β-catenin regulates Pax3 and Cdx2 for caudal neural tube closure and elongation

Ethionine-induced S-adenosylmethionine deficiency suppressed H3K27me3 and cell differentiation during neural tube development in mice

S-adenosylmethionine (SAM) as a major methyl donor plays a key role in methylation modification in vivo, and its disorder was closely related to neural tube defects (NTDs). However, the exact mechanism between SAM deficiency and NTDs remained unclearly. Hence, we investigated the association between histone methylation modification and cell differentiation in NTDs mice induced by SAM deficiency. The levels of SAM and SAH (S-adenosylhomocysteine) were determined by enzyme linked immunosorbent assay (ELISA). The level of histone methylation, β-catenin were analyzed by Western blot, reversing transcription and quantitative PCR (RT-qPCR) and immunofluorescence. The results showed that the incidence rate of NTDs induced by ethionine were 46.2%. Post treatment of ethionine combined with SAM, the incidence rate of NTDs was reduced to 26.2%. The level of SAM was significantly decreased (p < 0.05) and a reduction in the SAM/SAH ratio was observed after entionine treatment. The SAM deficiency caused the reduction of H3K27me3 modifications and the elevated UTX activity (p < 0.05), and inhibited the expressions of β-catenin. The differentiations of NSCs into neurons and oligodendrocytes were inhibited under SAM deficiency (p < 0.05). These results indicated that the SAM deficiency led to reduce H3K27me3 modifications, prevented the β-catenin signaling pathway and NSCs differentiation, which provided an understanding of the novel function of epigenetic regulation in NTDs.

TMEM132A regulates Wnt/β-catenin signaling through stabilizing LRP6 during mouse embryonic development

The Wnt/β-catenin signaling pathway is crucial for embryonic development and adult tissue homeostasis. Dysregulation of Wnt signaling is linked to various developmental anomalies and diseases, notably cancer. Although numerous regulators of the Wnt signaling pathway have been identified, their precise function during mouse embryo development remains unclear. Here, we revealed that TMEM132A is a crucial regulator of canonical Wnt/β-catenin signaling in mouse development. Mouse embryos lacking Tmem132a displayed a range of malformations, including open spina bifida, caudal truncation, syndactyly, and renal defects, similar to the phenotypes of Wnt/β-catenin mutants. Tmem132a knockdown in cultured cells suppressed canonical Wnt/β-catenin signaling. In developing mice, loss of Tmem132a also led to diminished Wnt/β-catenin signaling. Mechanistically, we showed that TMEM132A interacts with the Wnt co-receptor LRP6, thereby stabilizing it and preventing its lysosomal degradation. These findings shed light on a novel role for TMEM132A in regulating LRP6 stability and canonical Wnt/β-catenin signaling during mouse embryo development. This study provides valuable insights into the molecular intricacies of the Wnt signaling pathway. Further research may deepen our understanding of Wnt pathway regulation and offer its potential therapeutic applications.

Whole-Genome Scanning for Selection Signatures Reveals Candidate Genes Associated with Growth and Tail Length in Sheep

Compared to Chinese indigenous sheep, Western sheep have rapid growth rate, larger physique, and higher meat yield. These excellent Western sheep were introduced into China for crossbreeding to expedite the enhancement of production performance and mutton quality in local breeds. Here, we investigated population genetic structure and genome-wide selection signatures among the Chinese indigenous sheep and the introduced sheep based on whole-genome resequencing data. The PCA, N-J tree and ADMIXTURE results showed significant genetic difference between Chinese indigenous sheep and introduced sheep. The nucleotide diversity (π) and linkage disequilibrium (LD) decay results indicated that the genomic diversity of introduced breeds were lower. Then, Fst & π ratio, XP-EHH, and de-correlated composite of multiple signals (DCMS) methods were used to detect the selection signals. The results showed that we identified important candidate genes related to growth rate and body size in the introduced breeds. Selected genes with stronger selection signatures are associated with growth rate (CRADD), embryonic development (BVES, LIN28B, and WNT11), body size (HMGA2, MSRB3, and PTCH1), muscle development and fat metabolism (MSTN, PDE3A, LGALS12, GGPS1, and SAR1B), wool color (ASIP), and hair development (KRT71, KRT74, and IRF2BP2). Thus, these genes have the potential to serve as candidate genes for enhancing the growth traits of Chinese indigenous sheep. We also identified tail-length trait-related candidate genes (HOXB13, LIN28A, PAX3, and VEGFA) in Chinese long-tailed breeds. Among these genes, HOXB13 is the main candidate gene for sheep tail length phenotype. LIN28A, PAX3, and VEGFA are related to embryonic development and angiogenesis, so these genes may be candidate genes for sheep tail type traits. This study will serve as a foundation for further genetic improvement of Chinese indigenous sheep and as a reference for studies related to growth and development of sheep.

The Phylotypic Brain of Vertebrates, from Neural Tube Closure to Brain Diversification

BACKGROUND

The phylotypic or intermediate stages are thought to be the most evolutionary conserved stages throughout embryonic development. The contrast with divergent early and later stages derived from the concept of the evo-devo hourglass model. Nonetheless, this developmental constraint has been studied as a whole embryo process, not at organ level. In this review, we explore brain development to assess the existence of an equivalent brain developmental hourglass. In the specific case of vertebrates, we propose to split the brain developmental stages into: (1) Early: Neurulation, when the neural tube arises after gastrulation. (2) Intermediate: Brain patterning and segmentation, when the neuromere identities are established. (3) Late: Neurogenesis and maturation, the stages when the neurons acquire their functionality. Moreover, we extend this analysis to other chordates brain development to unravel the evolutionary origin of this evo-devo constraint.

SUMMARY

Based on the existing literature, we hypothesise that a major conservation of the phylotypic brain might be due to the pleiotropy of the inductive regulatory networks, which are predominantly expressed at this stage. In turn, earlier stages such as neurulation are rather mechanical processes, whose regulatory networks seem to adapt to environment or maternal geometries. The later stages are also controlled by inductive regulatory networks, but their effector genes are mostly tissue-specific and functional, allowing diverse developmental programs to generate current brain diversity. Nonetheless, all stages of the hourglass are highly interconnected: divergent neurulation must have a vertebrate shared end product to reproduce the vertebrate phylotypic brain, and the boundaries and transcription factor code established during the highly conserved patterning will set the bauplan for the specialised and diversified adult brain.

KEY MESSAGES

The vertebrate brain is conserved at phylotypic stages, but the highly conserved mechanisms that occur during these brain mid-development stages (Inducing Regulatory Networks) are also present during other stages. Oppositely, other processes as cell interactions and functional neuronal genes are more diverse and majoritarian in early and late stages of development, respectively. These phenomena create an hourglass of transcriptomic diversity during embryonic development and evolution, with a really conserved bottleneck that set the bauplan for the adult brain around the phylotypic stage.

Folate deficiency reduced aberrant level of DOT1L-mediated histone H3K79 methylation causes disruptive SHH gene expression involved in neural tube defects

BACKGROUND

Neural tube defects (NTDs) are one of the most severe congenital abnormalities characterized by failures of the neural tube to close during early embryogenesis. Maternal folate deficiency could impact the occurrence of NTDs, however, the mechanisms involved in the cause of NTDs are poorly defined.

RESULTS

Here, we report that histone H3 methyltransferase disruptor of telomeric silencing 1-like (DOT1L) expression was significantly downregulated, and low levels of H3K79me2 were found in the corresponding NTDs samples with their maternal serum folate under low levels. Using ChIP-seq assays, we found that a decrease of H3K79me2 downregulates the expression of Shh and Sufu in mouse embryonic stem cells (mESC) under folate deficiency. Interestingly, folate antagonist methotrexate treatment led to attenuation of H3K79me2 due to Dot1l, affecting Shh and Sufu genes regulation. Upon further analysis, we find that the genes Shh and Sufu are both downregulated in the brain tissues of mice and humans with NTDs. There was a positive correlation between the transcription levels of Shh, Sufu and the protein levels of DOT1L by Pearson correlation analysis.

CONCLUSION

Our results indicate that abnormal Shh and Sufu genes expression reduced by aberrant Dot1l-mediated H3K79me2 levels could be the cause of NTDs occurrence.

From signalling to form: the coordination of neural tube patterning

The development of the vertebrate spinal cord involves the formation of the neural tube and the generation of multiple distinct cell types. The process starts during gastrulation, combining axial elongation with specification of neural cells and the formation of the neuroepithelium. Tissue movements produce the neural tube which is then exposed to signals that provide patterning information to neural progenitors. The intracellular response to these signals, via a gene regulatory network, governs the spatial and temporal differentiation of progenitors into specific cell types, facilitating the assembly of functional neuronal circuits. The interplay between the gene regulatory network, cell movement, and tissue mechanics generates the conserved neural tube pattern observed across species. In this review we offer an overview of the molecular and cellular processes governing the formation and patterning of the neural tube, highlighting how the remarkable complexity and precision of vertebrate nervous system arises. We argue that a multidisciplinary and multiscale understanding of the neural tube development, paired with the study of species-specific strategies, will be crucial to tackle the open questions.

Pax3 lineage-specific deletion of Gpr161 is associated with spinal neural tube and craniofacial malformations during embryonic development

Sonic hedgehog (Shh) signaling is the morphogen signaling that regulates embryonic craniofacial and neural tube development. G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling, and its inactivation in mice results in embryo lethality associated with craniofacial defects and neural tube defects. However, the structural defects of later embryonic stages and cell lineages underlying abnormalities have not been well characterized due to the limited lifespan of Gpr161 null mice. We found that embryos with Pax3 lineage-specific deletion of Gpr161 presented with tectal hypertrophy (anterior dorsal neuroepithelium), cranial vault and facial bone hypoplasia (cranial neural crest), vertebral abnormalities (somite) and the closed form of spina bifida (posterior dorsal neuroepithelium). In particular, the closed form of spina bifida was partly due to reduced Pax3 and Cdx4 gene expression in the posterior dorsal neural tubes of Gpr161 mutant embryos with decreased Wnt signaling, whereas Shh signaling was increased. We describe a previously unreported role for Gpr161 in the development of posterior neural tubes and confirm its role in cranial neural crest- and somite-derived skeletogenesis and midbrain morphogenesis in mice.

ADAM11 a novel regulator of Wnt and BMP4 signaling in neural crest and cancer

Introduction: Cranial neural crest (CNC) cells are induced at the border of the neural plate by a combination of FGF, Wnt, and BMP4 signaling. CNC then migrate ventrally and invade ventral structures where they contribute to craniofacial development. Methods: We used loss and gain of function experiments to determine phenotypes associated with the perturbation of Adam11 expression in Xenopus Laevis. Mass spectrometry to identify partners of Adam11 and changes in protein expression in CNC lacking Adam11. We used mouse B16 melanoma to test the function of Adam11 in cancer cells, and published database analysis to study the expression of ADAM11 in human tumors. Results: Here we show that a non-proteolytic ADAM, Adam11, originally identified as a putative tumor suppressor binds to proteins of the Wnt and BMP4 signaling pathway. Mechanistic studies concerning these non-proteolytic ADAM lack almost entirely. We show that Adam11 positively regulates BMP4 signaling while negatively regulating β-catenin activity. In vivo, we show that Adam11 influences the timing of neural tube closure and the proliferation and migration of CNC. Using both human tumor data and mouse B16 melanoma cells, we further show that ADAM11 levels similarly correlate with Wnt or BMP4 activation levels. Discussion: We propose that ADAM11 preserves naïve cells by maintaining low Sox3 and Snail/Slug levels through stimulation of BMP4 and repression of Wnt signaling, while loss of ADAM11 results in increased Wnt signaling, increased proliferation and early epithelium to mesenchyme transition.

Brain-specific Pd1 deficiency leads to cortical neurogenesis defects and depressive-like behaviors in mice

Embryonic neurogenesis is tightly regulated by multiple factors to ensure the precise development of the cortex. Deficiency in neurogenesis may result in behavioral abnormalities. Pd1 is a well-known inhibitory immune molecule, but its function in brain development remains unknown. Here, we find brain specific deletion of Pd1 results in abnormal cortical neurogenesis, including enhanced proliferation of neural progenitors and reduced neuronal differentiation. In addition, neurons in Pd1 knockout mice exhibit abnormal morphology, both the total length and the number of primary dendrites were reduced. Moreover, Pd1cKO mice exhibit depressive-like behaviors, including immobility, despair, and anhedonia. Mechanistically, Pd1 regulates embryonic neurogenesis by targeting Pax3 through the β-catenin signaling pathway. The constitutive expression of Pax3 partly rescues the deficiency of neurogenesis in the Pd1 deleted embryonic brain. Besides, the administration of β-catenin inhibitor, XAV939, not only rescues abnormal brain development but also ameliorates depressive-like behaviors in Pd1cKO mice. Simultaneously, Pd1 plays a similar role in human neural progenitor cells (hNPCs) proliferation and differentiation. Taken together, our findings reveal the critical role and regulatory mechanism of Pd1 in embryonic neurogenesis and behavioral modulation, which could contribute to understanding immune molecules in brain development.

Pax3 lineage-specific deletion of Gpr161 is associated with spinal neural tube and craniofacial malformations during embryonic development

Shh signaling is the morphogen signaling that regulates embryonic craniofacial and neural tube development. G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling, and its inactivation in mice results in embryo lethality with craniofacial and neural tube defects (NTDs). However, the structural defects of later embryonic stages in Gpr161 null mice and cell lineages underlying abnormalities were not well characterized due to their limited lifespan. We found the Pax3 lineage-specific deletion of Gpr161 in mice presented with tectal hypertrophy (anterior dorsal neuroepithelium), cranial vault and facial bone hypoplasia (cranial neural crest (CNC)), vertebral abnormalities (somite), and the closed form of spina bifida (posterior dorsal neuroepithelium). In particular, the closed form of spina bifida is partly due to the reduced Pax3 and Cdx4 gene expression of the posterior dorsal neural tubes of Gpr161 mutant embryos involving decreased Wnt signaling whereas Shh signaling was increased. This study provides the novel role of Gpr161 in the posterior neural tube development and confirms its role on CNC- and somite-derived skeletogenesis and midbrain morphogenesis in mice.

ADAM11 a novel regulator of Wnt and BMP4 signaling in neural crest and cancer

Cranial neural crest (CNC) cells are induced at the border of the neural plate by a combination of FGF, Wnt, and BMP4 signaling. CNC then migrate ventrally and invade ventral structures where they contribute to craniofacial development. Here we show that a non-proteolytic ADAM, Adam11, originally identified as a putative tumor suppressor binds to proteins of the Wnt and BMP4 signaling pathway. Mechanistic studies concerning these non-proteolytic ADAM lack almost entirely. We show that Adam11 positively regulates BMP4 signaling while negatively regulating β-catenin activity. By modulating these pathways, Adam11 controls the timing of neural tube closure and the proliferation and migration of CNC. Using both human tumor data and mouse B16 melanoma cells, we further show that ADAM11 levels similarly correlate with Wnt or BMP4 activation levels. We propose that ADAM11 preserve naïve cells by maintaining low Sox3 and Snail/Slug levels through stimulation of BMP4 and repression of Wnt signaling, while loss of ADAM11 results in increased Wnt signaling, increased proliferation and early epithelium to mesenchyme transition.

Wnt Signaling in Brain Tumors: A Challenging Therapeutic Target

The involvement of Wnt signaling in normal tissue homeostasis and disease has been widely demonstrated over the last 20 years. In particular, dysregulation of Wnt pathway components has been suggested as a relevant hallmark of several neoplastic malignancies, playing a role in cancer onset, progression, and response to treatments. In this review, we summarize the current knowledge on the instructions provided by Wnt signaling during organogenesis and, particularly, brain development. Moreover, we recapitulate the most relevant mechanisms through which aberrant Wnt pathway activation may impact on brain tumorigenesis and brain tumor aggressiveness, with a particular focus on the mutual interdependency existing between Wnt signaling components and the brain tumor microenvironment. Finally, the latest anti-cancer therapeutic approaches employing the specific targeting of Wnt signaling are extensively reviewed and discussed. In conclusion, here we provide evidence that Wnt signaling, due to its pleiotropic involvement in several brain tumor features, may represent a relevant target in this context, although additional efforts will be needed to: (i) demonstrate the real clinical impact of Wnt inhibition in these tumors; (ii) overcome some still unsolved concerns about the potential systemic effects of such approaches; (iii) achieve efficient brain penetration.

Single-cell transcriptomic signatures and gene regulatory networks modulated by Wls in mammalian midline facial formation and clefts

Formation of highly unique and complex facial structures is controlled by genetic programs that are responsible for the precise coordination of three-dimensional tissue morphogenesis. However, the underlying mechanisms governing these processes remain poorly understood. We combined mouse genetic and genomic approaches to define the mechanisms underlying normal and defective midfacial morphogenesis. Conditional inactivation of the Wnt secretion protein Wls in Pax3-expressing lineage cells disrupted frontonasal primordial patterning, cell survival and directional outgrowth, resulting in altered facial structures, including midfacial hypoplasia and midline facial clefts. Single-cell RNA sequencing revealed unique transcriptomic atlases of mesenchymal subpopulations in the midfacial primordia, which are disrupted in the conditional Wls mutants. Differentially expressed genes and cis-regulatory sequence analyses uncovered that Wls modulates and integrates a core gene regulatory network, consisting of key midfacial regulatory transcription factors (including Msx1, Pax3 and Pax7) and their downstream targets (including Wnt, Shh, Tgfβ and retinoic acid signaling components), in a mesenchymal subpopulation of the medial nasal prominences that is responsible for midline facial formation and fusion. These results reveal fundamental mechanisms underlying mammalian midfacial morphogenesis and related defects at single-cell resolution.

Embryology of the Abdominal Wall and Associated Malformations—A Review

In humans, the incidence of congenital defects of the intraembryonic celom and its associated structures has increased over recent decades. Surgical treatment of abdominal and diaphragmatic malformations resulting in congenital hernia requires deep knowledge of ventral body closure and the separation of the primary body cavities during embryogenesis. The correct development of both structures requires the coordinated and fine-tuned synergy of different anlagen, including a set of molecules governing those processes. They have mainly been investigated in a range of vertebrate species (e.g., mouse, birds, and fish), but studies of embryogenesis in humans are rather rare because samples are seldom available. Therefore, we have to deal with a large body of conflicting data concerning the formation of the abdominal wall and the etiology of diaphragmatic defects. This review summarizes the current state of knowledge and focuses on the histological and molecular events leading to the establishment of the abdominal and thoracic cavities in several vertebrate species. In chronological order, we start with the onset of gastrulation, continue with the establishment of the three-dimensional body shape, and end with the partition of body cavities. We also discuss well-known human etiologies.

Towards clinical applications of in vitro-derived axial progenitors

The production of the tissues that make up the mammalian embryonic trunk takes place in a head-tail direction, via the differentiation of posteriorly-located axial progenitor populations. These include bipotent neuromesodermal progenitors (NMPs), which generate both spinal cord neurectoderm and presomitic mesoderm, the precursor of the musculoskeleton. Over the past few years, a number of studies have described the derivation of NMP-like cells from mouse and human pluripotent stem cells (PSCs). In turn, these have greatly facilitated the establishment of PSC differentiation protocols aiming to give rise efficiently to posterior mesodermal and neural cell types, which have been particularly challenging to produce using previous approaches. Moreover, the advent of 3-dimensional-based culture systems incorporating distinct axial progenitor-derived cell lineages has opened new avenues toward the functional dissection of early patterning events and cell vs non-cell autonomous effects. Here, we provide a brief overview of the applications of these cell types in disease modelling and cell therapy and speculate on their potential uses in the future.

Exploring epigenomic mechanisms of neural tube defects using multi-omics methods and data

Neural tube defects (NTDs) are a heterogeneous set of malformations attributed to disruption in normal neural tube closure during early embryogenesis. An in-depth understanding of NTD etiology and mechanisms remains elusive, however. Among the proposed mechanisms, epigenetic changes are thought to play an important role in the formation of NTDs. Epigenomics covers a wide spectrum of genomic DNA sequence modifications that can be investigated via high-throughput techniques. Recent advances in epigenomic technologies have enabled epigenetic studies of congenital malformations and facilitated the integration of big data into the understanding of NTDs. Herein, we review clinical epigenomic data that focuses on DNA methylation, histone modification, and miRNA alterations in human neural tissues, placental tissues, and leukocytes to explore potential mechanisms by which candidate genes affect human NTD pathogenesis. We discuss the links between epigenomics and gene regulatory mechanisms, and the effects of epigenetic alterations in human tissues on neural tube closure.

The role of Lrp6-mediated Wnt/β-catenin signaling in the development and intervention of spinal neural tube defects in mice

Neural tube defects (NTDs) are among the common and severe birth defects with poorly understood etiology. Mutations in the Wnt co-receptor LRP6 are associated with NTDs in humans. Either gain-of-function (GOF) or loss-of-function (LOF) mutations of Lrp6 can cause NTDs in mice. NTDs in Lrp6-GOF mutants may be attributed to altered β-catenin-independent noncanonical Wnt signaling. However, the mechanisms underlying NTDs in Lrp6-LOF mutants and the role of Lrp6-mediated canonical Wnt/β-catenin signaling in neural tube closure remain unresolved. We previously demonstrated that β-catenin signaling is required for posterior neuropore (PNP) closure. In the current study, conditional ablation of Lrp6 in dorsal PNP caused spinal NTDs with diminished activities of Wnt/β-catenin signaling and its downstream target gene Pax3, which is required for PNP closure. β-catenin-GOF rescued NTDs in Lrp6-LOF mutants. Moreover, maternal supplementation of a Wnt/β-catenin signaling agonist reduced the frequency and severity of spinal NTDs in Lrp6-LOF mutants by restoring Pax3 expression. Together, these results demonstrate the essential role of Lrp6-mediated Wnt/β-catenin signaling in PNP closure, which could also provide a therapeutic target for NTD intervention through manipulation of canonical Wnt/β-catenin signaling activities.

Summary: Conditional ablation of Lrp6 in dorsal neural folds causes spinal neural tube defects that can be rescued by genetic activation of β-catenin or maternal supplementation of Wnt signaling agonists.

Pax3 Hypomorphs Reveal Hidden Pax7 Functional Genetic Compensation in Utero

Pax3 and Pax7 transcription factors are paralogs within the Pax gene family that that are expressed in early embryos in partially overlapping expression domains and have distinct functions. Significantly, mammalian development is largely unaffected by Pax7 systemic deletion but systemic Pax3 deletion results in defects in neural tube closure, neural crest emigration, cardiac outflow tract septation, muscle hypoplasia and in utero lethality by E14. However, we previously demonstrated that Pax3 hypomorphs expressing only 20% functional Pax3 protein levels exhibit normal neural tube and heart development, but myogenesis is selectively impaired. To determine why only some Pax3-expressing cell lineages are affected and to further titrate Pax3 threshold levels required for neural tube and heart development, we generated hypomorphs containing both a hypomorphic and a null Pax3 allele. This resulted in mutants only expressing 10% functional Pax3 protein with exacerbated neural tube, neural crest and muscle defects, but still a normal heart. To examine why the cardiac neural crest appears resistant to very low Pax3 levels, we examined its paralog Pax7. Significantly, Pax7 expression is both ectopically expressed in Pax3-expressing dorsal neural tube cells and is also upregulated in the Pax3-expressing lineages. To test whether this compensatory Pax7 expression is functional, we deleted Pax7 both systemically and lineage-specifically in hypomorphs expressing only 10% Pax3. Removal of one Pax7 allele resulted in partial outflow tract defects, and complete loss of Pax7 resulted in full penetrance outflow tract defects and in utero lethality. Moreover, combinatorial loss of Pax3 and Pax7 resulted in severe craniofacial defects and a total block of neural crest cell emigration from the neural tube. Pax7^Cre lineage mapping revealed ectopic labeling of Pax3-derived neural crest tissues and within the outflow tract of the heart, experimentally confirming the observation of ectopic activation of Pax7 in 10% Pax3 hypomorphs. Finally, genetic cell ablation of Pax7^Cre-marked cells is sufficient to cause outflow tract defects in hypomorphs expressing only 10% Pax3, confirming that ectopic and induced Pax7 can play an overlapping functional genetic compensational role in both cardiac neural crest lineage and during craniofacial development, which is normally masked by the dominant role of Pax3.

Hinge point emergence in mammalian spinal neurulation

Neurulation is the process in early vertebrate embryonic development during which the neural plate folds to form the neural tube. Spinal neural tube folding in the posterior neuropore changes over time, first showing a median hinge point, then both the median hinge point and dorsolateral hinge points, followed by dorsolateral hinge points only. The biomechanical mechanism of hinge point formation in the mammalian neural tube is poorly understood. Here we employ a mechanical finite element model to study neural tube formation. The computational model mimics the mammalian neural tube using microscopy data from mouse and human embryos. While intrinsic curvature at the neural plate midline has been hypothesized to drive neural tube folding, intrinsic curvature was not sufficient for tube closure in our simulations. We achieved neural tube closure with an alternative model combining mesoderm expansion, nonneural ectoderm expansion, and neural plate adhesion to the notochord. Dorsolateral hinge points emerged in simulations with low mesoderm expansion and zippering. We propose that zippering provides the biomechanical force for dorsolateral hinge point formation in settings where the neural plate lateral sides extend above the mesoderm. Together, these results provide a perspective on the biomechanical and molecular mechanism of mammalian spinal neurulation.

Intestinal Wnt in the transition from physiology to oncology

Adult stem cells are necessary for self-renewal tissues and regeneration after damage. Especially in the intestine, which self-renews every few days, they play a key role in tissue homeostasis. Therefore, complex regulatory mechanisms are needed to prevent hyperproliferation, which can lead in the worst case to carcinogenesis or under-activation of stem cells, which can result in dysfunctional epithelial. One main regulatory signaling pathway is the Wnt/β-catenin signaling pathway. It is a highly conserved pathway, with β-catenin, a transcription factor, as target protein. Translocation of β-catenin from cytoplasm to nucleus activates the transcription of numerous genes involved in regulating stem cell pluripo-tency, proliferation, cell differentiation and regulation of cell death. This review presents a brief overview of the Wnt/β-catenin signaling pathway, the regulatory mechanism of this pathway and its role in intestinal homeostasis. Additionally, this review highlights the molecular mechanisms and the histomorphological features of Wnt hyperactivation. Furthermore, the central role of the Wnt signaling pathway in intestinal carcinogenesis as well as its clinical relevance in colorectal carcinoma are discussed.

Loss of Foxd4 Impacts Neurulation and Cranial Neural Crest Specification During Early Head Development

The specification of anterior head tissue in the late gastrulation mouse embryo relies on signaling cues from the visceral endoderm and anterior mesendoderm (AME). Genetic loss-of-function studies have pinpointed a critical requirement of LIM homeobox 1 (LHX1) transcription factor in these tissues for the formation of the embryonic head. Transcriptome analysis of embryos with gain-of-function LHX1 activity identified the forkhead box gene, Foxd4, as one downstream target of LHX1 in late-gastrulation E7.75 embryos. Our analysis of single-cell RNA-seq data show Foxd4 is co-expressed with Lhx1 and Foxa2 in the anterior midline tissue of E7.75 mouse embryos, and in the anterior neuroectoderm (ANE) at E8.25 alongside head organizer genes Otx2 and Hesx1. To study the role of Foxd4 during early development we used CRISPR-Cas9 gene editing in mouse embryonic stem cells (mESCs) to generate bi-allelic frameshift mutations in the coding sequence of Foxd4. In an in vitro model of the anterior neural tissues derived from Foxd4-loss of function (LOF) mESCs and extraembryonic endoderm cells, expression of head organizer genes as well as Zic1 and Zic2 was reduced, pointing to a need for FOXD4 in regulating early neuroectoderm development. Mid-gestation mouse chimeras harbouring Foxd4-LOF mESCs displayed craniofacial malformations and neural tube closure defects. Furthermore, our in vitro data showed a loss of FOXD4 impacts the expression of cranial neural crest markers Twist1 and Sox9. Our findings have demonstrated that FOXD4 is essential in the AME and later in the ANE for rostral neural tube closure and neural crest specification during head development.

Genetic interaction of Pax3 mutation and canonical Wnt signaling modulates neural tube defects and neural crest abnormalities

Mouse models provide opportunities to investigate genetic interactions that cause or modify the frequency of neural tube defects (NTDs). Mutation of the PAX3 transcription factor prevents neural tube closure, leading to cranial and spinal NTDs whose frequency is responsive to folate status. Canonical Wnt signalling is implicated both in regulation of Pax3 expression and as a target of PAX3. This study investigated potential interactions of Pax3 mutation and canonical Wnt signalling using conditional gain- and loss-of-function models of β-catenin. We found an additive effect of β-catenin gain of function and Pax3 loss of function on NTDs and neural crest defects. β-catenin gain of function in the Pax3 expression domain led to significantly increased frequency of cranial but not spinal NTDs in embryos that are heterozygous for Pax3 mutation, while both cranial and spinal neural tube closure were exacerbated in Pax3 homozygotes. Similarly, deficits of migrating neural crest cells were exacerbated by β-catenin gain of function, with almost complete ablation of spinal neural crest cells and derivatives in Pax3 homozygous mutants. Pax3 expression was not affected by β-catenin gain of function, while we confirmed that loss of function led to reduced Pax3 transcription. In contrast to gain of function, β-catenin knockout in the Pax3 expression domain lowered the frequency of cranial NTDs in Pax3 null embryos. However, loss of function of β-catenin and Pax3 resulted in spinal NTDs, suggesting differential regulation of cranial and spinal neural tube closure. In summary, β-catenin function modulates the frequency of PAX3-related NTDs in the mouse.

Ciliary and extraciliary Gpr161 pools repress hedgehog signaling in a tissue-specific manner

The role of compartmentalized signaling in primary cilia during tissue morphogenesis is not well understood. The cilia localized G protein-coupled receptor, Gpr161, represses hedgehog pathway via cAMP signaling. We engineered a knock-in at the Gpr161 locus in mice to generate a variant (Gpr161^mut1), which was ciliary localization defective but cAMP signaling competent. Tissue phenotypes from hedgehog signaling depend on downstream bifunctional Gli transcriptional factors functioning as activators or repressors. Compared to knockout (ko), Gpr161^mut1/ko had delayed embryonic lethality, moderately increased hedgehog targets, and partially down-regulated Gli3 repressor. Unlike ko, the Gpr161^mut1/ko neural tube did not show Gli2 activator-dependent expansion of ventral-most progenitors. Instead, the intermediate neural tube showed progenitor expansion that depends on loss of Gli3 repressor. Increased extraciliary receptor levels in Gpr161^mut1/mut1 prevented ventralization. Morphogenesis in limb buds and midface requires Gli repressor; these tissues in Gpr161^mut1/mut1 manifested hedgehog hyperactivation phenotypes—polydactyly and midfacial widening. Thus, ciliary and extraciliary Gpr161 pools likely establish tissue-specific Gli repressor thresholds in determining morpho-phenotypic outcomes.

Canonical Wnt Signaling Pathway on Polarity Formation of Utricle Hair Cells

As part of the inner ear, the vestibular system is responsible for sense of balance, which consists of three semicircular canals, the utricle, and the saccule. Increasing evidence has indicated that the noncanonical Wnt/PCP signaling pathway plays a significant role in the development of the polarity of the inner ear. However, the role of canonical Wnt signaling in the polarity of the vestibule is still not completely clear. In this study, we found that canonical Wnt pathway-related genes are expressed in the early stage of development of the utricle and change dynamically. We conditionally knocked out β-catenin, a canonical Wnt signaling core protein, and found that the cilia orientation of hair cells was disordered with reduced number of hair cells in the utricle. Moreover, regulating the canonical Wnt pathway (Licl and IWP2) in vitro also affected hair cell polarity and indicated that Axin2 may be important in this process. In conclusion, our results not only confirm that the regulation of canonical Wnt signaling affects the number of hair cells in the utricle but also provide evidence for its role in polarity development.

Understanding axial progenitor biology in vivo and in vitro

The generation of the components that make up the embryonic body axis, such as the spinal cord and vertebral column, takes place in an anterior-to-posterior (head-to-tail) direction. This process is driven by the coordinated production of various cell types from a pool of posteriorly-located axial progenitors. Here, we review the key features of this process and the biology of axial progenitors, including neuromesodermal progenitors, the common precursors of the spinal cord and trunk musculature. We discuss recent developments in the in vitro production of axial progenitors and their potential implications in disease modelling and regenerative medicine.

Snx3 is important for mammalian neural tube closure via its role in canonical and non-canonical WNT signaling

Disruptions in neural tube (NT) closure result in neural tube defects (NTDs). To understand the molecular processes required for mammalian NT closure, we investigated the role of Snx3, a sorting nexin gene. Snx3^-/- mutant mouse embryos display a fully-penetrant cranial NTD. In vivo, we observed decreased canonical WNT target gene expression in the cranial neural epithelium of the Snx3^-/- embryos and a defect in convergent extension of the neural epithelium. Snx3^-/- cells show decreased WNT secretion, and live cell imaging reveals aberrant recycling of the WNT ligand-binding protein WLS and mis-trafficking to the lysosome for degradation. The importance of SNX3 in WNT signaling regulation is demonstrated by rescue of NT closure in Snx3^-/- embryos with a WNT agonist. The potential for SNX3 to function in human neurulation is revealed by a point mutation identified in an NTD-affected individual that results in functionally impaired SNX3 that does not colocalize with WLS and the degradation of WLS in the lysosome. These data indicate that Snx3 is crucial for NT closure via its role in recycling WLS in order to control levels of WNT signaling.

Neural Tube Defects and ZIC4 Hypomethylation in Relation to Polycyclic Aromatic Hydrocarbon Exposure

Background

Epigenetic dysregulation is one of the postulated underlying mechanisms of neural tube defects (NTDs). Polycyclic aromatic hydrocarbons (PAHs), a group of environmental pollutants that are reported as a risk factor of NTDs, may cause decreased genome-wide DNA methylation. With DNA extracted from neural tissues, this study identified gene(s) whose hypomethylation was related to elevated risk for NTDs and examined whether its hypomethylation is related to PAH exposure.

Results

Using data profiled by Infinium HumanMethylation450 BeadChip array from 10 NTD cases and eight controls, ZIC4, CASP8, RAB32, RARA, and TRAF6 were identified to be the top five genes in NTD-related hypomethylated gene families. Among all identified genes, ZIC4 had the largest number of differently methylated CpG sites (n = 13) in the promoter region and 5′ UTR. Significantly decreased methylation in the ZIC4 promoter region and 5′ UTR was verified in an independent cohort of 80 cases and 32 controls (p < 0.001) utilizing the Sequenom EpiTYPER platform. Hypomethylation of ZIC4 was associated with a higher risk of NTDs [adjusted OR = 1.08; 95% confidence interval (CI): 1.03, 1.13] in a logistic regression model. Mean methylation levels in the promoter region and 5′ UTR of ZIC4 tended to be inversely associated with levels of high-molecular-weight PAHs in fetal liver among NTD fetuses (β [95% CI]: −0.045 [−0.091, 0.001], p = 0.054). Six and three CpG sites in the ZIC4 promoter region and 5′ UTR were inversely correlated with antioxidant indicators and protein oxidation markers (ρ: −0.45 to −0.75, p < 0.05) in fetal neural tissues, respectively. In a whole-embryo cultured mouse model, hypomethylation of the Zic4 promoter region and 5′ UTR and upregulation of Zic4 were observed, coupled with increased NTD rates after BaP exposure. The antioxidant N-acetyl-L-cysteine normalized the changes observed in the BaP exposure group.

Conclusion

Hypomethylation of the ZIC4 promoter region and 5′ UTR may increase the risk for NTDs; oxidative stress is likely to play a role in the methylation change of Zic4 in response to PAH exposure in NTD formation.

Nkx2.1 downregulation is involved in brain abnormality induced by excess retinoic acid

Abnormal development of central nervous system (CNS) caused by neural tube defects is not only a major contributor in the prevalence of stillbirths and neonatal deaths but also causes lifelong physical disability in surviving infants. Due to insufficient known investigated causes, CNS developmental abnormality has brought sever burden on health around the world. From previous results of high throughput transcriptome sequencing, we selected transcription factor Nkx2.1 as a candidate to investigate its role on brain abnormalities induced by excessive retinoic acid. The result of in situ hybridization showed that Nkx2.1 was mainly expressed in mouse brain. After the Nkx2.1 gene was silenced, retarded proliferation and accelerated apoptosis were found in mouse Neuro-2a (N2a) cells. Furthermore, our results indicated that the main components of sonic hedgehog (Shh) signaling pathway were affected in Nkx2.1-silenced cells, implying that Nkx2.1 plays an important role in the development of mouse brain by regulating Shh signaling pathway.

Retinal astrocytes transcriptome reveals Cyp1b1 regulates the expression of genes involved in cell adhesion and migration

Astrocytes (AC) are the most abundant cells in the central nervous system. In the retina, astrocytes play important roles in the development and integrity of the retinal neurovasculature. Astrocytes dysfunction contributes to pathogenesis of a variety of neurovascular diseases including diabetic retinopathy. Recent studies have demonstrated the expression of Cyp1b1 in the neurovascular cells of the central nervous system including AC. We recently showed retinal AC constitutively express Cyp1b1, and global Cyp1b1-deficiency (Cyp1b1-/-) attenuates retinal ischemia-mediated neovascularization in vivo and the pro-angiogenic activity of retinal vascular cells in vitro. We also demonstrated that Cyp1b1 expression is a key regulator of retinal AC function. However, the underlying mechanisms involved need further investigation. Here we determined changes in the transcriptome profiles of Cyp1b1+/+ and Cyp1b1-/- retinal AC by RNA sequencing. We identified 585 differentially expressed genes, whose pathway enrichment analysis revealed the most significant pathways impacted in Cyp1b1-/- AC. These genes included those of axon guidance, extracellular matrix proteins and their receptors, cancer, cell adhesion molecules, TGF-β signaling, and the focal adhesion modulation. The expression of a selected set of differentially expressed genes was confirmed by RT-qPCR analysis. To our knowledge, this is the first report of RNAseq investigation of the retinal AC transcriptome and the molecular pathways impacted by Cyp1b1 expression. These results demonstrated an important role for Cyp1b1 expression in the regulation of various retinal AC functions, which are important in neurovascular development and integrity.

Non-neural surface ectodermal rosette formation and F-actin dynamics drive mammalian neural tube closure

The mechanisms underlying mammalian neural tube closure remain poorly understood. We report a unique cellular process involving multicellular rosette formation, convergent cellular protrusions, and F-actin cable network of the non-neural surface ectodermal cells encircling the closure site of the posterior neuropore, which are demonstrated by scanning electron microscopy and genetic fate mapping analyses during mouse spinal neurulation. These unique cellular structures are severely disrupted in the surface ectodermal transcription factor Grhl3 mutants that exhibit fully penetrant spina bifida. We propose a novel model of mammalian neural tube closure driven by surface ectodermal dynamics, which is computationally visualized.

Astragaloside IV suppresses transforming growth factor-β1-induced epithelial-mesenchymal transition through inhibition of Wnt/β-catenin pathway in glioma U251 cells

Astragaloside IV (AS#IV) has previously demonstrated antitumoractivity. We investigated the effect and mechanisms of AS#IV in relation to epithelial-mesenchymal transition (EMT), viainterference with the Wnt/β-catenin signaling pathway in gliomaU251 cells. Induction of glioma U251 cells by transforming growthfactor (TGF)#β1 activated EMT, including switching E#cadherin toN-cadherin and altering the expression of Wnt/β-catenin signalingpathway components such as vimentin, β-catenin, and cyclin-D1.AS-IV inhibited the viability, invasion, and migration of TGF-β1-induced glioma U251 cells. AS-IV also interfered with the TGF#β1-induced Wnt/β-catenin signaling pathway in glioma U251 cells.These findings indicate that AS#IV prohibits TGF#β1-induced EMTby disrupting the Wnt/β-catenin pathway in glioma U251 cells. AS#IV may thus be a potential candidate agent for treating glioma andother central nervous system tumors.

Inhibiting MARSs reduces hyperhomocysteinemia-associated neural tube and congenital heart defects

Hyperhomocysteinemia is a common metabolic disorder that imposes major adverse health consequences. Reducing homocysteine levels, however, is not always effective against hyperhomocysteinemia-associated pathologies. Herein, we report the potential roles of methionyl-tRNA synthetase (MARS)-generated homocysteine signals in neural tube defects (NTDs) and congenital heart defects (CHDs). Increased copy numbers of MARS and/or MARS2 were detected in NTD and CHD patients. MARSs sense homocysteine and transmit its signal by inducing protein lysine (N)-homocysteinylation. Here, we identified hundreds of novel N-homocysteinylated proteins. N-homocysteinylation of superoxide dismutases (SOD1/2) provided new mechanistic insights for homocysteine-induced oxidative stress, apoptosis and Wnt signalling deregulation. Elevated MARS expression in developing and proliferating cells sensitizes them to the effects of homocysteine. Targeting MARSs using the homocysteine analogue acetyl homocysteine thioether (AHT) reversed MARS efficacy. AHT lowered NTD and CHD onsets in retinoic acid-induced and hyperhomocysteinemia-induced animal models without affecting homocysteine levels. We provide genetic and biochemical evidence to show that MARSs are previously overlooked genetic determinants and key pathological factors of hyperhomocysteinemia, and suggest that MARS inhibition represents an important medicinal approach for controlling hyperhomocysteinemia-associated diseases.

Olig2 regulates terminal differentiation and maturation of peripheral olfactory sensory neurons

The bHLH transcription factor Olig2 is required for sequential cell fate determination of both motor neurons and oligodendrocytes and for progenitor proliferation in the central nervous system. However, the role of Olig2 in peripheral sensory neurogenesis remains unknown. We report that Olig2 is transiently expressed in the newly differentiated olfactory sensory neurons (OSNs) and is down-regulated in the mature OSNs in mice from early gestation to adulthood. Genetic fate mapping demonstrates that Olig2-expressing cells solely give rise to OSNs in the peripheral olfactory system. Olig2 depletion does not affect the proliferation of peripheral olfactory progenitors and the fate determination of OSNs, sustentacular cells, and the olfactory ensheathing cells. However, the terminal differentiation and maturation of OSNs are compromised in either Olig2 single or Olig1/Olig2 double knockout mice, associated with significantly diminished expression of multiple OSN maturation and odorant signaling genes, including Omp, Gnal, Adcy3, and Olfr15. We further demonstrate that Olig2 binds to the E-box in the Omp promoter region to regulate its expression. Taken together, our results reveal a distinctly novel function of Olig2 in the periphery nervous system to regulate the terminal differentiation and maturation of olfactory sensory neurons.

A regulatory loop connecting WNT signaling and telomere capping: possible therapeutic implications for dyskeratosis congenita

The consequences of telomere dysfunction are most apparent in rare inherited syndromes caused by genetic deficiencies in factors that normally maintain telomeres. The principal disease is known as dyskeratosis congenita (DC), but other syndromes with similar underlying genetic defects share some clinical aspects with this disease. Currently, there are no curative therapies for these diseases of telomere dysfunction. Here, we review recent findings demonstrating that dysfunctional (i.e., uncapped) telomeres can downregulate the WNT pathway, and that restoration of WNT signaling helps to recap telomeres by increasing expression of shelterins, proteins that naturally bind and protect telomeres. We discuss how these findings are different from previous observations connecting WNT and telomere biology, and discuss potential links between WNT and clinical manifestations of the DC spectrum of diseases. Finally, we argue for exploring the use of WNT agonists, specifically lithium, as a possible therapeutic approach for patients with DC.

Impaired neurodevelopmental pathways in autism spectrum disorder: a review of signaling mechanisms and crosstalk

Background

The development of an autistic brain is a highly complex process as evident from the involvement of various genetic and non-genetic factors in the etiology of the autism spectrum disorder (ASD). Despite being a multifactorial neurodevelopmental disorder, autistic patients display a few key characteristics, such as the impaired social interactions and elevated repetitive behaviors, suggesting the perturbation of specific neuronal circuits resulted from abnormal signaling pathways during brain development in ASD. A comprehensive review for autistic signaling mechanisms and interactions may provide a better understanding of ASD etiology and treatment.

Main body

Recent studies on genetic models and ASD patients with several different mutated genes revealed the dysregulation of several key signaling pathways, such as WNT, BMP, SHH, and retinoic acid (RA) signaling. Although no direct evidence of dysfunctional FGF or TGF-β signaling in ASD has been reported so far, a few examples of indirect evidence can be found. This review article summarizes how various genetic and non-genetic factors which have been reported contributing to ASD interact with WNT, BMP/TGF-β, SHH, FGF, and RA signaling pathways. The autism-associated gene ubiquitin-protein ligase E3A (UBE3A) has been reported to influence WNT, BMP, and RA signaling pathways, suggesting crosstalk between various signaling pathways during autistic brain development. Finally, the article comments on what further studies could be performed to gain deeper insights into the understanding of perturbed signaling pathways in the etiology of ASD.

Conclusion

The understanding of mechanisms behind various signaling pathways in the etiology of ASD may help to facilitate the identification of potential therapeutic targets and design of new treatment methods.

PAX3 Promotes Proliferation of Human Glioma Cells by WNT/β-Catenin Signaling Pathways

The PAX3 (paired box 3) gene plays an important role in embryonic development, diseases, and cancer formation. Our preliminary studies have shown that PAX3 gene is upregulated in glioma cells, which is associated with a worse prognosis. Moreover, PAX3, by facilitating cell proliferation and invasion and inhibiting cell apoptosis, plays an oncogenic role in glioma. However, the specific molecular mechanism of PAX3 acting as an oncogene in glioma remains unclarified. In the present study, we have found that PAX3 overexpression was observed in high grade glioma and predicted a worse prognosis. PAX3 overexpression did not correlate significantly to IDH1 mutation and MGMT methylation. Moreover, the expression of PAX3 was positively correlated with that of β-catenin. In U87 glioma cells, PAX3 interacted with β-catenin, as was confirmed by CO-IP. Besides, PAX3 overexpression promoted cell proliferation and cell cycle progression, while it inhibited cell apoptosis by altering the expressions of important molecules associated with the Wnt signaling pathway, including β-catenin, Myc, VEGF, cyclinD1, MMP7, and Wnt1. In the meantime, it was also proved that PAX3 correlated to β-catenin through a negative regulatory mechanism with respect to the promotion of U87 glioma cell proliferation and cell cycle progression and inhibition of the cell apoptosis. Our experiment demonstrated the role of PAX3 in promoting glioma growth and development, possibly by interacting directly with β-catenin and regulating the Wnt signaling pathway.

A gene regulatory architecture that controls region-independent dynamics of oligodendrocyte differentiation

Oligodendrocytes (OLs) facilitate information processing in the vertebrate central nervous system via axonal ensheathment. The structure and dynamics of the regulatory network that mediates oligodendrogenesis are poorly understood. We employed bioinformatics and meta-analysis of high-throughput datasets to reconstruct a regulatory network underpinning OL differentiation. From this network, we identified families of feedforward loops comprising the transcription factors (TFs) Olig2, Sox10, and Tcf7l2 and their targets. Among the targets, we found eight other TFs related to OL differentiation, suggesting a hierarchical architecture in which some TFs (Olig2, Sox10, and Tcf7l2) regulate via feedforward loops the expression of others (Sox2, Sox6, Sox11, Nkx2-2, Nkx6-2, Hes5, Myt1, and Myrf). Model simulations with a kinetic model reproduced the mechanisms of OL differentiation only when in the model, Sox10-mediated repression of Tcf7l2 by miR-338/miR-155 was introduced, a prediction confirmed in genetic functional experiments. Additional model simulations suggested that OLs from dorsal regions emerge through BMP/Sox9 signaling.

Expression pattern of Wif1 and -catenin during development of anorectum in fetal rats with anorectal malformations

Purpose

This study was performed to investigate the expression pattern of Wnt inhibitory factor 1 (Wif1) and β-catenin during anorectal development in normal and anorectal malformation (ARM) embryos and the possible role of Wif1 and β-catenin in the pathogenesis of ARM.

Methods

ARM was induced with ethylenethiourea on the 10th gestational day in rat embryos. Cesarean deliveries were performed to harvest the embryos. The expression pattern of Wif1 and β-catenin protein and mRNA was evaluated in normal rat embryos (n = 288) and ARM rat embryos (n = 306) from GD13 to GD16 using immunohistochemical staining, Western blot, and real time RT-PCR.

Results

Immunohistochemical staining revealed that in normal embryos Wif1 was constantly expressed in the cloaca from GD13 to GD16. On GD13 and GD14, Wif1-immunopositive cells were extensively expressed in the cloaca. On GD15, the expression of Wif1 were mainly detected on the very thin anal membrane. In ARM embryos, the epithelium of the hindgut and urorectal septum demonstrated faint immunostaining for Wif1 from GD14 to GD16. Western blot and real time RT-PCR revealed that Wif1 and β-catenin protein and mRNA expression level was significantly decreased in the ARM groups compared with the normal group on GD14 and GD15 (p < 0.05).

Conclusions

This study demonstrated that the expression pattern of Wif1 and β-catenin was disrupted in ARM embryos during anorectal morphogenesis, which demonstrated that downregulation of Wif1 and β-catenin at the time of cloacal separation into the primitive rectum and urogenital septum might related to the development of ARM.

Anteroposterior polarity and elongation in the absence of extra-embryonic tissues and of spatially localised signalling in gastruloids: mammalian embryonic organoids

The establishment of the anteroposterior (AP) axis is a crucial step during animal embryo development. In mammals, genetic studies have shown that this process relies on signals spatiotemporally deployed in the extra-embryonic tissues that locate the position of the head and the onset of gastrulation, marked by T/Brachyury (T/Bra) at the posterior of the embryo. Here, we use gastruloids, mESC-based organoids, as a model system with which to study this process. We find that gastruloids localise T/Bra expression to one end and undergo elongation similar to the posterior region of the embryo, suggesting that they develop an AP axis. This process relies on precisely timed interactions between Wnt/β-catenin and Nodal signalling, whereas BMP signalling is dispensable. Additionally, polarised T/Bra expression occurs in the absence of extra-embryonic tissues or localised sources of signals. We suggest that the role of extra-embryonic tissues in the mammalian embryo might not be to induce the axes but to bias an intrinsic ability of the embryo to initially break symmetry. Furthermore, we suggest that Wnt signalling has a separable activity involved in the elongation of the axis.

Neural tube closure: cellular, molecular and biomechanical mechanisms

Neural tube closure has been studied for many decades, across a range of vertebrates, as a paradigm of embryonic morphogenesis. Neurulation is of particular interest in view of the severe congenital malformations - 'neural tube defects' - that result when closure fails. The process of neural tube closure is complex and involves cellular events such as convergent extension, apical constriction and interkinetic nuclear migration, as well as precise molecular control via the non-canonical Wnt/planar cell polarity pathway, Shh/BMP signalling, and the transcription factors Grhl2/3, Pax3, Cdx2 and Zic2. More recently, biomechanical inputs into neural tube morphogenesis have also been identified. Here, we review these cellular, molecular and biomechanical mechanisms involved in neural tube closure, based on studies of various vertebrate species, focusing on the most recent advances in the field.

Genome-wide identification of Wnt/β-catenin transcriptional targets during Xenopus gastrulation

The canonical Wnt/β-catenin signaling pathway plays multiple roles during Xenopus gastrulation, including posteriorization of the neural plate, patterning of the mesoderm, and induction of the neural crest. Wnt signaling stabilizes β-catenin, which then activates target genes. However, few targets of this signaling pathway that mediate early developmental processes are known. Here we sought to identify transcriptional targets of the Wnt/β-catenin signaling pathway using a genome-wide approach. We selected putative targets using the criteria of reduced expression upon zygotic Wnt knockdown, β-catenin binding within 50kb of the gene, and expression in tissues that receive Wnt signaling. Using these criteria, we found 21 novel direct transcriptional targets of Wnt/β-catenin signaling during gastrulation and in addition have identified putative regulatory elements for further characterization in future studies.

Armc5 deletion causes developmental defects and compromises T-cell immune responses

Armadillo repeat containing 5 (ARMC5) is a cytosolic protein with no enzymatic activities. Little is known about its function and mechanisms of action, except that gene mutations are associated with risks of primary macronodular adrenal gland hyperplasia. Here we map Armc5 expression by in situ hybridization, and generate Armc5 knockout mice, which are small in body size. Armc5 knockout mice have compromised T-cell proliferation and differentiation into Th1 and Th17 cells, increased T-cell apoptosis, reduced severity of experimental autoimmune encephalitis, and defective immune responses to lymphocytic choriomeningitis virus infection. These mice also develop adrenal gland hyperplasia in old age. Yeast 2-hybrid assays identify 16 ARMC5-binding partners. Together these data indicate that ARMC5 is crucial in fetal development, T-cell function and adrenal gland growth homeostasis, and that the functions of ARMC5 probably depend on interaction with multiple signalling pathways.

Mutations in ARMC5 are associated with risk of primary macronodular adrenal gland hyperplasia. Here the authors show that mice lacking Armc5 have adrenal gland hyperplasia and defective T-cell proliferation, differentiation, survival and in vivo T-cell-mediated immune responses.

MARK2/Par1b Insufficiency Attenuates DVL Gene Transcription via Histone Deacetylation in Lumbosacral Spina Bifida

Dishevelled (DVL/Dvl) genes play roles in canonical and noncanonical Wnt signaling, both of which are essential in neural tube closing and are involved in balancing neural progenitor growth and differentiation, or neuroepithelial cell polarity, respectively. In mouse Dvl haploinsufficiency leads to neural tube defects (NTDs), which represent the second most common birth defects. However, DVL genes' genetic contributions in human NTDs are modest. We sought to explore the molecular impact on such genes in human NTDs in a Han Chinese cohort. In 47 cases with NTDs and 61 matched controls, in brain tissues, the DVL1/2 mRNA levels were correlated with the levels of a serine/threonine protein kinase MARK2, and in 20 cases with lumbosacral spina bifida, the mRNA levels of DVL1 and MARK2 were significantly decreased; by contrast, only an intronic rare variant was found. Moreover, in an extended population, we found merely three novel rare missense variants in 1 % of individuals with NTDs. In cell-based assays, Mark2 depletion indeed reduces Dvl gene expression and interrupts neural stem cell (NSCs) growth and differentiation, which are likely to be mediated through a decrease in class IIa HDAC phosphorylation and reduced H3K4ac and H3K27ac occupancies at the Dvl1/2 promoters. Finally, the detections of folate concentration in human brain tissue and NSCs and MEF cells indicates that folate deficiency contributes to the observed decreases in Mark2 and Dvl1 expression. Our present study raises a potential common pathogenicity mechanism in human lumbosacral spina bifida about DVL genes rather than their genetic pathogenic role.

Tcf7l1 protects the anterior neural fold from adopting the neural crest fate

The neural crest (NC) is crucial for the evolutionary diversification of vertebrates. NC cells are induced at the neural plate border by the coordinated action of several signaling pathways, including Wnt/β-catenin. NC cells are normally generated in the posterior neural plate border, whereas the anterior neural fold is devoid of NC cells. Using the mouse model, we show here that active repression of Wnt/β-catenin signaling is required for maintenance of neuroepithelial identity in the anterior neural fold and for inhibition of NC induction. Conditional inactivation of Tcf7l1, a transcriptional repressor of Wnt target genes, leads to aberrant activation of Wnt/β-catenin signaling in the anterior neuroectoderm and its conversion into NC. This reduces the developing prosencephalon without affecting the anterior-posterior neural character. Thus, Tcf7l1 defines the border between the NC and the prospective forebrain via restriction of the Wnt/β-catenin signaling gradient.

Inhibition of glycogen synthase kinase-3 (GSK3) promotes the neural differentiation of full-term amniotic fluid-derived stem cells towards neural progenitor cells

The amniotic fluid has a heterogeneous population of cells. Some human amniotic fluid-derived stem (hAFS) cells have been shown to harbor the potential to differentiate into neural cells. However, the neural differentiation efficiency of hAFS cells remains low. In this study, we isolated CD117-positive hAFS cells from amniotic fluid and then examined the pluripotency of these cells through the formation of embryoid bodies (EBs). Additionally, we induced the neural differentiation of these cells using neuroectodermal medium. This study revealed that the GSK3-beta inhibitor SB216763 was able to stimulate the proliferation of CD117-positive hAFS cells without influencing their undifferentiated state. Moreover, SB216763 can efficiently promote the neural differentiation of CD117-positive hAFS cells towards neural progenitor cells in the presence of DMEM/F12 and N2 supplement. These findings provide an easy and low-cost method to maintain the proliferation of hAFS cells, as well as induce an efficacious generation of neural progenitor cells from hAFS cells. Such induction of the neural commitment of hAFS cells may provide an option for the treatment of neurodegenerative diseases by hAFS cells-based therapies.

A direct role for murine Cdx proteins in the trunk neural crest gene regulatory network

Numerous studies in chordates and arthropods currently indicate that Cdx proteins have a major ancestral role in the organization of post-head tissues. In urochordate embryos, Cdx loss-of-function has been shown to impair axial elongation, neural tube (NT) closure and pigment cell development. Intriguingly, in contrast to axial elongation and NT closure, a Cdx role in neural crest (NC)-derived melanocyte/pigment cell development has not been reported in any other chordate species. To address this, we generated a new conditional pan-Cdx functional knockdown mouse model that circumvents Cdx functional redundancy as well as the early embryonic lethality of Cdx mutants. Through directed inhibition in the neuroectoderm, we provide in vivo evidence that murine Cdx proteins impact melanocyte and enteric nervous system development by, at least in part, directly controlling the expression of the key early regulators of NC ontogenesis Pax3,Msx1 and Foxd3 Our work thus reveals a novel role for Cdx proteins at the top of the trunk NC gene regulatory network in the mouse, which appears to have been inherited from their ancestral ortholog.

Inhibition of β-catenin-TCF1 interaction delays differentiation of mouse embryonic stem cells

Blocking β-catenin/TCF1–mediated transcriptional activation with a specific small molecule or by TCF1 knockdown delays the mouse embryonic stem cell differentiation program and enhances pluripotency.

The ability of mouse embryonic stem cells (mESCs) to self-renew or differentiate into various cell lineages is regulated by signaling pathways and a core pluripotency transcriptional network (PTN) comprising Nanog, Oct4, and Sox2. The Wnt/β-catenin pathway promotes pluripotency by alleviating T cell factor TCF3-mediated repression of the PTN. However, it has remained unclear how β-catenin’s function as a transcriptional activator with TCF1 influences mESC fate. Here, we show that TCF1-mediated transcription is up-regulated in differentiating mESCs and that chemical inhibition of β-catenin/TCF1 interaction improves long-term self-renewal and enhances functional pluripotency. Genetic loss of TCF1 inhibited differentiation by delaying exit from pluripotency and conferred a transcriptional profile strikingly reminiscent of self-renewing mESCs with high Nanog expression. Together, our data suggest that β-catenin’s function in regulating mESCs is highly context specific and that its interaction with TCF1 promotes differentiation, further highlighting the need for understanding how its individual protein–protein interactions drive stem cell fate.

Restricted Pax3 Deletion within the Neural Tube Results in Congenital Hydrocephalus

Congenital hydrocephalus is a common birth-defect whose developmental origins are poorly understood. Pax3-null mutants show defects in myogenesis, neural tube closure, neural crest morphogenesis, and heart development that, consequently, results in embryonic lethality. Here we demonstrate that conditional deletion of the mouse Pax3 transcription factor results in fully-penetrant congenital obstructive hydrocephalus. To identify the role of Pax3 during cranial development, we deleted Pax3 within the neuroepithelium (via Pax7^−Cre), in the neural crest (via P0-Cre), and in both the neuroepithelium and the neural crest (via Wnt1-Cre). Only conditional mutants generated using Pax7^−Cre or Wnt1-Cre developed early onset congenital hydrocephalus due to stenosis of the third ventricle, suggesting that loss of neuroepithelial Pax3 is sufficient to disturb third ventricle morphogenesis. Dilation of lateral ventricles occurs as early as E14.5, and lineage-mapping revealed that the neuroepithelial cells in the conditional mutants are present, but fail to undergo normal differentiation at the stenotic site. Concomitant with a narrowing of the mutant third ventricle, we detected ectopic apoptosis, reduced proliferation, and abnormal β-catenin localization. Furthermore, consistent with the overlapping expression pattern of Pax3 and Pax7 in early cranial neuroepithelium, we demonstrated a combinatorial role, as compound Pax3/Pax7 heterozygotes display partially-penetrant congenital hydrocephalus. These murine data provide an experimental paradigm underpinning clinical observations of the presence of PAX3 mutations in some hydrocephalic patients.

Brain Transcriptomic Response to Social Eavesdropping in Zebrafish (Danio rerio)

Public information is widely available at low cost to animals living in social groups. For instance, bystanders may eavesdrop on signaling interactions between conspecifics and use it to adapt their subsequent behavior towards the observed individuals. This social eavesdropping ability is expected to require specialized mechanisms such as social attention, which selects social information available for learning. To begin exploring the genetic basis of social eavesdropping, we used a previously established attention paradigm in the lab to study the brain gene expression profile of male zebrafish (Danio rerio) in relation to the attention they paid towards conspecifics involved or not involved in agonistic interactions. Microarray gene chips were used to characterize their brain transcriptomes based on differential expression of single genes and gene sets. These analyses were complemented by promoter region-based techniques. Using data from both approaches, we further drafted protein interaction networks. Our results suggest that attentiveness towards conspecifics, whether interacting or not, activates pathways linked to neuronal plasticity and memory formation. The network analyses suggested that fos and jun are key players on this response, and that npas4a, nr4a1 and egr4 may also play an important role. Furthermore, specifically observing fighting interactions further triggered pathways associated to a change in the alertness status (dnajb5) and to other genes related to memory formation (btg2, npas4b), which suggests that the acquisition of eavesdropped information about social relationships activates specific processes on top of those already activated just by observing conspecifics.