Dorsal-ventral patterning of the neural tube: a tale of three signals

Mouse neural tube organoids self-organize floorplate through BMP-mediated cluster competition

During neural tube (NT) development, the notochord induces an organizer, the floorplate, which secretes Sonic Hedgehog (SHH) to pattern neural progenitors. Conversely, NT organoids (NTOs) from embryonic stem cells (ESCs) spontaneously form floorplates without the notochord, demonstrating that stem cells can self-organize without embryonic inducers. Here, we investigated floorplate self-organization in clonal mouse NTOs. Expression of the floorplate marker FOXA2 was initially spatially scattered before resolving into multiple clusters, which underwent competition and sorting, resulting in a stable "winning" floorplate. We identified that BMP signaling governed long-range cluster competition. FOXA2+ clusters expressed BMP4, suppressing FOXA2 in receiving cells while simultaneously expressing the BMP-inhibitor NOGGIN, promoting cluster persistence. Noggin mutation perturbed floorplate formation in NTOs and in the NT in vivo at mid/hindbrain regions, demonstrating how the floorplate can form autonomously without the notochord. Identifying the pathways governing organizer self-organization is critical for harnessing the developmental plasticity of stem cells in tissue engineering.

Production of Neuroepithelial Organoids from Human-Induced Pluripotent Stem Cells for Mimicking Early Neural Tube Development

Organoids have emerged as robust tools for unravelling the mechanisms that underly tissue development. They also serve as important in vitro systems for studying fundamentals of stem cell behavior and for building advanced disease models. During early development, a crucial step in the formation of the central nervous system is patterning of the neural tube dorsal-ventral (DV) axis. Here we describe a simple and rapid culture protocol to produce human neuroepithelial (NE) cysts and DV-patterned organoids from single human-induced pluripotent stem cells (hiPSCs). Rather than being embedded within a matrix, hiPSCs undergo a 5-day differentiation process in medium containing soluble extracellular matrix and are allowed to self-organize into 3D cysts with defined central lumen structures that express early neuroepithelial markers. Moreover, upon stimulation with sonic hedgehog proteins and all-trans retinoic acid, NE cysts further develop into NE organoids with DV patterning. This rapid generation of patterned NE organoids using simple culture conditions enables mimicking, monitoring, and longitudinal manipulation of NE cell behavior. This straightforward culture system makes NE organoids a tractable model for studying neural stem cell self-organization and early neural tube developmental events.

Decoding the spatiotemporal regulation of transcription factors during human spinal cord development

The spinal cord is a crucial component of the central nervous system that facilitates sensory processing and motor performance. Despite its importance, the spatiotemporal codes underlying human spinal cord development have remained elusive. In this study, we have introduced an image-based single-cell transcription factor (TF) expression decoding spatial transcriptome method (TF-seqFISH) to investigate the spatial expression and regulation of TFs during human spinal cord development. By combining spatial transcriptomic data from TF-seqFISH and single-cell RNA-sequencing data, we uncovered the spatial distribution of neural progenitor cells characterized by combinatorial TFs along the dorsoventral axis, as well as the molecular and spatial features governing neuronal generation, migration, and differentiation along the mediolateral axis. Notably, we observed a sandwich-like organization of excitatory and inhibitory interneurons transiently appearing in the dorsal horns of the developing human spinal cord. In addition, we integrated data from 10× Visium to identify early and late waves of neurogenesis in the dorsal horn, revealing the formation of laminas in the dorsal horns. Our study also illuminated the spatial differences and molecular cues underlying motor neuron (MN) diversification, and the enrichment of Amyotrophic Lateral Sclerosis (ALS) risk genes in MNs and microglia. Interestingly, we detected disease-associated microglia (DAM)-like microglia groups in the developing human spinal cord, which are predicted to be vulnerable to ALS and engaged in the TYROBP causal network and response to unfolded proteins. These findings provide spatiotemporal transcriptomic resources on the developing human spinal cord and potential strategies for spinal cord injury repair and ALS treatment.

Insights into the mechanisms of neuron generation and specification in the zebrafish ventral spinal cord

The vertebrate nervous system is composed of a wide range of neurons and complex synaptic connections, raising the intriguing question of how neuronal diversity is generated. The spinal cord provides an excellent model for exploring the mechanisms governing neuronal diversity due to its simple neural network and the conserved molecular processes involved in neuron formation and specification during evolution. This review specifically examines two distinct progenitor domains present in the zebrafish ventral spinal cord: the lateral floor plate (LFP) and the p2 progenitor domain. The LFP is responsible for the production of GABAergic Kolmer-Agduhr neurons (KA″), glutamatergic V3 neurons, and intraspinal serotonergic neurons, while the p2 domain generates V2 precursors that subsequently differentiate into three unique subpopulations of V2 neurons, namely glutamatergic V2a, GABAergic V2b, and glycinergic V2s. Based on recent findings, we will examine the fundamental signaling pathways and transcription factors that play a key role in the specification of these diverse neurons and neuronal subtypes derived from the LFP and p2 progenitor domains.

Cytoneme signaling provides essential contributions to mammalian tissue patterning

During development, morphogens pattern tissues by instructing cell fate across long distances. Directly visualizing morphogen transport in situ has been inaccessible, so the molecular mechanisms ensuring successful morphogen delivery remain unclear. To tackle this longstanding problem, we developed a mouse model for compromised sonic hedgehog (SHH) morphogen delivery and discovered that endocytic recycling promotes SHH loading into signaling filopodia called cytonemes. We optimized methods to preserve in vivo cytonemes for advanced microscopy and show endogenous SHH localized to cytonemes in developing mouse neural tubes. Depletion of SHH from neural tube cytonemes alters neuronal cell fates and compromises neurodevelopment. Mutation of the filopodial motor myosin 10 (MYO10) reduces cytoneme length and density, which corrupts neuronal signaling activity of both SHH and WNT. Combined, these results demonstrate that cytoneme-based signal transport provides essential contributions to morphogen dispersion during mammalian tissue development and suggest MYO10 is a key regulator of cytoneme function.

Regulation of progenitor cell survival by a novel chromatin remodeling factor during neural tube development

During development, progenitor cell survival is essential for proper tissue functions, but the underlying mechanisms are not fully understood. Here we show that ERCC6L2, a member of the Snf2 family of helicase-like proteins, plays an essential role in the survival of developing chick neural cells. ERCC6L2 expression is induced by the Sonic Hedgehog (Shh) signaling molecule by a mechanism similar to that of the known Shh target genes Ptch1 and Gli1. ERCC6L2 blocks programmed cell death induced by Shh inhibition and this inhibition is independent of neural tube patterning. ERCC6L2 knockdown by siRNA resulted in the aberrant appearance of apoptotic cells. Furthermore, ERCC6L2 cooperates with the Shh signal and plays an essential role in the induction of the anti-apoptotic factor Bcl-2. Taken together, ERCC6L2 acts as a key factor in ensuring the survival of neural progenitor cells.

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.

A phylogenetically-conserved axis of thalamocortical connectivity in the human brain

The thalamus enables key sensory, motor, emotive, and cognitive processes via connections to the cortex. These projection patterns are traditionally considered to originate from discrete thalamic nuclei, however recent work showing gradients of molecular and connectivity features in the thalamus suggests the organisation of thalamocortical connections occurs along a continuous dimension. By performing a joint decomposition of densely sampled gene expression and non-invasive diffusion tractography in the adult human thalamus, we define a principal axis of genetic and connectomic variation along a medial-lateral thalamic gradient. Projections along this axis correspond to an anterior-posterior cortical pattern and are aligned with electrophysiological properties of the cortex. The medial-lateral axis demonstrates phylogenetic conservation, reflects transitions in neuronal subtypes, and shows associations with neurodevelopment and common brain disorders. This study provides evidence for a supra-nuclear axis of thalamocortical organisation characterised by a graded transition in molecular properties and anatomical connectivity.

Accelerated differentiation of human induced pluripotent stem cells into regionally specific dorsal and ventral spinal neural progenitor cells for application in spinal cord therapeutics

Spinal cord injury can attenuate both motor and sensory function with minimal potential for full recovery. Research utilizing human induced pluripotent stem cell (hiPSC) -derived spinal cell types for in vivo remodeling and neuromodulation after spinal cord injury has grown substantially in recent years. However, the majority of protocols for the differentiation of spinal neurons are lengthy, lack the appropriate dorsoventral or rostrocaudal specification, and are not typically replicated in more than one cell line. Furthermore, most researchers currently utilize hiPSC-derived motor neurons for cell transplantation after injury, with very little exploration of spinal sensory neuron transplantation. The lack of studies that utilize sensory populations may be due in part to the relative scarcity of dorsal horn differentiation protocols. Building upon our previously published work that demonstrated the rapid establishment of a primitive ectoderm population from hiPSCs, we describe here the production of a diverse population of both ventral spinal and dorsal horn progenitor cells. Our work creates a novel system allowing dorsal and ventral spinal neurons to be differentiated from the same intermediate ectoderm population, making it possible to construct the dorsal and ventral domains of the spinal cord while decreasing variability. This technology can be used in tandem with biomaterials and pharmacology to improve cell transplantation for spinal cord injury, increasing the potential for neuroregeneration.

Single-cell sequencing of individual retinal organoids reveals determinants of cell-fate heterogeneity

With a critical need for more complete in vitro models of human development and disease, organoids hold immense potential. Their complex cellular composition makes single-cell sequencing of great utility; however, the limitation of current technologies to a handful of treatment conditions restricts their use in screens or studies of organoid heterogeneity. Here, we apply sci-Plex, a single-cell combinatorial indexing (sci)-based RNA sequencing (RNA-seq) multiplexing method to retinal organoids. We demonstrate that sci-Plex and 10× methods produce highly concordant cell-class compositions and then expand sci-Plex to analyze the cell-class composition of 410 organoids upon modulation of critical developmental pathways. Leveraging individual organoid data, we develop a method to measure organoid heterogeneity, and we identify that activation of Wnt signaling early in retinal organoid cultures increases retinal cell classes up to 6 weeks later. Our data show sci-Plex's potential to dramatically scale up the analysis of treatment conditions on relevant human models.

sox1a:eGFP transgenic line and single-cell transcriptomics reveal the origin of zebrafish intraspinal serotonergic neurons

Sox transcription factors are crucial for vertebrate nervous system development. In zebrafish embryo, sox1 genes are expressed in neural progenitor cells and neurons of ventral spinal cord. Our recent study revealed that the loss of sox1a and sox1b function results in a significant decline of V2 subtype neurons (V2s). Using single-cell RNA sequencing, we analyzed the transcriptome of sox1a lineage progenitors and neurons in the zebrafish spinal cord at four time points during embryonic development, employing the Tg(sox1a:eGFP) line. In addition to previously characterized sox1a-expressing neurons, we discovered the expression of sox1a in late-developing intraspinal serotonergic neurons (ISNs). Developmental trajectory analysis suggests that ISNs arise from lateral floor plate (LFP) progenitor cells. Pharmacological inhibition of the Notch signaling pathway revealed its role in negatively regulating LFP progenitor cell differentiation into ISNs. Our findings highlight the zebrafish LFP as a progenitor domain for ISNs, alongside known Kolmer-Agduhr (KA) and V3 interneurons.

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.

Single-cell chromatin accessibility profiling of cell-state-specific gene regulatory programs during mouse organogenesis

In mammals, early organogenesis begins soon after gastrulation, accompanied by specification of various type of progenitor/precusor cells. In order to reveal dynamic chromatin landscape of precursor cells and decipher the underlying molecular mechanism driving early mouse organogenesis, we performed single-cell ATAC-seq of E8.5-E10.5 mouse embryos. We profiled a total of 101,599 single cells and identified 41 specific cell types at these stages. Besides, by performing integrated analysis of scATAC-seq and public scRNA-seq data, we identified the critical cis-regulatory elements and key transcription factors which drving development of spinal cord and somitogenesis. Furthermore, we intersected accessible peaks with human diseases/traits-related loci and found potential clinical associated single nucleotide variants (SNPs). Overall, our work provides a fundamental source for understanding cell fate determination and revealing the underlying mechanism during postimplantation embryonic development, and expand our knowledge of pathology for human developmental malformations.

Single-cell sequencing of individual retinal organoids reveals determinants of cell fate heterogeneity

With a critical need for more complete in vitro models of human development and disease, organoids hold immense potential. Their complex cellular composition makes single-cell sequencing of great utility; however, the limitation of current technologies to a handful of treatment conditions restricts their use in screens or studies of organoid heterogeneity. Here, we apply sci-Plex, a single-cell combinatorial indexing (sci)-based RNA-seq multiplexing method to retinal organoids. We demonstrate that sci-Plex and 10x methods produce highly concordant cell class compositions and then expand sci-Plex to analyze the cell class composition of 410 organoids upon modulation of critical developmental pathways. Leveraging individual organoid data, we develop a method to measure organoid heterogeneity, and we identify that activation of Wnt signaling early in retinal organoid cultures increases retinal cell classes up to six weeks later. Our data show sci-Plex's potential to dramatically scale-up the analysis of treatment conditions on relevant human models.

A single-cell transcriptome atlas profiles early organogenesis in human embryos

The early window of human embryogenesis is largely a black box for developmental biologists. Here we probed the cellular diversity of 4-6 week human embryos when essentially all organs are just laid out. On the basis of over 180,000 single-cell transcriptomes, we generated a comprehensive atlas of 313 clusters in 18 developmental systems, which were annotated with a collection of ontology and markers from 157 publications. Together with spatial transcriptome on embryonic sections, we characterized the molecule and spatial architecture of previously unappreciated cell types. Combined with data from other vertebrates, the rich information shed light on spatial patterning of axes, systemic temporal regulation of developmental progression and potential human-specific regulation. Our study provides a compendium of early progenitor cells of human organs, which can serve as the root of lineage analysis in organogenesis.

Emerging mitochondrial-mediated mechanisms involved in oligodendrocyte development

Oligodendrocytes are the myelinating glia of the central nervous system and are generated after oligodendrocyte progenitor cells (OPCs) transition into pre-oligodendrocytes and then into myelinating oligodendrocytes. Myelin is essential for proper signal transmission within the nervous system and axonal metabolic support. Although the intrinsic and extrinsic factors that support the differentiation, survival, integration, and subsequent myelination of appropriate axons have been well investigated, little is known about how mitochondria-related pathways such as mitochondrial dynamics, bioenergetics, and apoptosis finely tune these developmental events. Previous findings suggest that changes to mitochondrial morphology act as an upstream regulatory mechanism of neural stem cell (NSC) fate decisions. Whether a similar mechanism is engaged during OPC differentiation has yet to be elucidated. Maintenance of mitochondrial dynamics is vital for regulating cellular bioenergetics, functional mitochondrial networks, and the ability of cells to distribute mitochondria to subcellular locations, such as the growing processes of oligodendrocytes. Myelination is an energy-consuming event, thus, understanding the interplay between mitochondrial dynamics, metabolism, and apoptosis will provide further insight into mechanisms that mediate oligodendrocyte development in healthy and disease states. Here we will provide a concise overview of oligodendrocyte development and discuss the potential contribution of mitochondrial mitochondrial-mediated mechanisms to oligodendrocyte bioenergetics and development.

3D Interfacial and Spatiotemporal Regulation of Human Neuroepithelial Organoids

Neuroepithelial (NE) organoids with dorsal-ventral patterning provide a useful three-dimensional (3D) in vitro model to interrogate neural tube formation during early development of the central nervous system. Understanding the fundamental processes behind the cellular self-organization in NE organoids holds the key to the engineering of organoids with higher, more in vivo-like complexity. However, little is known about the cellular regulation driving the NE development, especially in the presence of interfacial cues from the microenvironment. Here a simple 3D culture system that allows generation and manipulation of NE organoids from human-induced pluripotent stem cells (hiPSCs), displaying developmental phases of hiPSC differentiation and self-aggregation, first into NE cysts with lumen structure and then toward NE organoids with floor-plate patterning, is established. Longitudinal inhibition reveals distinct and dynamic roles of actomyosin contractility and yes-associated protein (YAP) signaling in governing these phases. By growing NE organoids on culture chips containing anisotropic surfaces or confining microniches, it is further demonstrated that interfacial cues can sensitively exert dimension-dependent influence on luminal cyst and organoid morphology, successful floor-plate patterning, as well as cytoskeletal regulation and YAP activity. This study therefore sheds new light on how organoid and tissue architecture can be steered through intracellular and extracellular means.

Temporal cell fate determination in the spinal cord is mediated by the duration of Notch signalling

During neural development, progenitor cells generate different types of neurons in specific time windows. Despite the characterisation of many of the transcription factor networks involved in these differentiation events, the mechanism behind their temporal regulation is poorly understood. To address this question, we studied the temporal differentiation of the simple lateral floor plate (LFP) domain in the zebrafish spinal cord. LFP progenitors generate both early-born Kolmer-Agduhr" (KA") interneuron and late-born V3 interneuron populations. Analysis using a Notch signalling reporter demonstrates that these cell populations have distinct Notch signalling profiles. Not only do V3 progenitors receive higher total levels of Notch response, but they collect this response over a longer duration compared to KA" progenitors. To test whether the duration of Notch signalling determines the temporal cell fate specification, we combined a transgene that constitutively activates Notch signalling in the ventral spinal cord with a heat shock inducible Notch signalling terminator to switch off Notch response at any given time. Sustained Notch signalling results in expanded LFP progenitors while KA" and V3 interneurons fail to specify. Early termination of Notch signalling leads to exclusively KA" cell fate, despite the high level of Notch signalling, whereas late attenuation of Notch signalling drives only V3 cell fate. This suggests that the duration of Notch signalling, not simply the level, mediates cell fate specification. Interestingly, knockdown experiments reveal a role for the Notch ligand Jag2b in maintaining LFP progenitors and limiting their differentiation into KA" and V3 interneurons. Our results indicate that Notch signalling is required for neural progenitor maintenance while a specific attenuation timetable defines the fate of the postmitotic progeny.

A Wnt/β-catenin signaling pathway is involved in early dopaminergic differentiation of trabecular meshwork-derived mesenchymal stem cells

Permanent degeneration and loss of dopaminergic (DA) neurons in substantia nigra is the main cause of Parkinson's disease. Considering the therapeutic application of stem cells in neurodegeneration, we sought to examine the neurogenic differentiation potential of the newly introduced neural crest originated mesenchymal stem cells (MSCs), namely, trabecular meshwork-derived mesenchymal stem cells (TM-MSCs) compared to two other sources of MSCs, adipose tissue-derived stem cells (ADSCs) and bone marrow-derived mesenchymal stem cells (BM-MSCs). The three types of cells were therefore cultured in the presence and absence of a neural induction medium followed by the analysis of their differentiation potentials. Our results showed that TM-MSCs exhibited enhanced neural morphologies as well as higher expressions of MAP2 as the general neuron marker and Nurr-1 as an early DA marker compared to the adipose tissue-derived mesenchymal stem cells (AD-MSCs) and bone marrow-derived stem cells (BMSCs). Also, analysis of Nurr-1 immunostaining showed more intense Nurr-1 stained nuclei in the neurally induced TM-MSCs compared to those in the AD-MSCs, BMSCs, and noninduced control TM-MSCs. To examine if Wnt/beta-catenin pathway drives TM-MSCs towards a DA fate, we treated them with the Wnt agonist (CHIR, 3 μM) and the Wnt antagonist (IWP-2, 3 μM). Our results showed that the expressions of Nurr-1 and MAP2, as well as the Wnt/beta-catenin target genes, c-Myc and Cyclin D1, were significantly increased in the CHIR-treated TM-MSCs, but significantly reduced in those treated with IWP-2. Altogether, we declare first a higher neural potency of TM-MSCs compared to the more commonly used MSCs, BMSCs and ADSCs, and second that Wnt/beta-catenin activation directs the neurally induced TM-MSCs towards a DA fate.

Rnf220 is Implicated in the Dorsoventral Patterning of the Hindbrain Neural Tube in Mice

Rnf220 is reported to regulate the patterning of the ventral spinal neural tube in mice. The brainstem has divergent connections with peripheral and central targets and contains unique internal neuronal groups, but the role of Rnf220 in the early development of the hindbrain has not been explored. In this study, Nestin-Cre-mediated conditional knockout (Rnf220^Nestin CKO) mice were used to examine if Rnf220 is involved in the early morphogenesis of the hindbrain. Rnf220 showed restricted expression in the ventral half of ventricular zone (VZ) of the hindbrain at embryonic day (E) 10.5, and as development progressed, Rnf220-expressing cells were also present in the mantle zone outside the VZ at E12.5. In Rnf220^Nestin CKO embryos, alterations of progenitor domains in the ventral VZ were observed at E10.5. There were significant reductions of the p1 and p2 domains shown by expression of Dbx1, Olig2, and Nkx6.1, accompanied by a ventral expansion of the Dbx1⁺ p0 domain and a dorsal expansion of the Nkx2.2⁺ p3 domain. Different from the case in the spinal cord, the Olig2⁺ pMN (progenitors of somatic motor neuron) domain shifted and expanded dorsally. Notably, the total range of the ventral VZ and the extent of the dorsal tube were unchanged. In addition, the post-mitotic cells derived from their corresponding progenitor domain, including oligodendrocyte precursor cells (OPCs) and serotonergic neurons (5-HTNs), were also changed in the same trend as the progenitor domains do in the CKO embryos at E12.5. In summary, our data suggest similar functions of Rnf220 in the hindbrain dorsoventral (DV) patterning as in the spinal cord with different effects on the pMN domain. Our work also reveals novel roles of Rnf220 in the development of 5-HTNs and OPCs.

BuMPing Into Neurogenesis: How the Canonical BMP Pathway Regulates Neural Stem Cell Divisions Throughout Space and Time

Bone morphogenetic proteins (BMPs) are secreted factors that contribute to many aspects of the formation of the vertebrate central nervous system (CNS), from the initial shaping of the neural primordium to the maturation of the brain and spinal cord. In particular, the canonical (SMAD1/5/8-dependent) BMP pathway appears to play a key role during neurogenesis, its activity dictating neural stem cell fate decisions and thereby regulating the growth and homeostasis of the CNS. In this mini-review, I summarize accumulating evidence demonstrating how the canonical BMP activity promotes the amplification and/or maintenance of neural stem cells at different times and in diverse regions of the vertebrate CNS, and highlight findings suggesting that this function is evolutionarily conserved.

N-Cadherin adhesive ligation regulates mechanosensitive neural stem cell lineage commitment in 3D matrices

We demonstrate a novel strategy to independently control cell–ECM and cell–cell interactions of neural stem cells in a 3D matrix.

Emerging Methods in Modeling Brain Development and Disease with Human Pluripotent Stem Cells

The Nobel Prize-winning discovery that human somatic cells can be readily reprogrammed into pluripotent cells has revolutionized our potential to understand the human brain. The rapid technological progression of this field has made it possible to easily obtain human neural cells and even intact tissues, offering invaluable resources to model human brain development. In this chapter, we present a brief history of hPSC-based approaches to study brain development and then, provide new insights into neurological diseases, focusing on those driven by aberrant cell death. Furthermore, we will shed light on the latest technologies and highlight the methods that researchers can use to employ established hPSC approaches in their research. Our intention is to demonstrate that hPSC-based modeling is a technical approach accessible to all researchers who seek a deeper understanding of the human brain.

Wnt1 Lineage Specific Deletion of Gpr161 Results in Embryonic Midbrain Malformation and Failure of Craniofacial Skeletal Development

Sonic hedgehog (Shh) signaling regulates multiple morphogenetic processes during embryonic neurogenesis and craniofacial skeletal development. Gpr161 is a known negative regulator of Shh signaling. Nullizygous Gpr161 mice are embryonic lethal, presenting with structural defects involving the neural tube and the craniofacies. However, the lineage specific role of Gpr161 in later embryonic development has not been thoroughly investigated. We studied the Wnt1-Cre lineage specific role of Gpr161 during mouse embryonic development. We observed three major gross morphological phenotypes in Gpr161 cKO (Gpr161 f/f; Wnt1-Cre) fetuses; protrusive tectum defect, encephalocele, and craniofacial skeletal defect. The overall midbrain tissues were expanded and cell proliferation in ventricular zones of midbrain was increased in Gpr161 cKO fetuses, suggesting that protrusive tectal defects in Gpr161 cKO are secondary to the increased proliferation of midbrain neural progenitor cells. Shh signaling activity as well as upstream Wnt signaling activity were increased in midbrain tissues of Gpr161 cKO fetuses. RNA sequencing further suggested that genes in the Shh, Wnt, Fgf and Notch signaling pathways were differentially regulated in the midbrain of Gpr161 cKO fetuses. Finally, we determined that cranial neural crest derived craniofacial bone formation was significantly inhibited in Gpr161 cKO fetuses, which partly explains the development of encephalocele. Our results suggest that Gpr161 plays a distinct role in midbrain development and in the formation of the craniofacial skeleton during mouse embryogenesis.

A shared transcriptional code orchestrates temporal patterning of the central nervous system

The molecular mechanisms that produce the full array of neuronal subtypes in the vertebrate nervous system are incompletely understood. Here, we provide evidence of a global temporal patterning program comprising sets of transcription factors that stratifies neurons based on the developmental time at which they are generated. This transcriptional code acts throughout the central nervous system, in parallel to spatial patterning, thereby increasing the diversity of neurons generated along the neuraxis. We further demonstrate that this temporal program operates in stem cell-derived neurons and is under the control of the TGFβ signaling pathway. Targeted perturbation of components of the temporal program, Nfia and Nfib, reveals their functional requirement for the generation of late-born neuronal subtypes. Together, our results provide evidence for the existence of a previously unappreciated global temporal transcriptional program of neuronal subtype identity and suggest that the integration of spatial and temporal patterning mechanisms diversifies and organizes neuronal subtypes in the vertebrate nervous system.

Differentiation of Human Pluripotent Stem Cells Into Specific Neural Lineages

Human pluripotent stem cells (hPSCs) are sources of several somatic cell types for human developmental studies, in vitro disease modeling, and cell transplantation therapy. Improving strategies of derivation of high-purity specific neural and glial lineages from hPSCs is critical for application to the study and therapy of the nervous system. Here, we will focus on the principles behind establishment of neuron and glia differentiation methods according to developmental studies. We will also highlight the limitations and challenges associated with the differentiation of several “difficult” neural lineages and delay in neuronal maturation and functional integration. To overcome these challenges, we will introduce strategies and novel technologies aimed at improving the differentiation of various neural lineages to expand the application potential of hPSCs to the study of the nervous system.

Introducing dorsoventral patterning in adult regenerating lizard tails with gene-edited embryonic neural stem cells

Lizards regenerate amputated tails but fail to recapitulate the dorsoventral patterning achieved during embryonic development. Regenerated lizard tails form ependymal tubes (ETs) that, like embryonic tail neural tubes (NTs), induce cartilage differentiation in surrounding cells via sonic hedgehog (Shh) signaling. However, adult ETs lack characteristically roof plate-associated structures and express Shh throughout their circumferences, resulting in the formation of unpatterned cartilage tubes. Both NTs and ETs contain neural stem cells (NSCs), but only embryonic NSC populations differentiate into roof plate identities when protected from endogenous Hedgehog signaling. NSCs were isolated from parthenogenetic lizard embryos, rendered unresponsive to Hedgehog signaling via CRISPR/Cas9 gene knockout of smoothened (Smo), and implanted back into clonally-identical adults to regulate tail regeneration. Here we report that Smo knockout embryonic NSCs oppose cartilage formation when engrafted to adult ETs, representing an important milestone in the creation of regenerated lizard tails with dorsoventrally patterned skeletal tissues.

Organisms with regenerative capacity typically regrow organs with correct axial patterning, however, regrown lizard tails lack this feature. Here the authors used neural stem cells to induce patterning in regenerating lizard tails and rescued normal skeletal morphology.

Deciphering and reconstitution of positional information in the human brain development

Organoid has become a novel in vitro model to research human development and relevant disorders in recent years. With many improvements on the culture protocols, current brain organoids could self-organize into a complicated three-dimensional organization that mimics most of the features of the real human brain at the molecular, cellular, and further physiological level. However, lacking positional information, an important characteristic conveyed by gradients of signaling molecules called morphogens, leads to the deficiency of spatiotemporally regulated cell arrangements and cell–cell interactions in the brain organoid development. In this review, we will overview the role of morphogen both in the vertebrate neural development in vivo as well as the brain organoid culture in vitro, the strategies to apply morphogen concentration gradients in the organoid system and future perspectives of the brain organoid technology.

Pharmacological Manipulation of Wnt/β-Catenin Signaling Pathway in Human Neural Precursor Cells Alters Their Differentiation Potential and Neuronal Yield

The canonical Wnt/β-catenin pathway is a master-regulator of cell fate during embryonic and adult neurogenesis and is therefore a major pharmacological target in basic and clinical research. Chemical manipulation of Wnt signaling during in vitro neuronal differentiation of stem cells can alter both the quantity and the quality of the derived neurons. Accordingly, the use of Wnt activators and blockers has become an integral part of differentiation protocols applied to stem cells in recent years. Here, we investigated the effects of the glycogen synthase kinase-3β inhibitor CHIR99021, which upregulates β-catenin agonizing Wnt; and the tankyrase-1/2 inhibitor XAV939, which downregulates β-catenin antagonizing Wnt. Both drugs and their potential neurogenic and anti-neurogenic effects were studied using stable lines human neural precursor cells (hNPCs), derived from embryonic stem cells, which can be induced to generate mature neurons by chemically-defined conditions. We found that Wnt-agonism by CHIR99021 promotes induction of neural differentiation, while also reducing cell proliferation and survival. This effect was not synergistic with those of pro-neural growth factors during long-term neuronal differentiation. Conversely, antagonism of Wnt by XAV939 consistently prevented neuronal progression of hNPCs. We show here how these two drugs can be used to manipulate cell fate and how self-renewing hNPCs can be used as reliable human in vitro drug-screening platforms.

Towards a physical understanding of developmental patterning

The temporal coordination of events at cellular and tissue scales is essential for the proper development of organisms, and involves cell-intrinsic processes that can be coupled by local cellular signalling and instructed by global signalling, thereby creating spatial patterns of cellular states that change over time. The timing and structure of these patterns determine how an organism develops. Traditional developmental genetic methods have revealed the complex molecular circuits regulating these processes but are limited in their ability to predict and understand the emergent spatio-temporal dynamics. Increasingly, approaches from physics are now being used to help capture the dynamics of the system by providing simplified, generic descriptions. Combined with advances in imaging and computational power, such approaches aim to provide insight into timing and patterning in developing systems.

Induction of the CD24 Surface Antigen in Primary Undifferentiated Human Adipose Progenitor Cells by the Hedgehog Signaling Pathway

In the murine model system of adipogenesis, the CD24 cell surface protein represents a valuable marker to label undifferentiated adipose progenitor cells. Indeed, when injected into the residual fat pads of lipodystrophic mice, these CD24 positive cells reconstitute a normal white adipose tissue (WAT) depot. Unluckily, similar studies in humans are rare and incomplete. This is because it is impossible to obtain large numbers of primary CD24 positive human adipose stem cells (hASCs). This study shows that primary hASCs start to express the glycosylphosphatidylinositol (GPI)-anchored CD24 protein when cultured with a chemically defined medium supplemented with molecules that activate the Hedgehog (Hh) signaling pathway. Therefore, this in vitro system may help understand the biology and role in adipogenesis of the CD24-positive hASCs. The induced cells’ phenotype was studied by flow cytometry, Real-Time Quantitative Polymerase Chain Reaction (RT-qPCR) techniques, and their secretion profile. The results show that CD24 positive cells are early undifferentiated progenitors expressing molecules related to the angiogenic pathway.

Cerebellar Differentiation from Human Stem Cells Through Retinoid, Wnt, and Sonic Hedgehog Pathways

Differentiating cerebellar organoids can be challenging due to complex cell organization and structure in the cerebellum. Different approaches were investigated to recapitulate differentiation process of the cerebellum from human-induced pluripotent stem cells (hiPSCs) without high efficiency. This study was carried out to test the hypothesis that the combination of different signaling factors including retinoic acid (RA), Wnt activator, and sonic hedgehog (SHH) activator promotes the cerebellar differentiation of hiPSCs. Wnt, RA, and SHH pathways were activated by CHIR99021 (CHIR), RA, and purmorphamine (PMR), respectively. Different combinations of the morphogens (RA/CHIR, RA/PMR, CHIR/PMR, and RA/CHIR/PMR) were utilized, and the spheroids (day 35) were characterized for the markers of three cerebellum layers (the molecular layer, the Purkinje cell layer, and the granule cell layer). Of all the combinations tested, RA/CHIR/PMR promoted both the Purkinje cell layer and the granule cell layer differentiation. The cells also exhibited electrophysiological characteristics using whole-cell patch clamp recording, especially demonstrating Purkinje cell electrophysiology. This study should advance the understanding of different signaling pathways during cerebellar development to engineer cerebellum organoids for drug screening and disease modeling. Impact statement This study investigated the synergistic effects of retinoic acid, Wnt activator, and sonic hedgehog activator on cerebellar patterning of human-induced pluripotent stem cell (hiPSC) spheroids and organoids. The results indicate that the combination promotes the differentiation of the Purkinje cell layer and the granule cell layer. The cells also exhibit electrophysiological characteristics using whole-cell patch clamp recording, especially demonstrating Purkinje cell electrophysiology. The findings are significant for understanding the biochemical signaling of three-dimensional microenvironment on neural patterning of hiPSCs for applications in organoid engineering, disease modeling, and drug screening.

Neural specification, targeting, and circuit formation during visual system assembly

Like other sensory systems, the visual system is topographically organized: Its sensory neurons, the photoreceptors, and their targets maintain point-to-point correspondence in physical space, forming a retinotopic map. The iterative wiring of circuits in the visual system conveniently facilitates the study of its development. Over the past few decades, experiments in Drosophila have shed light on the principles that guide the specification and connectivity of visual system neurons. In this review, we describe the main findings unearthed by the study of the Drosophila visual system and compare them with similar events in mammals. We focus on how temporal and spatial patterning generates diverse cell types, how guidance molecules distribute the axons and dendrites of neurons within the correct target regions, how vertebrates and invertebrates generate their retinotopic map, and the molecules and mechanisms required for neuronal migration. We suggest that basic principles used to wire the fly visual system are broadly applicable to other systems and highlight its importance as a model to study nervous system development.

Modeling Neurological Disorders in 3D Organoids Using Human-Derived Pluripotent Stem Cells

Modeling neurological disorders is challenging because they often have both endogenous and exogenous causes. Brain organoids consist of three-dimensional (3D) self-organizing brain tissue which increasingly is being used to model various aspects of brain development and disorders, such as the generation of neurons, neuronal migration, and functional networks. These organoids have been recognized as important in vitro tools to model developmental features of the brain, including neurological disorders, which can provide insights into the molecular mechanisms involved in those disorders. In this review, we describe recent advances in the generation of two-dimensional (2D), 3D, and blood-brain barrier models that were derived from induced pluripotent stem cells (iPSCs) and we discuss their advantages and limitations in modeling diseases, as well as explore the development of a vascularized and functional 3D model of brain processes. This review also examines the applications of brain organoids for modeling major neurodegenerative diseases and neurodevelopmental disorders.

Notch signaling is a critical initiator of roof plate formation as revealed by the use of RNA profiling of the dorsal neural tube

Background

The dorsal domain of the neural tube is an excellent model to investigate the generation of complexity during embryonic development. It is a highly dynamic and multifaceted region being first transiently populated by prospective neural crest (NC) cells that sequentially emigrate to generate most of the peripheral nervous system. Subsequently, it becomes the definitive roof plate (RP) of the central nervous system. The RP, in turn, constitutes a patterning center for dorsal interneuron development. The factors underlying establishment of the definitive RP and its segregation from NC and dorsal interneurons are currently unknown.

Results

We performed a transcriptome analysis at trunk levels of quail embryos comparing the dorsal neural tube at premigratory NC and RP stages. This unraveled molecular heterogeneity between NC and RP stages, and within the RP itself. By implementing these genes, we asked whether Notch signaling is involved in RP development. First, we observed that Notch is active at the RP-interneuron interface. Furthermore, gain and loss of Notch function in quail and mouse embryos, respectively, revealed no effect on early NC behavior. Constitutive Notch activation caused a local downregulation of RP markers with a concomitant development of dI1 interneurons, as well as an ectopic upregulation of RP markers in the interneuron domain. Reciprocally, in mice lacking Notch activity, both the RP and dI1 interneurons failed to form and this was associated with expansion of the dI2 population.

Conclusions

Collectively, our results offer a new resource for defining specific cell types, and provide evidence that Notch is required to establish the definitive RP, and to determine the choice between RP and interneuron fates, but not the segregation of RP from NC.

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.

HoxB genes regulate neuronal delamination in the trunk neural tube by controlling the expression of Lzts1

Differential Hox gene expression is central for specification of axial neuronal diversity in the spinal cord. Here, we uncover an additional function of Hox proteins in the developing spinal cord, restricted to B cluster Hox genes. We found that members of the HoxB cluster are expressed in the trunk neural tube of chicken embryo earlier than Hox from the other clusters, with poor antero-posterior axial specificity and with overlapping expression in the intermediate zone (IZ). Gain-of-function experiments of HoxB4, HoxB8 and HoxB9, respectively, representative of anterior, central and posterior HoxB genes, resulted in ectopic progenitor cells in the mantle zone. The search for HoxB8 downstream targets in the early neural tube identified the leucine zipper tumor suppressor 1 gene (Lzts1), the expression of which is also activated by HoxB4 and HoxB9. Gain- and loss-of-function experiments showed that Lzts1, which is expressed endogenously in the IZ, controls neuronal delamination. These data collectively indicate that HoxB genes have a generic function in the developing spinal cord, controlling the expression of Lzts1 and neuronal delamination.

Congenital Malformations in Sea Turtles: Puzzling Interplay between Genes and Environment

Simple Summary

Congenital malformations can lead to embryonic mortality in many species, and sea turtles are no exception. Genetic and/or environmental alterations occur during early development in the embryo, and may produce aberrant phenotypes, many of which are incompatible with life. Causes of malformations are multifactorial; genetic factors may include mutations, chromosomal aberrations, and inbreeding effects, whereas non-genetic factors may include nutrition, hyperthermia, low moisture, radiation, and contamination. It is possible to monitor and control some of these factors (such as temperature and humidity) in nesting beaches, and toxic compounds in feeding areas, which can be transferred to the embryo through their lipophilic properties. In this review, we describe possible causes of different types of malformations observed in sea turtle embryos, as well as some actions that may help reduce embryonic mortality.

Abstract

The completion of embryonic development depends, in part, on the interplay between genetic factors and environmental conditions, and any alteration during development may affect embryonic genetic and epigenetic regulatory pathways leading to congenital malformations, which are mostly incompatible with life. Oviparous reptiles, such as sea turtles, that produce numerous eggs in a clutch that is buried on the beach provide an opportunity to study embryonic mortality associated with malformations that occur at different times during development, or that prevent the hatchling from emerging from the nest. In sea turtles, the presence of congenital malformations frequently leads to mortality. A few years ago, a detailed study was performed on external congenital malformations in three species of sea turtles from the Mexican Pacific and Caribbean coasts, the hawksbill turtle, Eretmochelys imbricata (n = 23,559 eggs), the green turtle, Chelonia mydas (n = 17,690 eggs), and the olive ridley, Lepidochelys olivacea (n = 20,257 eggs), finding 63 types of congenital malformations, of which 38 were new reports. Of the three species, the olive ridley showed a higher incidence of severe anomalies in the craniofacial region (49%), indicating alterations of early developmental pathways; however, several malformations were also observed in the body, including defects in the carapace (45%) and limbs (33%), as well as pigmentation disorders (20%), indicating that deviations occurred during the middle and later stages of development. Although intrinsic factors (i.e., genetic mutations or epigenetic modifications) are difficult to monitor in the field, some environmental factors (such as the incubation temperature, humidity, and probably the status of feeding areas) are, to some extent, less difficult to monitor and/or control. In this review, we describe the aetiology of different malformations observed in sea turtle embryos, and provide some actions that can reduce embryonic mortality.

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.

Complex crosstalk of Notch and Hedgehog signalling during the development of the central nervous system

The development of the vertebrate central nervous system (CNS) is tightly regulated by many highly conserved cell signalling pathways. These pathways ensure that differentiation and migration events occur in a specific and spatiotemporally restricted manner. Two of these pathways, Notch and Hedgehog (Hh) signalling, have been shown to form a complex web of interaction throughout different stages of CNS development. Strikingly, some processes employ Notch signalling to regulate Hh response, while others utilise Hh signalling to modulate Notch response. Notch signalling functions upstream of Hh response through controlling the trafficking of integral pathway components as well as through modulating protein levels and transcription of downstream transcriptional factors. In contrast, Hh signalling regulates Notch response by either indirectly controlling expression of key Notch ligands and regulatory proteins or directly through transcriptional control of canonical Notch target genes. Here, we review these interactions and demonstrate the level of interconnectivity between the pathways, highlighting context-dependent modes of crosstalk. Since many other developmental signalling pathways are active in these tissues, it is likely that the interplay between Notch and Hh signalling is not only an example of signalling crosstalk but also functions as a component of a wider, multi-pathway signalling network.

Diversification of molecularly defined myenteric neuron classes revealed by single-cell RNA sequencing

Autonomous regulation of the intestine requires the combined activity of functionally distinct neurons of the enteric nervous system (ENS). However, the variety of enteric neuron types and how they emerge during development remain largely unknown. Here, we define a molecular taxonomy of twelve enteric neuron classes within the myenteric plexus of the mouse small intestine using single cell RNA-sequencing. We present cell-cell communication features, histochemical markers for motor, sensory, and interneurons together with transgenic tools for class-specific targeting. Transcriptome analysis of embryonic ENS uncovers a novel principle of neuronal diversification, where two neuron classes arise through a binary neurogenic branching, and all other identities emerge through subsequent post-mitotic differentiation. We identify generic and class-specific transcriptional regulators and functionally connect Pbx3 to a post-mitotic fate transition. Our results offer a conceptual and molecular resource for dissecting ENS circuits, and predicting key regulators for directed differentiation of distinct enteric neuron classes.

High-throughput 3D screening for differentiation of hPSC-derived cell therapy candidates

The emergence of several cell therapy candidates in the clinic is an encouraging sign for human diseases/disorders that currently have no effective treatment; however, scalable production of these cell therapies has become a bottleneck. To overcome this barrier, three-dimensional (3D) cell culture strategies have been considered for enhanced cell production. Here, we demonstrate a high-throughput 3D culture platform used to systematically screen 1200 culture conditions with varying doses, durations, dynamics, and combinations of signaling cues to derive oligodendrocyte progenitor cells and midbrain dopaminergic neurons from human pluripotent stem cells (hPSCs). Statistical models of the robust dataset reveal previously unidentified patterns about cell competence to Wnt, retinoic acid, and sonic hedgehog signals, and their interactions, which may offer insights into the combinatorial roles these signals play in human central nervous system development. These insights can be harnessed to optimize production of hPSC-derived cell replacement therapies for a range of neurological indications.

Brain organoids: Human 3D models to investigate neuronal circuits assembly, function and dysfunction

The human brain is characterized by an extraordinary complexity of neuronal and nonneuronal cell types, wired together into patterned neuronal circuits, which represent the anatomical substrates for the execution of high-order cognitive functions. Brain circuits' development and function is metabolically supported by an intricate network of selectively permeable blood vessels and finely tuned by short-range interactions with immune factors and immune cells. The coordinated cellular and molecular events governing the assembly of this unique and complex structure are at the core of intense investigation and pose legitimate questions about the best modeling strategies. Unceasing advancements in stem cell technologies coupled with recent demonstration of cell self-assembly capacity have enabled the exponential growth of brain organoid protocols in the past decade. This provides a compelling solution to investigate human brain development, a quest often halted by the inaccessibility of brain tissues and the lack of suitable models. We review the current state-of-the-art on the generation of brain organoids, describing the latest progresses in unguided, guided, and assembloids protocols, as well as organoid-on-a-chip strategies and xenograft approaches. High resolution genome wide sequencing technologies, both at the transcriptional and epigenomic level, enable the molecular comparative analysis of multiple brain organoid protocols, as well as to benchmark them against the human fetal brain. Coupling the molecular profiling with increasingly detailed analyses of the electrophysiological properties of several of these systems now allows a more accurate estimation of the protocol of choice for a given biological question. Thus, we summarize strengths and weaknesses of several brain organoid protocols and further speculate on some potential future endeavors to model human brain development, evolution and neurodevelopmental and neuropsychiatric diseases.

FZD10 regulates cell proliferation and mediates Wnt1 induced neurogenesis in the developing spinal cord

Wnt/FZD signalling activity is required for spinal cord development, including the dorsal-ventral patterning of the neural tube, where it affects proliferation and specification of neurons. Wnt ligands initiate canonical, β -catenin-dependent, signaling by binding to Frizzled receptors. However, in many developmental contexts the cognate FZD receptor for a particular Wnt ligand remains to be identified. Here, we characterized FZD10 expression in the dorsal neural tube where it overlaps with both Wnt1 and Wnt3a, as well as markers of dorsal progenitors and interneurons. We show FZD10 expression is sensitive to Wnt1, but not Wnt3a expression, and FZD10 plays a role in neural tube patterning. Knockdown approaches show that Wnt1 induced ventral expansion of dorsal neural markes, Pax6 and Pax7, requires FZD10. In contrast, Wnt3a induced dorsalization of the neural tube is not affected by FZD10 knockdown. Gain of function experiments show that FZD10 is not sufficient on its own to mediate Wnt1 activity in vivo. Indeed excess FZD10 inhibits the dorsalizing activity of Wnt1. However, addition of the Lrp6 co-receptor dramatically enhances the Wnt1/FZD10 mediated activation of dorsal markers. This suggests that the mechanism by which Wnt1 regulates proliferation and patterning in the neural tube requires both FZD10 and Lrp6.

The Role of Tissue-Specific Ubiquitin Ligases, RNF183, RNF186, RNF182 and RNF152, in Disease and Biological Function

Ubiquitylation plays multiple roles not only in proteasome-mediated protein degradation but also in various other cellular processes including DNA repair, signal transduction, and endocytosis. Ubiquitylation is mediated by ubiquitin ligases, which are predicted to be encoded by more than 600 genes in humans. RING finger (RNF) proteins form the majority of these ubiquitin ligases. It has also been predicted that there are 49 RNF proteins containing transmembrane regions in humans, several of which are specifically localized to membrane compartments in the secretory and endocytic pathways. Of these, RNF183, RNF186, RNF182, and RNF152 are closely related genes with high homology. These genes share a unique common feature of exhibiting tissue-specific expression patterns, such as in the kidney, nervous system, and colon. The products of these genes are also reported to be involved in various diseases such as cancers, inflammatory bowel disease, Alzheimer's disease, and chronic kidney disease, and in various biological functions such as apoptosis, endoplasmic reticulum stress, osmotic stress, nuclear factor-kappa B (NF-κB), mammalian target of rapamycin (mTOR), and Notch signaling. This review summarizes the current knowledge of these tissue-specific ubiquitin ligases, focusing on their physiological roles and significance in diseases.

Induced Pluripotent Stem Cell (iPSC)-Based Neurodegenerative Disease Models for Phenotype Recapitulation and Drug Screening

Neurodegenerative diseases represent a significant unmet medical need in our aging society. There are no effective treatments for most of these diseases, and we know comparatively little regarding pathogenic mechanisms. Among the challenges faced by those involved in developing therapeutic drugs for neurodegenerative diseases, the syndromes are often complex, and small animal models do not fully recapitulate the unique features of the human nervous system. Human induced pluripotent stem cells (iPSCs) are a novel technology that ideally would permit us to generate neuronal cells from individual patients, thereby eliminating the problem of species-specificity inherent when using animal models. Specific phenotypes of iPSC-derived cells may permit researchers to identify sub-types and to distinguish among unique clusters and groups. Recently, iPSCs were used for drug screening and testing for neurologic disorders including Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), spinocerebellar atrophy (SCA), and Zika virus infection. However, there remain many challenges still ahead, including how one might effectively recapitulate sporadic disease phenotypes and the selection of ideal phenotypes and for large-scale drug screening. Fortunately, quite a few novel strategies have been developed that might be combined with an iPSC-based model to solve these challenges, including organoid technology, single-cell RNA sequencing, genome editing, and deep learning artificial intelligence. Here, we will review current applications and potential future directions for iPSC-based neurodegenerative disease models for critical drug screening.

Modeling Psychiatric Disorder Biology with Stem Cells

PURPOSE OF REVIEW

We review the ways in which stem cells are used in psychiatric disease research, including the related advances in gene editing and directed cell differentiation.

RECENT FINDINGS

The recent development of induced pluripotent stem cell (iPSC) technologies has created new possibilities for the study of psychiatric disease. iPSCs can be derived from patients or controls and differentiated to an array of neuronal and non-neuronal cell types. Their genomes can be edited as desired, and they can be assessed for a variety of phenotypes. This makes them especially interesting for studying genetic variation, which is particularly useful today now that our knowledge on the genetics of psychiatric disease is quickly expanding. The recent advances in cell engineering have led to powerful new methods for studying psychiatric illness including schizophrenia, bipolar disorder, and autism. There is a wide array of possible applications as illustrated by the many examples from the literature, most of which are cited here.

Recapitulating complex biological signaling environments using a multiplexed, DNA-patterning approach

Elucidating how the spatial organization of extrinsic signals modulates cell behavior and drives biological processes remains largely unexplored because of challenges in controlling spatial patterning of multiple microenvironmental cues in vitro. Here, we describe a high-throughput method that directs simultaneous assembly of multiple cell types and solid-phase ligands across length scales within minutes. Our method involves lithographically defining hierarchical patterns of unique DNA oligonucleotides to which complementary strands, attached to cells and ligands-of-interest, hybridize. Highlighting our method's power, we investigated how the spatial presentation of self-renewal ligand fibroblast growth factor-2 (FGF-2) and differentiation signal ephrin-B2 instruct single adult neural stem cell (NSC) fate. We found that NSCs have a strong spatial bias toward FGF-2 and identified an unexpected subpopulation exhibiting high neuronal differentiation despite spatially occupying patterned FGF-2 regions. Overall, our broadly applicable, DNA-directed approach enables mechanistic insight into how tissues encode regulatory information through the spatial presentation of heterogeneous signals.