A mitogen gradient of dorsal midline Wnts organizes growth in the CNS

Adherent junctions: Physiology, role in hydrocephalus and potential therapeutic targets

In all epithelial cells, the adherent junctions (AJs) with cadherin as the core play an important role in the maintenance of the connection and the formation of apical-basal polarity. The ependymal cells close to the ventricular system rely on AJs with N-cadherin at the core to maintain their normal morphology and function. Therefore, it has an important impact on the function and disease of the central nervous system. Hydrocephalus is a pathological phenomenon of excessive cerebrospinal fluid accumulating in the ventricular system accompanied by continuous ventricular dilatation, which can be divided into obstructive hydrocephalus and communicating hydrocephalus according to the pathogenesis. Obstructive hydrocephalus is often associated with excessive ependymal cells produced by differentiation of radial glial cells. The etiology of communicating hydrocephalus is mainly related to the dyskinesia of cerebrospinal fluid. In addition, the damage of the brain barrier can lead to brain edema and aggravate the symptoms. At present, the researches on the pathogenesis of hydrocephalus are mainly focused on the development of ependymal cells and cilia, while less attention has been paid to molecules such as AJs, which play an important role in maintaining the polarity of ependymal cells. This paper discusses the formation and function of AJs and their role in preventing hydrocephalus by preserving the polarity of ependymal cilia, regulating the number of ependymal cells, and upholding the brain barrier integrity to impede hydrocephalus exacerbation, which provides a new direction for the study of hydrocephalus.

Early spinal cord development: from neural tube formation to neurogenesis

As one of the simplest and most evolutionarily conserved parts of the vertebrate nervous system, the spinal cord serves as a key model for understanding the principles of nervous system construction. During embryonic development, the spinal cord originates from a population of bipotent stem cells termed neuromesodermal progenitors, which are organized within a transient embryonic structure known as the neural tube. Neural tube morphogenesis differs along its anterior-to-posterior axis: most of the neural tube (including the regions that will develop into the brain and the anterior spinal cord) forms via the bending and dorsal fusion of the neural groove, but the establishment of the posterior region of the neural tube involves de novo formation of a lumen within a solid medullary cord. The early spinal cord primordium consists of highly polarized neural progenitor cells organized into a pseudostratified epithelium. Tight regulation of the cell division modes of these progenitors drives the embryonic growth of the neural tube and initiates primary neurogenesis. A rich history of observational and functional studies across various vertebrate models has advanced our understanding of the cellular events underlying spinal cord development, and these foundational studies are beginning to inform our knowledge of human spinal cord development.

CRISPR-Cas13d as a molecular tool to achieve targeted gene expression knockdown in chick embryos

The chick embryo is a classical model system commonly used in developmental biology due to its amenability to gene perturbation experiments. Pairing this powerful model organism with cutting-edge technology can significantly expand the range of experiments that can be performed. Recently, the CRISPR-Cas13d system has been successfully adapted for use in zebrafish, medaka, killifish, and mouse embryos to achieve targeted gene expression knockdown. Despite its success in other animal models, no prior study has explored the potential of CRISPR-Cas13d in the chick. Here, we present an adaptation of the CRISPR-Cas13d system to achieve targeted gene expression knockdown in the chick embryo. As proof-of-principle, we demonstrate the knockdown of PAX7, an early neural crest marker. Application of this adapted CRISPR-Cas13d technique resulted in effective knockdown of PAX7 expression and function, comparable to knockdown achieved by translation-blocking morpholino. CRISPR-Cas13d complements preexisting knockdown tools such as CRISPR-Cas9 and morpholinos, thereby expanding the experimental potential and versatility of the chick model system.

The transmembrane protein TMEM196 controls cell proliferation and determines the floor plate cell lineage

The neural tube, the embryonic precursor to the vertebrate central nervous system, comprises distinct progenitor and neuronal domains, each with specific proliferation programs. In this study, we identified TMEM196, a novel transmembrane protein that plays a crucial role in regulating cell proliferation in the floor plate in chick embryos. TMEM196 is expressed in the floor plate, and its overexpression leads to reduced cell proliferation without affecting the pattern formation of the neural tube. We also established the floor plate differentiation protocol of the mouse embryonic stem cells, and analyzed the function of TMEM196 with this system. Mutating the Tmem196 gene does not alter cell division and overall differentiation remains unchanged within the neural cells. However, TMEM196 inhibits Wnt signaling, and Tmem196 mutant cells exhibit aberrant paraxial mesoderm differentiation, suggesting that TMEM196 selects the floor plate cell fate at the binary decision of the neuromesodermal cells. These findings highlight TMEM196 as a key regulator of both cell proliferation and floor plate determination, contributing to proper regionalization during embryogenesis.

Neural plate pre-patterning enables specification of intermediate neural progenitors in the spinal cord

Dorsal-ventral patterning of neural progenitors in the posterior neural tube, which gives rise to the spinal cord, has served as a model system to understand how extracellular signals organize developing tissues. While previous work has shown that signaling gradients diversify progenitor fates at the dorsal and ventral ends of the tissue, the basis of fate specification in intermediate regions has remained unclear. Here we use zebrafish to investigate the neural plate, which precedes neural tube formation, and show that its pre-patterning by a distinct signaling environment enables intermediate fate specification. Systematic spatial analysis of transcription factor (TF) expression and signaling activity using a reference-based mapping approach shows that the neural plate is partitioned into a striking complexity of TF co-expression states that, in part, correspond to the activity of gastrulation signals such as FGF and Wnt that persist through axis extension. Using in toto analysis of cellular movement combined with fate mapping, we find that dbx1b-expressing intermediate progenitors (p0) originate from a neural-plate specific state characterized by transient co-expression of the TFs pax3a, olig4 and her3. Finally, we show that this state is defined by Wnt signaling in the posterior neural plate and that ectopic Wnt activation within pax3a/olig4+ cells is sufficient to promote dbx1b expression. Our data broadly support a model in which neural progenitor specification occurs through the sequential use of multiple signals to progressively diversify the neural tissue as it develops. This has implications for in vitro differentiation of spinal cord cell types and for understanding signal-based patterning in other developmental contexts.

Deleterious ZNRF3 germline variants cause neurodevelopmental disorders with mirror brain phenotypes via domain-specific effects on Wnt/β-catenin signaling

Zinc and RING finger 3 (ZNRF3) is a negative-feedback regulator of Wnt/β-catenin signaling, which plays an important role in human brain development. Although somatically frequently mutated in cancer, germline variants in ZNRF3 have not been established as causative for neurodevelopmental disorders (NDDs). We identified 12 individuals with ZNRF3 variants and various phenotypes via GeneMatcher/Decipher and evaluated genotype-phenotype correlation. We performed structural modeling and representative deleterious and control variants were assessed using in vitro transcriptional reporter assays with and without Wnt-ligand Wnt3a and/or Wnt-potentiator R-spondin (RSPO). Eight individuals harbored de novo missense variants and presented with NDD. We found missense variants associated with macrocephalic NDD to cluster in the RING ligase domain. Structural modeling predicted disruption of the ubiquitin ligase function likely compromising Wnt receptor turnover. Accordingly, the functional assays showed enhanced Wnt/β-catenin signaling for these variants in a dominant negative manner. Contrarily, an individual with microcephalic NDD harbored a missense variant in the RSPO-binding domain predicted to disrupt binding affinity to RSPO and showed attenuated Wnt/β-catenin signaling in the same assays. Additionally, four individuals harbored de novo truncating or de novo or inherited large in-frame deletion variants with non-NDD phenotypes, including heart, adrenal, or nephrotic problems. In contrast to NDD-associated missense variants, the effects on Wnt/β-catenin signaling were comparable between the truncating variant and the empty vector and between benign variants and the wild type. In summary, we provide evidence for mirror brain size phenotypes caused by distinct pathomechanisms in Wnt/β-catenin signaling through protein domain-specific deleterious ZNRF3 germline missense variants.

CRISPR-Cas13d as a molecular tool to achieve targeted gene expression knockdown in chick embryos

The chick embryo is a classical model system commonly used in developmental biology due to its amenability to gene perturbation experiments. Pairing this powerful model organism with cutting-edge technology can significantly expand the range of experiments that can be performed. Recently, the CRISPR-Cas13d system has been successfully adapted for use in zebrafish, medaka, killifish, and mouse embryos to achieve targeted gene expression knockdown. Despite its success in other animal models, no prior study has explored the potential of CRISPR-Cas13d in the chick. Here, we present an adaptation of the CRISPR-Cas13d system to achieve targeted gene expression knockdown in the chick embryo. As proof-of-principle, we demonstrate the knockdown of PAX7, an early neural crest marker. Application of this adapted CRISPR-Cas13d technique resulted in effective knockdown of PAX7 expression and function, comparable to knockdown achieved by translation-blocking morpholino. CRISPR-Cas13d complements preexisting knockdown tools such as CRISPR-Cas9 and morpholinos, thereby expanding the experimental potential and versatility of the chick model system.

Semaphorin Receptors Antagonize Wnt Signaling Through Beta-Catenin Degradation

Precise control of morphogen signaling levels is essential for proper development. An outstanding question is: what mechanisms ensure proper morphogen activity and correct cellular responses? Previous work has identified Semaphorin (SEMA) receptors, Neuropilins (NRPs) and Plexins (PLXNs), as positive regulators of the Hedgehog (HH) signaling pathway. Here, we provide evidence that NRPs and PLXNs antagonize Wnt signaling in both fibroblasts and epithelial cells. Further, Nrp1/2 deletion in fibroblasts results in elevated baseline Wnt pathway activity and increased maximal responses to Wnt stimulation. Notably, and in contrast to HH signaling, SEMA receptor-mediated Wnt antagonism is independent of primary cilia. Mechanistically, PLXNs and NRPs act downstream of Dishevelled (DVL) to destabilize β-catenin (CTNNB1) in a proteosome-dependent manner. Further, NRPs, but not PLXNs, act in a GSK3β/CK1-dependent fashion to antagonize Wnt signaling, suggesting distinct repressive mechanisms for these SEMA receptors. Overall, this study identifies SEMA receptors as novel Wnt pathway antagonists that may also play larger roles integrating signals from multiple inputs.

Investigating the basis of lineage decisions and developmental trajectories in the dorsal spinal cord through pseudotime analyses

Dorsal interneurons (dIs) in the spinal cord encode the perception of touch, pain, heat, itchiness and proprioception. Previous studies using genetic strategies in animal models have revealed important insights into dI development, but the molecular details of how dIs arise as distinct populations of neurons remain incomplete. We have developed a resource to investigate dI fate specification by combining a single-cell RNA-Seq atlas of mouse embryonic stem cell-derived dIs with pseudotime analyses. To validate this in silico resource as a useful tool, we used it to first identify genes that are candidates for directing the transition states that lead to distinct dI lineage trajectories, and then validated them using in situ hybridization analyses in the developing mouse spinal cord in vivo. We have also identified an endpoint of the dI5 lineage trajectory and found that dIs become more transcriptionally homogeneous during terminal differentiation. This study introduces a valuable tool for further discovery about the timing of gene expression during dI differentiation and demonstrates its utility in clarifying dI lineage relationships.

Investigating the basis of lineage decisions and developmental trajectories in the dorsal spinal cord through pseudotime analyses

Dorsal interneurons (dIs) in the spinal cord encode the perception of touch, pain, heat, itch, and proprioception. While previous studies using genetic strategies in animal models have revealed important insights into dI development, the molecular details by which dIs arise as distinct populations of neurons remain incomplete. We have developed a resource to investigate dI fate specification by combining a single-cell RNA-Seq atlas of mouse ESC-derived dIs with pseudotime analyses. To validate this in silico resource as a useful tool, we used it to first identify novel genes that are candidates for directing the transition states that lead to distinct dI lineage trajectories, and then validated them using in situ hybridization analyses in the developing mouse spinal cord in vivo . We have also identified a novel endpoint of the dI5 lineage trajectory and found that dIs become more transcriptionally homogenous during terminal differentiation. Together, this study introduces a valuable tool for further discovery about the timing of gene expression during dI differentiation and demonstrates its utility clarifying dI lineage relationships.

Summary statement

Pseudotime analyses of embryonic stem cell-derived dorsal spinal interneurons reveals both novel regulators and lineage relationships between different interneuron populations.

Toward a better understanding of how a gyrified brain develops

The size and shape of the cerebral cortex have changed dramatically across evolution. For some species, the cortex remains smooth (lissencephalic) throughout their lifetime, while for other species, including humans and other primates, the cortex increases substantially in size and becomes folded (gyrencephalic). A folded cortex boasts substantially increased surface area, cortical thickness, and neuronal density, and it is therefore associated with higher-order cognitive abilities. The mechanisms that drive gyrification in some species, while others remain lissencephalic despite many shared neurodevelopmental features, have been a topic of investigation for many decades, giving rise to multiple perspectives of how the gyrified cerebral cortex acquires its unique shape. Recently, a structurally unique germinal layer, known as the outer subventricular zone, and the specialized cell type that populates it, called basal radial glial cells, were identified, and these have been shown to be indispensable for cortical expansion and folding. Transcriptional analyses and gene manipulation models have provided an invaluable insight into many of the key cellular and genetic drivers of gyrification. However, the degree to which certain biomechanical, genetic, and cellular processes drive gyrification remains under investigation. This review considers the key aspects of cerebral expansion and folding that have been identified to date and how theories of gyrification have evolved to incorporate this new knowledge.

Neuroepithelial organoid patterning is mediated by a neighborhood watch mechanism

During epithelial tissue patterning, morphogens operate across multiple length scales to instruct cell identities. However, how cell fate changes are coordinated over these scales to establish spatial organization remains poorly understood. Here, we use human neural tube organoids as models of epithelial patterning and develop an in silico approach to define conditions permissive to patterning. By systematically varying morphogen position, diffusivity, and fate-inducing concentration levels, we show that cells follow a "neighborhood watch" (NW) mechanism that is deterministically dictated by initial morphogen source positions, reflecting scale-invariant in vitro phenotypes. We define how the frequency and local bias of morphogen sources stabilize pattern orientation. The model predicts enhanced patterning through floor plate inhibition, and receptor-ligand interaction analysis of single-cell RNA sequencing (scRNA-seq) data identifies wingless-related integration site (WNT) and bone morphogenic protein (BMP) as inhibition modulators, which we validate in vitro. These results suggest that developing neuroepithelia employ NW-based mechanisms to organize morphogen sources, define cellular identity, and establish patterns.

Lmx1a is a master regulator of the cortical hem

Development of the nervous system depends on signaling centers - specialized cellular populations that produce secreted molecules to regulate neurogenesis in the neighboring neuroepithelium. In some cases, signaling center cells also differentiate to produce key types of neurons. The formation of a signaling center involves its induction, the maintenance of expression of its secreted molecules, and cell differentiation and migration events. How these distinct processes are coordinated during signaling center development remains unknown. By performing studies in mice, we show that Lmx1a acts as a master regulator to orchestrate the formation and function of the cortical hem (CH), a critical signaling center that controls hippocampus development. Lmx1a co-regulates CH induction, its Wnt signaling, and the differentiation and migration of CH-derived Cajal-Retzius neurons. Combining RNAseq, genetic, and rescue experiments, we identified major downstream genes that mediate distinct Lmx1a-dependent processes. Our work revealed that signaling centers in the mammalian brain employ master regulatory genes and established a framework for analyzing signaling center development.

Protocol for a Wnt reporter assay to measure its activity in human neural stem cells derived from induced pluripotent stem cells

The canonical Wnt signaling is an essential pathway that regulates cellular proliferation, maturation, and differentiation during neurodevelopment and maintenance of adult tissue homeostasis. This pathway has been implicated with the pathophysiology of neuropsychiatric disorders and was associated with cognitive processes, such as learning and memory. However, the molecular investigation of the Wnt signaling in functional human neural cell lines might be challenging since brain biopsies are not possible and animal models may not represent the polygenic profile of some neurological and neurodevelopmental disorders. In this context, using induced pluripotent stem cells (iPSCs) has become a powerful tool to model disorders that affect the Central Nervous System (CNS) in vitro, by maintaining patients' genetic backgrounds. In this method paper, we report the development of a virus-free Wnt reporter assay in neural stem cells (NSCs) derived from human iPSCs from two healthy individuals, by using a vector containing a reporter gene (luc2P) under the control of a TCF/LEF (T-cell factor/lymphoid enhancer factor) responsive element. Dose-response curve analysis from this luciferase-based method might be useful when testing the activity of the Wnt signaling pathway after agonists (e.g. Wnt3a) or antagonists (e.g. DKK1) administration, comparing activity between cases and controls in distinct disorders. Using such a reporter assay method may help to elucidate whether neurological or neurodevelopmental mental disorders show alterations in this pathway, and testing whether targeted treatment may reverse these. Therefore, our established assay aims to help researchers on the functional and molecular investigation of the Wnt pathway in patient-specific cell types comprising several neuropsychiatric disorders.

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.

Factors affecting the role of canonical Wnt inhibitor Dickkopf-1 in cancer progression

Background

The canonical Wnt inhibitor Dickkopf-1 (Dkk-1) has the capacity to modulate homeostasis between canonical and non-canonical Wnt pathways and also signal independently of Wnt. The specific effects of Dkk-1 activity on tumor physiology are therefore unpredictable with examples of Dkk-1 serving as either a driver or suppressor of malignancy. Given that Dkk-1 blockade may serve as a potential treatment for some types of cancer, we questioned whether it is possible to predict the role of Dkk-1 on tumor progression based on the tissue origin of the tumor.

Methods

Original research articles that described Dkk-1 in terms a tumor suppressor or driver of cancer growth were identified. To determine the association between tumor developmental origin and the role of Dkk-1, a logistic regression was performed. The Cancer Genome Atlas database was interrogated for survival statistics based on tumor Dkk-1 expression.

Results

We report that Dkk-1 is statistically more likely to serve as a suppressor in tumors arising from the ectoderm (p = 0.0198) or endoderm (p = 0.0334) but more likely to serve as a disease driver in tumors of mesodermal origin (p = 0.0155). Survival analyses indicated that in cases where Dkk-1 expression could be stratified, high Dkk-1 expression is usually associated with poor prognosis. This in part may be due to pro-tumorigenic role Dkk-1 plays on tumor cells but also through its influence on immunomodulatory and angiogenic processes in the tumor stroma.

Conclusion

Dkk-1 has a context-specific dual role as a tumor suppressor or driver. Dkk-1 is significantly more likely to serve as a tumor suppressor in tumors arising from ectoderm and endoderm while the converse is true for mesodermal tumors. Patient survival data indicated high Dkk-1 expression is generally a poor prognostic indicator. These findings provide further support for the importance of Dkk-1 as a therapeutic cancer target in some cases.

Progress and challenges in directing the differentiation of human iPSCs into spinal motor neurons

Motor neuron (MN) diseases, including amyotrophic lateral sclerosis, progressive bulbar palsy, primary lateral sclerosis and spinal muscular atrophy, cause progressive paralysis and, in many cases, death. A better understanding of the molecular mechanisms of pathogenesis is urgently needed to identify more effective therapies. However, studying MNs has been extremely difficult because they are inaccessible in the spinal cord. Induced pluripotent stem cells (iPSCs) can generate a theoretically limitless number of MNs from a specific patient, making them powerful tools for studying MN diseases. However, to reach their potential, iPSCs need to be directed to efficiently differentiate into functional MNs. Here, we review the reported differentiation protocols for spinal MNs, including induction with small molecules, expression of lineage-specific transcription factors, 2-dimensional and 3-dimensional cultures, as well as the implementation of microfluidics devices and co-cultures with other cell types, including skeletal muscle. We will summarize the advantages and disadvantages of each strategy. In addition, we will provide insights into how to address some of the remaining challenges, including reproducibly obtaining mature and aged MNs.

Balancing WNT signalling in early forebrain development: The role of LRP4 as a modulator of LRP6 function

The specification of the forebrain relies on the precise regulation of WNT/ß-catenin signalling to support neuronal progenitor cell expansion, patterning, and morphogenesis. Imbalances in WNT signalling activity in the early neuroepithelium lead to congenital disorders, such as neural tube defects (NTDs). LDL receptor-related protein (LRP) family members, including the well-studied receptors LRP5 and LRP6, play critical roles in modulating WNT signalling capacity through tightly regulated interactions with their co-receptor Frizzled, WNT ligands, inhibitors and intracellular WNT pathway components. However, little is known about the function of LRP4 as a potential modulator of WNT signalling in the central nervous system. In this study, we investigated the role of LRP4 in the regulation of WNT signalling during early mouse forebrain development. Our results demonstrate that LRP4 can modulate LRP5- and LRP6-mediated WNT signalling in the developing forebrain prior to the onset of neurogenesis at embryonic stage 9.5 and is therefore essential for accurate neural tube morphogenesis. Specifically, LRP4 functions as a genetic modifier for impaired mitotic activity and forebrain hypoplasia, but not for NTDs in LRP6-deficient mutants. In vivo and in vitro data provide evidence that LRP4 is a key player in fine-tuning WNT signalling capacity and mitotic activity of mouse neuronal progenitors and of human retinal pigment epithelial (hTERT RPE-1) cells. Our data demonstrate the crucial roles of LRP4 and LRP6 in regulating WNT signalling and forebrain development and highlight the need to consider the interaction between different signalling pathways to understand the underlying mechanisms of disease. The findings have significant implications for our mechanistic understanding of how LRPs participate in controlling WNT signalling.

Lhx2 is a progenitor-intrinsic modulator of Sonic Hedgehog signaling during early retinal neurogenesis

An important question in organogenesis is how tissue-specific transcription factors interact with signaling pathways. In some cases, transcription factors define the context for how signaling pathways elicit tissue- or cell-specific responses, and in others, they influence signaling through transcriptional regulation of signaling components or accessory factors. We previously showed that during optic vesicle patterning, the Lim-homeodomain transcription factor Lhx2 has a contextual role by linking the Sonic Hedgehog (Shh) pathway to downstream targets without regulating the pathway itself. Here, we show that during early retinal neurogenesis in mice, Lhx2 is a multilevel regulator of Shh signaling. Specifically, Lhx2 acts cell autonomously to control the expression of pathway genes required for efficient activation and maintenance of signaling in retinal progenitor cells. The Shh co-receptors Cdon and Gas1 are candidate direct targets of Lhx2 that mediate pathway activation, whereas Lhx2 directly or indirectly promotes the expression of other pathway components important for activation and sustained signaling. We also provide genetic evidence suggesting that Lhx2 has a contextual role by linking the Shh pathway to downstream targets. Through these interactions, Lhx2 establishes the competence for Shh signaling in retinal progenitors and the context for the pathway to promote early retinal neurogenesis. The temporally distinct interactions between Lhx2 and the Shh pathway in retinal development illustrate how transcription factors and signaling pathways adapt to meet stage-dependent requirements of tissue formation.

WNT signaling at the intersection between neurogenesis and brain tumorigenesis

Neurogenesis and tumorigenesis share signaling molecules/pathways involved in cell proliferation, differentiation, migration, and death. Self-renewal of neural stem cells is a tightly regulated process that secures the accuracy of cell division and eliminates cells that undergo mitotic errors. Abnormalities in the molecular mechanisms controlling this process can trigger aneuploidy and genome instability, leading to neoplastic transformation. Mutations that affect cell adhesion, polarity, or migration enhance the invasive potential and favor the progression of tumors. Here, we review recent evidence of the WNT pathway’s involvement in both neurogenesis and tumorigenesis and discuss the experimental progress on therapeutic opportunities targeting components of this pathway.

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.

In vitro atlas of dorsal spinal interneurons reveals Wnt signaling as a critical regulator of progenitor expansion

Restoring sensation after injury or disease requires a reproducible method for generating large quantities of bona fide somatosensory interneurons. Toward this goal, we assess the mechanisms by which dorsal spinal interneurons (dIs; dI1-dI6) can be derived from mouse embryonic stem cells (mESCs). Using two developmentally relevant growth factors, retinoic acid (RA) and bone morphogenetic protein (BMP) 4, we recapitulate the complete in vivo program of dI differentiation through a neuromesodermal intermediate. Transcriptional profiling reveals that mESC-derived dIs strikingly resemble endogenous dIs, with the correct molecular and functional signatures. We further demonstrate that RA specifies dI4-dI6 fates through a default multipotential state, while the addition of BMP4 induces dI1-dI3 fates and activates Wnt signaling to enhance progenitor proliferation. Constitutively activating Wnt signaling permits the dramatic expansion of neural progenitor cultures. These cultures retain the capacity to differentiate into diverse populations of dIs, thereby providing a method of increasing neuronal yield.

Restricted Proliferation During Neurogenesis Contributes to Regionalisation of the Amphioxus Nervous System

The central nervous system of the cephalochordate amphioxus consists of a dorsal neural tube with an anterior brain. Two decades of gene expression analyses in developing amphioxus embryos have shown that, despite apparent morphological simplicity, the amphioxus neural tube is highly regionalised at the molecular level. However, little is known about the morphogenetic mechanisms regulating the spatiotemporal emergence of cell types at distinct sites of the neural axis and how their arrangements contribute to the overall neural architecture. In vertebrates, proliferation is key to provide appropriate cell numbers of specific types to particular areas of the nervous system as development proceeds, but in amphioxus proliferation has never been studied at this level of detail, nor in the specific context of neurogenesis. Here, we describe the dynamics of cell division during the formation of the central nervous system in amphioxus embryos, and identify specific regions of the nervous system that depend on proliferation of neuronal precursors at precise time-points for their maturation. By labelling proliferating cells in vivo at specific time points in development, and inhibiting cell division during neurulation, we demonstrate that localised proliferation in the anterior cerebral vesicle is required to establish the full cell type repertoire of the frontal eye complex and the putative hypothalamic region of the amphioxus brain, while posterior proliferating progenitors, which were found here to derive from the dorsal lip of the blastopore, contribute to elongation of the caudal floor plate. Between these proliferative domains, we find that trunk nervous system differentiation is independent from cell division, in which proliferation decreases during neurulation and resumes at the early larval stage. Taken together, our results highlight the importance of proliferation as a tightly controlled mechanism for shaping and regionalising the amphioxus neural axis during development, by addition of new cells fated to particular types, or by influencing tissue geometry.

Dysregulation of multiple signaling pathways: A possible cause of cerebral palsy

Cerebral palsy (CP) is a lifelong disability characterized by the impairment of brain functions that result in improper posture and abnormal motor patterns. Understanding this brain abnormality and the role of genetic, epigenetic, and non-genetic factors such as signaling pathway dysregulation and cytokine dysregulation in the pathogenesis of CP is a complex process. Hypoxic-ischemic injury and prematurity are two well-known contributors of CP. Like in the case of other neurodevelopmental disorders such as intellectual disability and autism, the genomic constituents in CP are highly complex. The neuroinflammation that is triggered by maternal cytokine response plays a critical role in the pathogenesis of fetal inflammation response, which is one of the contributing factors of CP, and it continues even after the birth of children suffering from CP. Canonical Wnt signaling pathway is important for the development of mammalian fetal brain and it regulates distinct processes including neurogenesis. The glycogen synthase kinase-3 (GSK-3) antagonistic activity in the Wnt signaling pathway plays a crucial role in neurogenesis and neural development. In this review, we investigated several genetic and non-genetic pathways that are involved in the pathogenesis of CP and their regulation, impairment, and implications for causing CP during embryonic growth and developmental period. Investigating the role of these pathways help to develop novel therapeutic interventions and biomarkers for early diagnosis and treatment. This review also helps us to comprehend the mechanical approach of various signaling pathways, as well as their consequences and relevance in the understanding of CP.

Regulatory mechanisms of cytoneme-based morphogen transport

During development and tissue homeostasis, cells must communicate with their neighbors to ensure coordinated responses to instructional cues. Cues such as morphogens and growth factors signal at both short and long ranges in temporal- and tissue-specific manners to guide cell fate determination, provide positional information, and to activate growth and survival responses. The precise mechanisms by which such signals traverse the extracellular environment to ensure reliable delivery to their intended cellular targets are not yet clear. One model for how this occurs suggests that specialized filopodia called cytonemes extend between signal-producing and -receiving cells to function as membrane-bound highways along which information flows. A growing body of evidence supports a crucial role for cytonemes in cell-to-cell communication. Despite this, the molecular mechanisms by which cytonemes are initiated, how they grow, and how they deliver specific signals are only starting to be revealed. Herein, we discuss recent advances toward improved understanding of cytoneme biology. We discuss similarities and differences between cytonemes and other types of cellular extensions, summarize what is known about how they originate, and discuss molecular mechanisms by which their activity may be controlled in development and tissue homeostasis. We conclude by highlighting important open questions regarding cytoneme biology, and comment on how a clear understanding of their function may provide opportunities for treating or preventing disease.

T-Cell Factors as Transcriptional Inhibitors: Activities and Regulations in Vertebrate Head Development

Since its first discovery in the late 90s, Wnt canonical signaling has been demonstrated to affect a large variety of neural developmental processes, including, but not limited to, embryonic axis formation, neural proliferation, fate determination, and maintenance of neural stem cells. For decades, studies have focused on the mechanisms controlling the activity of β-catenin, the sole mediator of Wnt transcriptional response. More recently, the spotlight of research is directed towards the last cascade component, the T-cell factor (TCF)/Lymphoid-Enhancer binding Factor (LEF), and more specifically, the TCF/LEF-mediated switch from transcriptional activation to repression, which in both embryonic blastomeres and mouse embryonic stem cells pushes the balance from pluri/multipotency towards differentiation. It has been long known that Groucho/Transducin-Like Enhancer of split (Gro/TLE) is the main co-repressor partner of TCF/LEF. More recently, other TCF/LEF-interacting partners have been identified, including the pro-neural BarH-Like 2 (BARHL2), which belongs to the evolutionary highly conserved family of homeodomain-containing transcription factors. This review describes the activities and regulatory modes of TCF/LEF as transcriptional repressors, with a specific focus on the functions of Barhl2 in vertebrate brain development. Specific attention is given to the transcriptional events leading to formation of the Organizer, as well as the roles and regulations of Wnt/β-catenin pathway in growth of the caudal forebrain. We present TCF/LEF activities in both embryonic and neural stem cells and discuss how alterations of this pathway could lead to tumors.

GAS1 is required for NOTCH-dependent facilitation of SHH signaling in the ventral forebrain neuroepithelium

Growth arrest-specific 1 (GAS1) acts as a co-receptor to patched 1, promoting sonic hedgehog (SHH) signaling in the developing nervous system. GAS1 mutations in humans and animal models result in forebrain and craniofacial malformations, defects ascribed to a function for GAS1 in SHH signaling during early neurulation. Here, we confirm loss of SHH activity in the forebrain neuroepithelium in GAS1-deficient mice and in induced pluripotent stem cell-derived cell models of human neuroepithelial differentiation. However, our studies document that this defect can be attributed, at least in part, to a novel role for GAS1 in facilitating NOTCH signaling, which is essential to sustain a persistent SHH activity domain in the forebrain neuroepithelium. GAS1 directly binds NOTCH1, enhancing ligand-induced processing of the NOTCH1 intracellular domain, which drives NOTCH pathway activity in the developing forebrain. Our findings identify a unique role for GAS1 in integrating NOTCH and SHH signal reception in neuroepithelial cells, and they suggest that loss of GAS1-dependent NOTCH1 activation contributes to forebrain malformations in individuals carrying GAS1 mutations.

Mosmo Is Required for Zebrafish Craniofacial Formation

Hedgehog (Hh) signaling is a highly regulated molecular pathway implicated in many developmental and homeostatic events. Mutations in genes encoding primary components or regulators of the pathway cause an array of congenital malformations or postnatal pathologies, the extent of which is not yet fully defined. Mosmo (Modulator of Smoothened) is a modulator of the Hh pathway, which encodes a membrane tetraspan protein. Studies in cell lines have shown that Mosmo promotes the internalization and degradation of the Hh signaling transducer Smoothened (Smo), thereby down-modulating pathway activation. Whether this modulation is essential for vertebrate embryonic development remains poorly explored. Here, we have addressed this question and show that in zebrafish embryos, the two mosmo paralogs, mosmoa and mosmob, are expressed in the head mesenchyme and along the entire ventral neural tube. At the cellular level, Mosmoa localizes at the plasma membrane, cytoplasmic vesicles and primary cilium in both zebrafish and chick embryos. CRISPR/Cas9 mediated inactivation of both mosmoa and mosmob in zebrafish causes frontonasal hypoplasia and craniofacial skeleton defects, which become evident in the adult fish. We thus suggest that MOSMO is a candidate to explain uncharacterized forms of human congenital craniofacial malformations, such as those present in the 16p12.1 chromosomal deletion syndrome encompassing the MOSMO locus.

Cell-to-cell heterogeneity in Sox2 and Bra expression guides progenitor motility and destiny

Although cell-to-cell heterogeneity in gene and protein expression within cell populations has been widely documented, we know little about its biological functions. By studying progenitors of the posterior region of bird embryos, we found that expression levels of transcription factors Sox2 and Bra, respectively involved in neural tube (NT) and mesoderm specification, display a high degree of cell-to-cell heterogeneity. By combining forced expression and downregulation approaches with time-lapse imaging, we demonstrate that Sox2-to-Bra ratio guides progenitor's motility and their ability to stay in or exit the progenitor zone to integrate neural or mesodermal tissues. Indeed, high Bra levels confer high motility that pushes cells to join the paraxial mesoderm, while high levels of Sox2 tend to inhibit cell movement forcing cells to integrate the NT. Mathematical modeling captures the importance of cell motility regulation in this process and further suggests that randomness in Sox2/Bra cell-to-cell distribution favors cell rearrangements and tissue shape conservation.

Neurophysiologic implications of neuronal nitric oxide synthase

The molecular and chemical properties of neuronal nitric oxide synthase (nNOS) have made it a key mediator in many physiological functions and signaling transduction. The NOS monomer is inactive, but the dimer form is active. There are three forms of NOS, which are neuronal (nNOS), inducible (iNOS), and endothelial (eNOS) nitric oxide synthase. nNOS regulates nitric oxide (NO) synthesis which is the mechanism used mostly by neurons to produce NO. nNOS expression and activation is regulated by some important signaling proteins, such as cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB), calmodulin (CaM), heat shock protein 90 (HSP90)/HSP70. nNOS-derived NO has been implicated in modulating many physiological functions, such as synaptic plasticity, learning, memory, neurogenesis, etc. In this review, we have summarized recent studies that have characterized structural features, subcellular localization, and factors that regulate nNOS function. Finally, we have discussed the role of nNOS in the developing brain under a wide range of physiological conditions, especially long-term potentiation and depression.

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

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

Nicotine Neurotoxicity Involves Low Wnt1 Signaling in Spinal Locomotor Networks of the Postnatal Rodent Spinal Cord

The postnatal rodent spinal cord in-vitro is a useful model to investigate early pathophysiological changes after injury. While low dose nicotine (1 µM) induces neuroprotection, how higher doses affect spinal networks is unknown. Using spinal preparations of postnatal wild-type Wistar rat and Wnt1Cre2:Rosa26Tom double-transgenic mouse, we studied the effect of nicotine (0.5–10 µM) on locomotor networks in-vitro. Nicotine 10 µM induced motoneuron depolarization, suppressed monosynaptic reflexes, and decreased fictive locomotion in rat spinal cord. Delayed fall in neuronal numbers (including motoneurons) of central and ventral regions emerged without loss of dorsal neurons. Conversely, nicotine (0.5–1 µM) preserved neurons throughout the spinal cord and strongly activated the Wnt1 signaling pathway. High-dose nicotine enhanced expression of S100 and GFAP in astrocytes indicating a stress response. Excitotoxicity induced by kainate was contrasted by nicotine (10 µM) in the dorsal area and persisted in central and ventral regions with no change in basal Wnt signaling. When combining nicotine with kainate, the activation of Wnt1 was reduced compared to kainate/sham. The present results suggest that high dose nicotine was neurotoxic to central and ventral spinal neurons as the neuroprotective role of Wnt signaling became attenuated. This also corroborates the risk of cigarette smoking for the foetus/newborn since tobacco contains nicotine.

Emerging roles for the YAP/TAZ transcriptional regulators in brain tumour pathology and targeting options

The transcriptional co-activators Yes-associated protein 1/transcriptional co-activator with PDZ-binding motif (YAP/TAZ) have emerged as significant regulators of a wide variety of cellular and organ functions with impact in early embryonic development, especially during the expansion of the neural progenitor cell pool. YAP/TAZ signalling regulates organ size development, tissue homeostasis, wound healing and angiogenesis by participating in a complex network of various pathways. However, recent evidence suggests an association of these physiologic regulatory effects of YAP/TAZ with pro-oncogenic activities. Herein, we discuss the physiological functions of YAP/TAZ as well as the extensive network of signalling pathways that control their expression and activity, leading to brain tumour development and progression. Furthermore, we describe current targeting approaches and drug options including direct YAP/TAZ and YAP-TEA domain transcription factor (TEAD) interaction inhibitors, G-protein coupled receptors (GPCR) signalling modulators and kinase inhibitors, which may be used to successfully attack YAP/TAZ-dependent tumours.

Morphological and Functional Changes of Roof Plate Cells in Spinal Cord Development

The most dorsal region, or roof plate, is the dorsal organizing center of developing spinal cord. This region is also involved in development of neural crest cells, which are the source of migratory neural crest cells. During early development of the spinal cord, roof plate cells secrete signaling molecules, such as Wnt and BMP family proteins, which regulate development of neural crest cells and dorsal spinal cord. After the dorso-ventral pattern is established, spinal cord dynamically changes its morphology. With this morphological transformation, the lumen of the spinal cord gradually shrinks to form the central canal, a cavity filled with cerebrospinal fluid that is connected to the ventricular system of the brain. The dorsal half of the spinal cord is separated by a glial structure called the dorsal (or posterior) median septum. However, underlying mechanisms of such morphological transformation are just beginning to be understood. Recent studies reveal that roof plate cells dramatically stretch along the dorso-ventral axis, accompanied by reduction of the spinal cord lumen. During this stretching process, the tips of roof plate cells maintain contact with cells surrounding the shrinking lumen, eventually exposed to the inner surface of the central canal. Interestingly, Wnt expression remains in stretched roof plate cells and activates Wnt/β-catenin signaling in ependymal cells surrounding the central canal. Wnt/β-catenin signaling in ependymal cells promotes proliferation of neural progenitor and stem cells in embryonic and adult spinal cord. In this review, we focus on the role of the roof plate, especially that of Wnt ligands secreted by roof plate cells, in morphological changes occurring in the spinal cord.

Oscillating field stimulation promotes recovery from spinal cord injury in rats by regulating the differentiation of endogenous neural stem cells

The mammalian spinal cord (SC) has a limited self-repair capacity and exogenous treatments are yet to produce substantial functional recovery following SC injury (SCI). The SC contains endogenous neural stem cells (NSCs) with multi-lineage differentiation potential and it may be possible to restore function via interventions that promote NSC differentiation following SCI. Oscillating field stimulation (OFS) has been reported to regulate the Wnt signaling pathway, a known modulator of NSC differentiation. However, the effects of OFS on NSC differentiation following SCI and associated functional recovery have not been previously examined. In the current study, the Basso-Beattie-Bresnahan (BBB) score was used to assess locomotion recovery following SCI in rats and immunofluorescence double-staining was used to examine the regeneration of neurons and oligodendrocytes derived from NSCs. Furthermore, Nissl staining was performed to assess the viability and survival of neurons following SCI, while recovery of the myelin sheath was examined by uranium-lead staining under transmission electron microscopy. OFS delivered via an implanted stimulator enhanced the differentiation of NSCs into neurons and oligodendrocytes and accelerated the regeneration of myelinated axons. Additionally, BBB scores revealed superior locomotion recovery in OFS-treated rats compared with SCI controls. Collectively, these results indicated that OFS may be a feasible strategy to promote SCI recovery by regulating the differentiation of endogenous NSCs.

Sox14 is essential for initiation of neuronal differentiation in the chick spinal cord

BACKGROUND

The neural tube comprises several different types of progenitors and postmitotic neurons that co-ordinately act with each other to play integrated functions. Its development consists of two phases: proliferation of progenitor cells and differentiation into postmitotic neurons. How progenitor cells differentiate into each corresponding neuron is an important question for understanding the mechanisms of neuronal development.

RESULTS

Here we introduce one of the Sox transcription factors, Sox14, which plays an essential role in the promotion of neuronal differentiation. Sox14 belongs to the SoxB2 subclass and its expression starts in the progenitor regions before neuronal differentiation is initiated at the trunk level of the neural tube. After neuronal differentiation is initiated, Sox14 expression gradually becomes confined to the V2a region of the neural tube, where Chx10 is co-expressed. Overexpression of Sox14 restricts progenitor cell proliferation. Conversely, the blockade of Sox14 expression by the RNAi strategy inhibits V2a neuron differentiation and causes expansion of the progenitor domain. We further found that Sox14 acted as a transcriptional activator.

CONCLUSIONS

Sox14 acts as a modulator of cell proliferation and is essential for initiation of neuronal differentiation in the chick neural tube.

Dbnl and β-catenin promote pro-N-cadherin processing to maintain apico-basal polarity

The neural tube forms when neural stem cells arrange into a pseudostratified, single-cell–layered epithelium, with a marked apico-basal polarity, and in which adherens junctions (AJs) concentrate in the subapical domain. We previously reported that sustained β-catenin expression promotes the formation of enlarged apical complexes (ACs), enhancing apico-basal polarity, although the mechanism through which this occurs remained unclear. Here, we show that β-catenin interacts with phosphorylated pro-N-cadherin early in its transit through the Golgi apparatus, promoting propeptide excision and the final maturation of N-cadherin. We describe a new β-catenin–dependent interaction of N-cadherin with Drebrin-like (Dbnl), an actin-binding protein that is involved in anterograde Golgi trafficking of proteins. Notably, Dbnl knockdown led to pro-N-cadherin accumulation and limited AJ formation. In brief, we demonstrate that Dbnl and β-catenin assist in the maturation of pro-N-cadherin, which is critical for AJ formation and for the recruitment AC components like aPKC and, consequently, for the maintenance of apico-basal polarity.

The multi-functional RNA-binding protein G3BP1 and its potential implication in neurodegenerative disease

Ras-GTPase-activating protein (GAP)-binding protein 1 (G3BP1) is a multi-functional protein that is best known for its role in the assembly and dynamics of stress granules. Recent studies have highlighted that G3BP1 also has other functions related to RNA metabolism. In the context of disease, G3BP1 has been therapeutically targeted in cancers because its over-expression is correlated with proliferation of cancerous cells and metastasis. However, evidence suggests that G3BP1 is essential for neuronal development and possibly neuronal maintenance. In this review, we will examine the many functions that are carried out by G3BP1 in the context of neurons and speculate how these functions are critical to the progression of neurodegenerative diseases. Additionally, we will highlight the similarities and differences between G3BP1 and the closely related protein G3BP2, which is frequently overlooked. Although G3BP1 and G3BP2 have both been deemed important for stress granule assembly, their roles may differ in other cellular pathways, some of which are specific to the CNS, and presents an opportunity for further exploration.

Cell intercalation driven by SMAD3 underlies secondary neural tube formation

Body axis elongation is a hallmark of the vertebrate embryo, involving the architectural remodeling of the tail bud. Although it is clear how neuromesodermal progenitors (NMPs) contribute to embryo elongation, the dynamic events that lead to de novo lumen formation and that culminate in the formation of a 3-dimensional, neural tube from NMPs, are poorly understood. Here, we used in vivo imaging of the chicken embryo to show that cell intercalation downstream of TGF-β/SMAD3 signaling is required for secondary neural tube formation. Our analysis describes the events in embryo elongation including lineage restriction, the epithelial-to-mesenchymal transition of NMPs, and the initiation of lumen formation. We show that the resolution of a single, centrally positioned lumen, which occurs through the intercalation of central cells, requires SMAD3/Yes-associated protein (YAP) activity. We anticipate that these findings will be relevant to understand caudal, skin-covered neural tube defects, among the most frequent birth defects detected in humans.

Mutation in the Ciliary Protein C2CD3 Reveals Organ-Specific Mechanisms of Hedgehog Signal Transduction in Avian Embryos

Primary cilia are ubiquitous microtubule-based organelles that serve as signaling hubs for numerous developmental pathways, most notably the Hedgehog (Hh) pathway. Defects in the structure or function of primary cilia result in a class of diseases called ciliopathies. It is well known that primary cilia participate in transducing a Hh signal, and as such ciliopathies frequently present with phenotypes indicative of aberrant Hh function. Interestingly, the exact mechanisms of cilia-dependent Hh signaling transduction are unclear as some ciliopathic animal models simultaneously present with gain-of-Hh phenotypes in one organ system and loss-of-Hh phenotypes in another. To better understand how Hh signaling is perturbed across different tissues in ciliopathic conditions, we examined four distinct Hh-dependent signaling centers in the naturally occurring avian ciliopathic mutant talpid² (ta²). In addition to the well-known and previously reported limb and craniofacial malformations, we observed dorsal-ventral patterning defects in the neural tube, and a shortened gastrointestinal tract. Molecular analyses for elements of the Hh pathway revealed that the loss of cilia impact transduction of an Hh signal in a tissue-specific manner at variable levels of the pathway. These studies will provide increased knowledge into how impaired ciliogenesis differentially regulates Hh signaling across tissues and will provide potential avenues for future targeted therapeutic treatments.

Foxp1 Regulates Neural Stem Cell Self-Renewal and Bias Toward Deep Layer Cortical Fates

The laminar architecture of the mammalian neocortex depends on the orderly generation of distinct neuronal subtypes by apical radial glia (aRG) during embryogenesis. Here, we identify critical roles for the autism risk gene Foxp1 in maintaining aRG identity and gating the temporal competency for deep-layer neurogenesis. Early in development, aRG express high levels of Foxp1 mRNA and protein, which promote self-renewing cell divisions and deep-layer neuron production. Foxp1 levels subsequently decline during the transition to superficial-layer neurogenesis. Sustained Foxp1 expression impedes this transition, preserving a population of cells with aRG identity throughout development and extending the early neurogenic period into postnatal life. FOXP1 expression is further associated with the initial formation and expansion of basal RG (bRG) during human corticogenesis and can promote the formation of cells exhibiting characteristics of bRG when misexpressed in the mouse cortex. Together, these findings reveal broad functions for Foxp1 in cortical neurogenesis.

Expression analysis of chick Frizzled receptors during spinal cord development

Frizzleds (Fzds) are transmembrane receptors that can transduce signals dependent upon binding of Wnts, a large family of secreted glycoproteins homologous to the Drosophila wingless gene. FZDs are critical for a wide variety of normal and pathological developmental processes. In the nervous system, Wnts and Frizzleds play an important role in anterior-posterior patterning, cell fate decisions, proliferation, and synaptogenesis. Here, we preformed a comprehensive expression profile of Wnt receptors (FZD) by using situ hybridization to identify FZDs that are expressed in dorsal-ventral regions of the neural tube development. Our data show specific expression for FZD1,2,3,7,9 and 10 in the chick developing spinal cord. This expression profile of cFZD receptors offers the basis for functional studies in the future to determine roles for the different FZD receptors and their interactions with Wnts during dorsal-ventral neural tube development in vivo. Furthermore, we also show that co-overexpression of Wnt1/3a by in vivo electroporation affects FZD7/10 expression in the neural tube. This illustrates an example of Wnts-FZDs interactions during spinal cord neurogenesis.

Abnormal neocortex arealization and Sotos-like syndrome-associated behavior in Setd2 mutant mice

Proper formation of area identities of the cerebral cortex is crucial for cognitive functions and social behaviors of the brain. It remains largely unknown whether epigenetic mechanisms, including histone methylation, regulate cortical arealization. Here, we removed SETD2, the methyltransferase for histone 3 lysine-36 trimethylation (H3K36me3), in the developing dorsal forebrain in mice and showed that Setd2 is required for proper cortical arealization and the formation of cortico-thalamo-cortical circuits. Moreover, Setd2 conditional knockout mice exhibit defects in social interaction, motor learning, and spatial memory, reminiscent of patients with the Sotos-like syndrome bearing SETD2 mutations. SETD2 maintains the expression of clustered protocadherin (cPcdh) genes in an H3K36me3 methyltransferase-dependent manner. Aberrant cortical arealization was recapitulated in cPcdh heterozygous mice. Together, our study emphasizes epigenetic mechanisms underlying cortical arealization and pathogenesis of the Sotos-like syndrome.

Dorsal commissural axon guidance in the developing spinal cord

Commissural axons have been a key model system for identifying axon guidance signals in vertebrates. This review summarizes the current thinking about the molecular and cellular mechanisms that establish a specific commissural neural circuit: the dI1 neurons in the developing spinal cord. We assess the contribution of long- and short-range signaling while sequentially following the developmental timeline from the birth of dI1 neurons, to the extension of commissural axons first circumferentially and then contralaterally into the ventral funiculus.

Characterization of an eutherian gene cluster generated after transposon domestication identifies Bex3 as relevant for advanced neurological functions

BACKGROUND

One of the most unusual sources of phylogenetically restricted genes is the molecular domestication of transposable elements into a host genome as functional genes. Although these kinds of events are sometimes at the core of key macroevolutionary changes, their origin and organismal function are generally poorly understood.

RESULTS

Here, we identify several previously unreported transposable element domestication events in the human and mouse genomes. Among them, we find a remarkable molecular domestication that gave rise to a multigenic family in placental mammals, the Bex/Tceal gene cluster. These genes, which act as hub proteins within diverse signaling pathways, have been associated with neurological features of human patients carrying genomic microdeletions in chromosome X. The Bex/Tceal genes display neural-enriched patterns and are differentially expressed in human neurological disorders, such as autism and schizophrenia. Two different murine alleles of the cluster member Bex3 display morphological and physiopathological brain modifications, such as reduced interneuron number and hippocampal electrophysiological imbalance, alterations that translate into distinct behavioral phenotypes.

CONCLUSIONS

We provide an in-depth understanding of the emergence of a gene cluster that originated by transposon domestication and gene duplication at the origin of placental mammals, an evolutionary process that transformed a non-functional transposon sequence into novel components of the eutherian genome. These genes were integrated into existing signaling pathways involved in the development, maintenance, and function of the CNS in eutherians. At least one of its members, Bex3, is relevant for higher brain functions in placental mammals and may be involved in human neurological disorders.

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

Yin Yang 1 is critical for mid-hindbrain neuroepithelium development and involved in cerebellar agenesis

The highly conserved and ubiquitously expressed transcription factor Yin Yang 1 (Yy1), was named after its dual functions of both activating and repressing gene transcription. Yy1 plays complex roles in various fundamental biological processes such as the cell cycle progression, cell proliferation, survival, and differentiation. Patients with dominant Yy1 mutations suffer from central nervous system (CNS) developmental defects. However, the role of Yy1 in mammalian CNS development remains to be fully elucidated. The isthmus organizer locates to the mid-hindbrain (MHB) boundary region and serves as the critical signaling center during midbrain and cerebellar early patterning. To study the function of Yy1 in mesencephalon/ rhombomere 1 (mes/r1) neuroepithelium development, we utilized the tissue-specific Cre-LoxP system and generated a conditional knockout mouse line to inactivate Yy1 in the MHB region. Mice with Yy1 deletion in the mes/r1 region displayed cerebellar agenesis and dorsal midbrain hypoplasia. The Yy1 deleted neuroepithelial cells underwent cell cycle arrest and apoptosis, with the concurrent changes of cell cycle regulatory genes expression, as well as activation of the p53 pathway. Moreover, we found that Yy1 is involved in the transcriptional activation of Wnt1 in neural stem cells. Thus, our work demonstrates the involvement of Yy1 in cerebellar agenesis and the critical function of Yy1 in mouse early MHB neuroepithelium maintenance and development.

A SMAD1/5-YAP signalling module drives radial glia self-amplification and growth of the developing cerebral cortex

The growth and evolutionary expansion of the cerebral cortex are defined by the spatial-temporal production of neurons, which itself depends on the decision of radial glial cells (RGCs) to self-amplify or to switch to neurogenic divisions. The mechanisms regulating these RGC fate decisions are still incompletely understood. Here, we describe a novel and evolutionarily conserved role of the canonical BMP transcription factors SMAD1/5 in controlling neurogenesis and growth during corticogenesis. Reducing the expression of both SMAD1 and SMAD5 in neural progenitors at early mouse cortical development caused microcephaly and an increased production of early-born cortical neurons at the expense of late-born ones, which correlated with the premature differentiation and depletion of the pool of cortical progenitors. Gain- and loss-of-function experiments performed during early cortical neurogenesis in the chick revealed that SMAD1/5 activity supports self-amplifying RGC divisions and restrains the neurogenic ones. Furthermore, we demonstrate that SMAD1/5 stimulate RGC self-amplification through the positive post-transcriptional regulation of the Hippo signalling effector YAP. We anticipate this SMAD1/5-YAP signalling module to be fundamental in controlling growth and evolution of the amniote cerebral cortex.