-range, directed migration led to the transition from neural crest-like cells in invertebrate chordates to multipotent migratory neural crest in the first vertebrates.

Wnt Signaling in Neural Crest Ontogenesis and Oncogenesis

Neural crest (NC) cells are a temporary population of multipotent stem cells that generate a diverse array of cell types, including craniofacial bone and cartilage, smooth muscle cells, melanocytes, and peripheral neurons and glia during embryonic development. Defective neural crest development can cause severe and common structural birth defects, such as craniofacial anomalies and congenital heart disease. In the early vertebrate embryos, NC cells emerge from the dorsal edge of the neural tube during neurulation and then migrate extensively throughout the anterior-posterior body axis to generate numerous derivatives. Wnt signaling plays essential roles in embryonic development and cancer. This review summarizes current understanding of Wnt signaling in NC cell induction, delamination, migration, multipotency, and fate determination, as well as in NC-derived cancers.

Sequence alteration in the enhancer contributes to the heterochronic Sox9 expression in marsupial cranial neural crest

Neonates of marsupial mammals are altricial at birth, because their gestation period is relatively short compared to placental mammals. Yet, as they need to travel to the teat from the birth canal, and suckle on the mother's milk, forelimbs and jaws develop significantly early. Previous studies in opossum (Monodelphis domestica), an experimental marsupial model, have revealed that cranial neural crest cells are generated significantly early compared to those in placental mammals, such as mouse, leading to an early development of jaw primordia. We have previously found that Sox9, an important neural crest-specifier gene, is expressed in the future cranial neural crest of the opossum embryonic ectoderm significantly earlier than that in mouse or quail embryos. As Sox9 is essential for neural crest formation in various vertebrates, it seems likely that the heterochronic expression of Sox9 is critical for the early cranial neural crest formation in the marsupial embryos. In this study, we show a marsupial-specific sequence in the Sox9 neural crest enhancer E3. We also reveal that the mouse E3 enhancer is activated in the cranial neural crest cells of quail embryos, that the E3 enhancer with marsupial-specific sequence is activated earlier in the Pax7-expressing neural border prior to the onset of endogenous Sox9 expression, and that a misexpression of cMyb, which is also a transcriptional activator of Pax7, in the neural border can ectopically activate the "marsupialized" enhancer. Thus, we suggest that the modification of the E3 enhancer sequence in the marsupial ancestor would have promoted the early expression of Sox9 in the neural border, facilitating the early formation of the cranial neural crest cells and the subsequent heterochronic development of the jaw primordia.

Stage-dependent differential gene expression profiles of cranial neural crest-like cells derived from mouse-induced pluripotent stem cells

Cranial neural crest cells are multipotent cells that migrate into the pharyngeal arches of the vertebrate embryo and differentiate into various craniofacial organ derivatives. Therefore, migrating cranial neural crest cells are considered one of the most attractive candidate cell sources in regenerative medicine. We generated cranial neural crest like cell (cNCCs) using mouse-induced pluripotent stem cells cultured in neural crest-inducing medium for 14 days. Subsequently, we conducted RNA sequencing experiments to analyze gene expression profiles of cNCCs at different time points after induction. cNCCs expressed several neural crest specifier genes; however, some previously reported specifier genes such as paired box 3 and Forkhead box D3, which are essential for embryonic neural crest development, were not expressed. Moreover, ETS proto-oncogene 1, transcription factor and sex-determining region Y-box 10 were only expressed after 14 days of induction. Finally, cNCCs expressed multiple protocadherins and a disintegrin and metalloproteinase with thrombospondin motifs enzymes, which may be crucial for their migration.

Evolution of Snail-mediated regulation of neural crest and placodes from an ancient role in bilaterian neurogenesis

A major challenge in vertebrate evolution is to identify the gene regulatory mechanisms that facilitated the origin of neural crest cells and placodes from ancestral precursors in invertebrates. Here, we show in lamprey, a primitively jawless vertebrate, that the transcription factor Snail is expressed simultaneously throughout the neural plate, neural plate border, and pre-placodal ectoderm in the early embryo and is then upregulated in the CNS throughout neurogenesis. Using CRISPR/Cas9-mediated genome editing, we demonstrate that Snail plays functional roles in all of these embryonic domains or their derivatives. We first show that Snail patterns the neural plate border by repressing lateral expansion of Pax3/7 and activating nMyc and ZicA. We also present evidence that Snail is essential for DlxB-mediated establishment of the pre-placodal ectoderm but is not required for SoxB1a expression during formation of the neural plate proper. At later stages, Snail regulates formation of neural crest-derived and placode-derived PNS neurons and controls CNS neural differentiation in part by promoting cell survival. Taken together with established functions of invertebrate Snail genes, we identify a pan-bilaterian mechanism that extends to jawless vertebrates for regulating neurogenesis that is dependent on Snail transcription factors. We propose that ancestral vertebrates deployed an evolutionarily conserved Snail expression domain in the CNS and PNS for neurogenesis and then acquired derived functions in neural crest and placode development by recruitment of regulatory genes downstream of neuroectodermal Snail activity. Our results suggest that Snail regulatory mechanisms in vertebrate novelties such as the neural crest and placodes may have emerged from neurogenic roles that originated early in bilaterian evolution.

Using Amphioxus as a Basal Chordate Model to Study BMP Signaling Pathway

The BMP signaling pathway has been shown to be involved in different aspects of embryonic development across diverse metazoan phyla. Comparative studies on the roles of the BMP signaling pathway provide crucial insights into the evolution of the animal body plans. In this chapter, we present the general workflow on how to investigate the roles of BMP signaling pathway during amphioxus embryonic development. As amphioxus are basal invertebrate chordates, studies on the BMP signaling pathway in amphioxus could elucidate the functional evolution of BMP pathway in the chordate group. Here, we describe methods for animal husbandry, spawning induction, and manipulation of the BMP signaling pathway during embryonic development through drug inhibitors and recombinant proteins. We also introduce an efficient method of using mesh baskets to handle amphioxus embryos for fluorescence immunostaining and multicolor fluorescence in situ hybridization and to assay the effects of manipulating BMP signaling pathway during amphioxus embryogenesis.

Loss of the small GTPase Arl8b results in abnormal development of the roof plate in mouse embryos

Lysosomes are acidic organelles responsible for degrading both exogenous and endogenous materials. The small GTPase Arl8 localizes primarily to lysosomes and is involved in lysosomal function. In the present study, using Arl8b gene-trapped mutant (Arl8b^-/- ) mice, we show that Arl8b is required for the development of dorsal structures of the neural tube, including the thalamus and hippocampus. In embryonic day (E) 10.5 Arl8b^-/- embryos, Sox1 (a neuroepithelium marker) was ectopically expressed in the roof plate, whereas the expression of Gdf7 and Msx1 (roof plate markers) was reduced in the dorsal midline of the midbrain. Ectopic expression of Sox1 in Arl8b^-/- embryos was detected also at E9.0 in the neural fold, which gives rise to the roof plate. In addition, the levels of Bmp receptor IA and phosphorylated Smad 1/5/8 (downstream of BMP signaling) were increased in the neural fold of E9.0 Arl8b^-/- embryos. These results suggest that Arl8b is involved in the development of the neural fold and the subsequently formed roof plate, possibly via control of BMP signaling.

From neural crest cells to melanocytes: cellular plasticity during development and beyond

Here, we review melanocyte development and how the embryonic melanoblast, although specified to become a melanocyte, is prone to cellular plasticity and is not fully committed to the melanocyte lineage. Even fully differentiated and pigment-producing melanocytes do not always have a stable phenotype. The gradual lineage restriction of neural crest cells toward the melanocyte lineage is determined by both cell-intrinsic and extracellular signals in which differentiation and pathfinding ability reciprocally influence each other. These signals are leveraged by subtle differences in timing and axial positioning. The most extensively studied migration route is the dorsolateral path between the dermomyotome and the prospective epidermis, restricted to melanoblasts. In addition, the embryonic origin of the skin dermis through which neural crest derivatives migrate may also affect the segregation between melanogenic and neurogenic cells in embryos. It is widely accepted that, irrespective of the model organism studied, the immediate precursor of both melanoblast and neurogenic populations is a glial-melanogenic bipotent progenitor. Upon exposure to different conditions, melanoblasts may differentiate into other neural crest-derived lineages such as neuronal cells and vice versa. Key factors that regulate melanoblast migration and patterning will regulate melanocyte homeostasis during different stages of hair cycling in postnatal hair follicles.

BMP4 and Neuregulin regulate the direction of mouse neural crest cell differentiation

The neural crest is a transient embryonic tissue that initially generates neural crest stem cells, which then migrate throughout the body to give rise to a variety of mature tissues. It was proposed that the fate of neural crest cells is gradually determined via environmental cues from the surrounding tissues. In the present study, neural crest cells were isolated and identified from mouse embryos. Bone morphogenetic protein 4 (BMP4) and Neuregulin (NRG) were employed to induce the differentiation of neural crest cells. Treatment with BMP4 revealed neuron-associated differentiation; cells treated with NRG exhibited differentiation into the Schwann cell lineage, a type of glia. Soft agar clonogenic and neurosphere formation assays were conducted to investigate the effects of N-Myc (MYCN) overexpression in neural crest cells; the number of colonies and neurospheres notably increased after 14 days. These findings demonstrated that the direction of cell differentiation may be affected by altering the factors present in the surrounding environment. In addition, MYCN may serve a key role in regulating neural crest cell differentiation.

Posterior axis formation requires Dlx5/Dlx6 expression at the neural plate border

Neural tube defects (NTDs), one of the most common birth defects in human, present a multifactorial etiology with a poorly defined genetic component. The Dlx5 and Dlx6 bigenic cluster encodes two evolutionary conserved homeodomain transcription factors, which are necessary for proper vertebrate development. It has been shown that Dlx5/6 genes are essential for anterior neural tube closure, however their role in the formation of the posterior structures has never been described. Here, we show that Dlx5/6 expression is required during vertebrate posterior axis formation. Dlx5 presents a similar expression pattern in neural plate border cells during posterior neurulation of zebrafish and mouse. Dlx5/6-inactivation in the mouse results in a phenotype reminiscent of NTDs characterized by open thoracic and lumbar vertebral arches and failure of epaxial muscle formation at the dorsal midline. The dlx5a/6a zebrafish morphants present posterior NTDs associated with abnormal delamination of neural crest cells showing altered expression of cell adhesion molecules and defects of motoneuronal development. Our findings provide new molecular leads to decipher the mechanisms of vertebrate posterior neurulation and might help to gather a better understanding of human congenital NTDs etiology.

Human Pluripotent Stem Cell-Derived Neural Crest Cells for Tissue Regeneration and Disease Modeling

Neural crest cells (NCCs) are a multipotent and migratory cell population in the developing embryo that contribute to the formation of a wide range of tissues. Defects in the development, differentiation and migration of NCCs give rise to a class of syndromes and diseases that are known as neurocristopathies. NCC development has historically been studied in a variety of animal models, including xenopus, chick and mouse. In the recent years, there have been efforts to study NCC development and disease in human specific models, with protocols being established to derive NCCs from human pluripotent stem cells (hPSCs), and to further differentiate these NCCs to neural, mesenchymal and other lineages. These in vitro differentiation platforms are a valuable tool to gain a better understanding of the molecular mechanisms involved in human neural crest development. The use of induced pluripotent stem cells (iPSCs) derived from patients afflicted with neurocristopathies has also enabled the study of defective human NCC development using these in vitro platforms. Here, we review the various in vitro strategies that have been used to derive NCCs from hPSCs and to specify NCCs into cranial, trunk, and vagal subpopulations and their derivatives. We will also discuss the potential applications of these human specific NCC platforms, including the use of iPSCs for disease modeling and the potential of NCCs for future regenerative applications.

Comparative Analysis of Biological Properties of Large-Scale Expanded Adult Neural Crest-Derived Stem Cells Isolated from Human Hair Follicle and Skin Dermis

Introduction

The adult neural crest-derived stem cells (NCSCs) have significant perspectives for use in regenerative medicine. The most attractive sources for adult NCSC isolation are the hair follicles (HF) and skin dermis (SD) because of easy access and minimally invasive biopsy. The aim of this study was to compare the biological properties of HF- and SD-derived NCSCs after their large-scale expansion.

Methods

The conventional explant method was used to obtain HF NCSCs. For the isolation of SD NCSCs, a new combined technique consisting of preplating and subsequent culturing in 3D blood plasma-derived fibrin hydrogel was applied. The studied cells were characterized by flow cytometry, ICC, qPCR, Bio-Plex multiplex assay, and directed multilineage differentiation assays.

Results

We have obtained both adult SD and HF NCSCs from each skin sample (n = 5). Adult SD and HF NCSCs were positive for key neural crest markers: SOX10, P75 (CD271), NESTIN, SOX2, and CD349. SD NCSCs showed a higher growth rate during the large-scale expansion compared to HF NCSCs (p < 0.01). Final population of SD NCSCs also contained more clonogenic cells (p < 0.01) and SOX10⁺, CD271⁺, CD105⁺, CD140a⁺, CD146⁺, CD349⁺ cells (p < 0.01). Both HF and SD NCSCs had similar gene expression profiling and produced growth factors, but some quantitative differences were detected. Adult HF and SD NCSCs were able to undergo directed differentiation into neurons, Schwann cells, adipocytes, and osteoblasts.

Conclusion

The HF and SD are suitable sources for large-scale manufacturing of adult NCSCs with similar biological properties. We demonstrated that the NCSC population from SD was homogenous and displayed significantly higher growth rate than HF NCSCs. Moreover, SD NCSC isolation is cheaper, easier, and minimally time-consuming method.

Head to Knee: Cranial Neural Crest-Derived Cells as Promising Candidates for Human Cartilage Repair

A large array of therapeutic procedures is available to treat cartilage disorders caused by trauma or inflammatory disease. Most are invasive and may result in treatment failure or development of osteoarthritis due to extensive cartilage damage from repeated surgery. Despite encouraging results of early cell therapy trials that used chondrocytes collected during arthroscopic surgery, these approaches have serious disadvantages, including morbidity associated with cell harvesting and low predictive clinical outcomes. To overcome these limitations, adult stem cells derived from bone marrow and subsequently from other tissues are now considered as preferred sources of cells for cartilage regeneration. Moreover, with new evidence showing that the choice of cell source is one of the most important factors for successful cell therapy, there is growing interest in neural crest-derived cells in both the research and clinical communities. Neural crest-derived cells such as nasal chondrocytes and oral stem cells that exhibit chondrocyte-like properties seem particularly promising in cartilage repair. Here, we review the types of cells currently available for cartilage cell therapy, including articular chondrocytes and various mesenchymal stem cells, and then highlight recent developments in the use of neural crest-derived chondrocytes and oral stem cells for repair of cartilage lesions.

A Novel Human Pluripotent Stem Cell-Derived Neural Crest Model of Treacher Collins Syndrome Shows Defects in Cell Death and Migration

The neural crest (NC) is a transient multipotent cell population present during embryonic development. The NC can give rise to multiple cell types and is involved in a number of different diseases. Therefore, the development of new strategies to model NC in vitro enables investigations into the mechanisms involved in NC development and disease. In this study, we report a simple and efficient protocol to differentiate human pluripotent stem cells (HPSC) into NC using a chemically defined media, with basic fibroblast growth factor 2 (FGF2) and the transforming growth factor-β inhibitor SB-431542. The cell population generated expresses a range of NC markers, including P75, TWIST1, SOX10, and TFAP2A. NC purification was achieved in vitro through serial passaging of the population, recreating the developmental stages of NC differentiation. The generated NC cells are highly proliferative, capable of differentiating to their derivatives in vitro and engraft in vivo to NC specific locations. In addition, these cells could be frozen for storage and thawed with no loss of NC properties, nor the ability to generate cellular derivatives. We assessed the potential of the derived NC population to model the neurocristopathy, Treacher Collins Syndrome (TCS), using small interfering RNA (siRNA) knockdown of TCOF1 and by creating different TCOF1+/- HPSC lines through CRISPR/Cas9 technology. The NC cells derived from TCOF1+/- HPSC recapitulate the phenotype of the reported TCS murine model. We also report for the first time an impairment of migration in TCOF1+/- NC and mesenchymal stem cells. In conclusion, the developed protocol permits the generation of the large number of NC cells required for developmental studies, disease modeling, and for drug discovery platforms in vitro.

In Vivo and In Vitro Quantitative Analysis of Neural Crest Cell Migration

The neural crest is an embryonic cell population induced at the border of the neural plate from where it delaminates and migrates long distances across the embryo. Due to its extraordinary migratory capabilities, the neural crest has become a powerful system to study cellular and molecular aspects of collective and single cell migration both in vivo and in vitro. Here we provide detailed protocols used to perform quantitative analysis of molecular and cellular aspects of Xenopus laevis neural crest cell migration, both in vivo and in vitro.

MAPK and PI3K signaling: At the crossroads of neural crest development

Receptor tyrosine kinase-mediated growth factor signaling is essential for proper formation and development of the neural crest. The many ligands and receptors implicated in these processes signal through relatively few downstream pathways, frequently converging on the MAPK and PI3K pathways. Despite decades of study, there is still considerable uncertainty about where and when these signaling pathways are required and how they elicit particular responses. This review summarizes our current understanding of growth factor-induced MAPK and PI3K signaling in the neural crest.