Endogenous WNT signaling regulates hPSC-derived neural progenitor cell heterogeneity and specifies their regional identity

Patient-derived induced pluripotent stem cells: Tools to advance the understanding and drug discovery in Major Depressive Disorder

Major Depressive Disorder (MDD) is a pleomorphic disease with substantial patterns of symptoms and severity with mensurable deficits in several associated domains. The broad spectrum of phenotypes observed in patients diagnosed with depressive disorders is the reflection of a very complex disease where clusters of biological and external factors (e.g., response/processing of life events, intrapsychic factors) converge and mediate pathogenesis, clinical presentation/phenotypes and trajectory. Patient-derived induced pluripotent stem cells (iPSCs) enable their differentiation into specialised cell types in the central nervous system to explore the pathophysiological substrates of MDD. These models may complement animal models to advance drug discovery and identify therapeutic approaches, such as cell therapy, drug repurposing, and elucidation of drug metabolism, toxicity, and mechanisms of action at the molecular/cellular level, to pave the way for precision psychiatry. Despite the remarkable scientific and clinical progress made over the last few decades, the disease is still poorly understood, the incidence and prevalence continue to increase, and more research is needed to meet clinical demands. This review aims to summarise and provide a critical overview of the research conducted thus far using patient-derived iPSCs for the modelling of psychiatric disorders, with a particular emphasis on MDD.

Nkd1 functions downstream of Axin2 to attenuate Wnt signaling

Wnt signaling is a crucial developmental pathway involved in early development as well as stem-cell maintenance in adults and its misregulation leads to numerous diseases. Thus, understanding the regulation of this pathway becomes vitally important. Axin2 and Nkd1 are widely utilized negative feedback regulators in Wnt signaling where Axin2 functions to destabilize cytoplasmic β-catenin, and Nkd1 functions to inhibit the nuclear localization of β-catenin. Here, we set out to further understand how Axin2 and Nkd1 regulate Wnt signaling by creating axin2gh1/gh1, nkd1gh2/gh2 single mutants and axin2gh1/gh1;nkd1gh2/gh2 double mutant zebrafish using sgRNA/Cas9. All three Wnt regulator mutants were viable and had impaired heart looping, neuromast migration defects, and behavior abnormalities in common, but there were no signs of synergy in the axin2gh1/gh1;nkd1gh2/gh2 double mutants. Further, Wnt target gene expression by qRT-PCR and RNA-seq, and protein expression by mass spectrometry demonstrated that the double axin2gh1/gh1;nkd1gh2/gh2 mutant resembled the nkd1gh2/gh2 phenotype demonstrating that Nkd1 functions downstream of Axin2. In support of this, the data further demonstrates that Axin2 uniquely alters the properties of β-catenin-dependent transcription having novel readouts of Wnt activity compared with nkd1gh2/gh2 or the axin2gh1/gh1;nkd1gh2/gh2 double mutant. We also investigated the sensitivity of the Wnt regulator mutants to exacerbated Wnt signaling, where the single mutants displayed characteristic heightened Wnt sensitivity, resulting in an eyeless phenotype. Surprisingly, this phenotype was rescued in the double mutant, where we speculate that cross-talk between Wnt/β-catenin and Wnt/Planar Cell Polarity pathways could lead to altered Wnt signaling in some scenarios. Collectively, the data emphasizes both the commonality and the complexity in the feedback regulation of Wnt signaling.

Wnt signal-dependent antero-posterior specification of early-stage CNS primordia modeled in EpiSC-derived neural stem cells

The specification of the embryonic central nervous system (CNS) into future brain (forebrain, midbrain, or hindbrain) and spinal cord (SC) regions is a critical step of CNS development. A previous chicken embryo study indicated that anterior epiblast cells marked by Sox2 N2 enhancer activity are specified to the respective brain regions during the transition phase of the epiblast to the neural plate-forming neural primordium. The present study showed that the SC precursors positioned posterior to the hindbrain precursors in the anterior epiblast migrated posteriorly in contrast to the anterior migration of brain precursors. The anteroposterior specification of the CNS precursors occurs at an analogous time (∼E7.5) in mouse embryos, in which an anterior-to-posterior incremental gradient of Wnt signal strength was observed. To examine the possible Wnt signal contribution to the anteroposterior CNS primordium specification, we utilized mouse epiblast stem cell (EpiSC)-derived neurogenesis in culture. EpiSCs maintained in an activin- and FGF2-containing medium start neural development after the removal of activin, following a day in a transitory state. We placed activin-free EpiSCs in EGF- and FGF2-containing medium to arrest neural development and expand the cells into neural stem cells (NSCs). Simultaneously, a Wnt antagonist or agonist was added to the culture, with the anticipation that different levels of Wnt signals would act on the transitory cells to specify CNS regionality; then, the Wnt-treated cells were expanded as NSCs. Gene expression profiles of six NSC lines were analyzed using microarrays and single-cell RNA-seq. The NSC lines demonstrated anteroposterior regional specification in response to increasing Wnt signal input levels: forebrain-midbrain-, hindbrain-, cervical SC-, and thoracic SC-like lines. The regional coverage of these NSC lines had a range; for instance, the XN1 line expressed Otx2 and En2, indicating midbrain characteristics, but additionally expressed the SC-characteristic Hoxa5. The ranges in the anteroposterior specification of neural primordia may be narrowed as neural development proceeds. The thoracic SC is presumably the posterior limit of the contribution by anterior epiblast-derived neural progenitors, as the characteristics of more posterior SC regions were not displayed.

High density SNP array and reanalysis of genome sequencing uncovers CNVs associated with neurodevelopmental disorders in KOLF2.1J iPSCs

The KOLF2.1J iPSC line was recently proposed as a reference iPSC to promote the standardization of research studies in the stem cell field. Due to overall good performance differentiating to neural cell lineages, high gene editing efficiency, and absence of genetic variants associated to neurological disorders KOLF2.1J iPSC line was particularly recommended for neurodegenerative disease modeling. However, our work uncovers that KOLF2.1J hPSCs carry heterozygous small copy number variants (CNVs) that cause DTNBP1, JARID2 and ASTN2 haploinsufficiencies, all of which are associated with neurological disorders. We further determine that these CNVs arose in vitro over the course of KOLF2.1J iPSC generation from a healthy donor-derived KOLF2 iPSC line and affect the expression of DNTBP1, JARID2 and ASTN2 proteins in KOLF2.1J iPSCs and neural progenitors. Therefore, our study suggests that KOLF2.1J iPSCs carry genetic variants that may be deleterious for neural cell lineages. This data is essential for a careful interpretation of neural cell studies derived from KOLF2.1J iPSCs and highlights the need for a catalogue of iPSC lines that includes a comprehensive genome characterization analysis.

MeCP2 dysfunction prevents proper BMP signaling and neural progenitor expansion in brain organoid

OBJECTIVES

Sporadic mutations in MeCP2 are a hallmark of Rett syndrome (RTT). Many RTT brain organoid models have exhibited pathogenic phenotypes such as decreased spine density and small size of soma with altered electrophysiological signals. However, previous models are mainly focused on the phenotypes observed in the late phase and rarely provide clues for the defect of neural progenitors which generate different types of neurons and glial cells.

METHODS

We newly established the RTT brain organoid model derived from MeCP2-truncated iPS cells which were genetically engineered by CRISPR/Cas9 technology. By immunofluorescence imaging, we studied the development of NPC pool and its fate specification into glutamatergic neurons or astrocytes in RTT organoids. By total RNA sequencing, we investigated which signaling pathways were altered during the early brain development in RTT organoids.

RESULTS

Dysfunction of MeCP2 caused the defect of neural rosette formation in the early phase of cortical development. In total transcriptome analysis, BMP pathway-related genes are highly associated with MeCP2 depletion. Moreover, levels of pSMAD1/5 and BMP target genes are excessively increased, and treatment of BMP inhibitors partially rescues the cell cycle progression of neural progenitors. Subsequently, MeCP2 dysfunction reduced the glutamatergic neurogenesis and induced overproduction of astrocytes. Nevertheless, early inhibition of BMP pathway rescued VGLUT1 expression and suppressed astrocyte maturation.

INTERPRETATION

Our results demonstrate that MeCP2 is required for the expansion of neural progenitor cells by modulating BMP pathway at early stages of development, and this influence persists during neurogenesis and gliogenesis at later stages of brain organoid development.

Applications of Boolean modeling to study the dynamics of a complex disease and therapeutics responses

Computational modeling has emerged as a critical tool in investigating the complex molecular processes involved in biological systems and diseases. In this study, we apply Boolean modeling to uncover the molecular mechanisms underlying Parkinson's disease (PD), one of the most prevalent neurodegenerative disorders. Our approach is based on the PD-map, a comprehensive molecular interaction diagram that captures the key mechanisms involved in the initiation and progression of PD. Using Boolean modeling, we aim to gain a deeper understanding of the disease dynamics, identify potential drug targets, and simulate the response to treatments. Our analysis demonstrates the effectiveness of this approach in uncovering the intricacies of PD. Our results confirm existing knowledge about the disease and provide valuable insights into the underlying mechanisms, ultimately suggesting potential targets for therapeutic intervention. Moreover, our approach allows us to parametrize the models based on omics data for further disease stratification. Our study highlights the value of computational modeling in advancing our understanding of complex biological systems and diseases, emphasizing the importance of continued research in this field. Furthermore, our findings have potential implications for the development of novel therapies for PD, which is a pressing public health concern. Overall, this study represents a significant step forward in the application of computational modeling to the investigation of neurodegenerative diseases, and underscores the power of interdisciplinary approaches in tackling challenging biomedical problems.

Inflammatory Response and Exosome Biogenesis of Choroid Plexus Organoids Derived from Human Pluripotent Stem Cells

The choroid plexus (ChP) is a complex structure in the human brain that is responsible for the secretion of cerebrospinal fluid (CSF) and forming the blood-CSF barrier (B-CSF-B). Human-induced pluripotent stem cells (hiPSCs) have shown promising results in the formation of brain organoids in vitro; however, very few studies to date have generated ChP organoids. In particular, no study has assessed the inflammatory response and the extracellular vesicle (EV) biogenesis of hiPSC-derived ChP organoids. In this study, the impacts of Wnt signaling on the inflammatory response and EV biogenesis of ChP organoids derived from hiPSCs was investigated. During days 10-15, bone morphogenetic protein 4 was added along with (+/-) CHIR99021 (CHIR, a small molecule GSK-3β inhibitor that acts as a Wnt agonist). At day 30, the ChP organoids were characterized by immunocytochemistry and flow cytometry for TTR (~72%) and CLIC6 (~20%) expression. Compared to the -CHIR group, the +CHIR group showed an upregulation of 6 out of 10 tested ChP genes, including CLIC6 (2-fold), PLEC (4-fold), PLTP (2-4-fold), DCN (~7-fold), DLK1 (2-4-fold), and AQP1 (1.4-fold), and a downregulation of TTR (0.1-fold), IGFBP7 (0.8-fold), MSX1 (0.4-fold), and LUM (0.2-0.4-fold). When exposed to amyloid beta 42 oligomers, the +CHIR group had a more sensitive response as evidenced by the upregulation of inflammation-related genes such as TNFα, IL-6, and MMP2/9 when compared to the -CHIR group. Developmentally, the EV biogenesis markers of ChP organoids showed an increase over time from day 19 to day 38. This study is significant in that it provides a model of the human B-CSF-B and ChP tissue for the purpose of drug screening and designing drug delivery systems to treat neurological disorders such as Alzheimer's disease and ischemic stroke.

Rapid and Robust Multi-Phenotypic Assay System for ALS Using Human iPS Cells with Mutations in Causative Genes

Amyotrophic lateral sclerosis (ALS) is a major life-threatening disease caused by motor neuron degeneration. More effective treatments through drug discovery are urgently needed. Here, we established an effective high-throughput screening system using induced pluripotent stem cells (iPSCs). Using a Tet-On-dependent transcription factor expression system carried on the PiggyBac vector, motor neurons were efficiently and rapidly generated from iPSCs by a single-step induction method. Induced iPSC transcripts displayed characteristics similar to those of spinal cord neurons. iPSC-generated motor neurons carried a mutation in fused in sarcoma (FUS) and superoxide dismutase 1 (SOD1) genes and had abnormal protein accumulation corresponding to each mutation. Calcium imaging and multiple electrode array (MEA) recordings demonstrated that ALS neurons were abnormally hyperexcitable. Noticeably, protein accumulation and hyperexcitability were ameliorated by treatment with rapamycin (mTOR inhibitor) and retigabine (Kv7 channel activator), respectively. Furthermore, rapamycin suppressed ALS neuronal death and hyperexcitability, suggesting that protein aggregate clearance through the activation of autophagy effectively normalized activity and improved neuronal survival. Our culture system reproduced several ALS phenotypes, including protein accumulation, hyperexcitability, and neuronal death. This rapid and robust phenotypic screening system will likely facilitate the discovery of novel ALS therapeutics and stratified and personalized medicine for sporadic motor neuron diseases.

A reference human induced pluripotent stem cell line for large-scale collaborative studies

Human induced pluripotent stem cell (iPSC) lines are a powerful tool for studying development and disease, but the considerable phenotypic variation between lines makes it challenging to replicate key findings and integrate data across research groups. To address this issue, we sub-cloned candidate human iPSC lines and deeply characterized their genetic properties using whole genome sequencing, their genomic stability upon CRISPR-Cas9-based gene editing, and their phenotypic properties including differentiation to commonly used cell types. These studies identified KOLF2.1J as an all-around well-performing iPSC line. We then shared KOLF2.1J with groups around the world who tested its performance in head-to-head comparisons with their own preferred iPSC lines across a diverse range of differentiation protocols and functional assays. On the strength of these findings, we have made KOLF2.1J and its gene-edited derivative clones readily accessible to promote the standardization required for large-scale collaborative science in the stem cell field.

Deciphering the Transcriptional Landscape of Human Pluripotent Stem Cell-Derived GnRH Neurons: The Role of Wnt Signaling in Patterning the Neural Fate

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons lay the foundation for human development and reproduction; however, the critical cell populations and the entangled mechanisms underlying the development of human GnRH neurons remain poorly understood. Here, by using our established human pluripotent stem cell-derived GnRH neuron model, we decoded the cellular heterogeneity and differentiation trajectories at the single-cell level. We found that a glutamatergic neuron population, which generated together with GnRH neurons, showed similar transcriptomic properties with olfactory sensory neuron and provided the migratory path for GnRH neurons. Through trajectory analysis, we identified a specific gene module activated along the GnRH neuron differentiation lineage, and we examined one of the transcription factors, DLX5, expression in human fetal GnRH neurons. Furthermore, we found that Wnt inhibition could increase DLX5 expression and improve the GnRH neuron differentiation efficiency through promoting neurogenesis and switching the differentiation fates of neural progenitors into glutamatergic neurons/GnRH neurons. Our research comprehensively reveals the dynamic cell population transition and gene regulatory network during GnRH neuron differentiation.

Interventional Strategies for Parkinson Disease: Can Neural Precursor Cells Forge a Path Ahead?

Neural precursor cells (NPCs), derived from pluripotent stem cells (PSCs), with their unique ability to generate multiple neuronal and glial cell types are extremely useful for understanding biological mechanisms in normal and diseased states. However, generation of specific neuronal subtypes (mature) from NPCs in large numbers adequate for cell therapy is challenging due to lack of a thorough understanding of the cues that govern their differentiation. Interestingly, neural stem cells (NSCs) themselves are in consideration for therapy given their potency to form different neural cell types, release of trophic factors, and immunomodulatory effects that confer neuroprotection. With the recent COVID-19 outbreak and its accompanying neurological indications, the immunomodulatory role of NSCs may gain additional significance in the prevention of disease progression in vulnerable populations. In this regard, small-molecule mediated NPC generation from PSCs via NSC formation has become an important strategy that ensures consistency and robustness of the process. The development of the mammalian brain occurs along the rostro-caudal axis, and the establishment of anterior identity is an early event. Wnt signaling, along with fibroblast growth factor and retinoic acid, acts as a caudalization signal. Further, the increasing amount of epigenetic data available from human fetal brain development has enhanced both our understanding of and ability to experimentally manipulate these developmental regulatory programs in vitro. However, the impact on homing and engraftment after transplantation and subsequently on therapeutic efficacy of NPCs based on their derivation strategy is not yet clear. Another formidable challenge in cell replacement therapy for neurodegenerative disorders is the mode of delivery. In this Perspective, we discuss these core ideas with insights from our preliminary studies exploring the role of PSC-derived NPCs in rat models of MPTP-induced Parkinson's disease following intranasal injections.

Depletion of Demethylase KDM6 Enhances Early Neuroectoderm Commitment of Human PSCs

Epigenetic modifications play a crucial role in neurogenesis, learning, and memory, but the study of their role in early neuroectoderm commitment from pluripotent inner cell mass is relatively lacking. Here we utilized the system of directed neuroectoderm differentiation from human embryonic stem cells and identified that KDM6B, an enzyme responsible to erase H3K27me3, was the most upregulated enzyme of histone methylation during neuroectoderm differentiation by transcriptome analysis. We then constructed KDM6B-null embryonic stem cells and found strikingly that the pluripotent stem cells with KDM6B knockout exhibited much higher neuroectoderm induction efficiency. Furthermore, we constructed a series of embryonic stem cell lines knocking out the other H3K27 demethylase KDM6A, and depleting both KDM6A and KDM6B, respectively. These cell lines together confirmed that KDM6 impeded early neuroectoderm commitment. By RNA-seq, we found that the expression levels of a panel of WNT genes were significantly affected upon depletion of KDM6. Importantly, the result that WNT agonist and antagonist could abolish the differential neuroectoderm induction due to manipulating KDM6 further demonstrated that WNT was the major downstream of KDM6 during early neural induction. Moreover, we found that the chemical GSK-J1, an inhibitor of KDM6, could enhance neuroectoderm induction from both embryonic stem cells and induced pluripotent stem cells. Taken together, our findings not only illustrated the important role of the histone methylation modifier KDM6 in early neurogenesis, providing insights into the precise epigenetic regulation in cell fate determination, but also showed that the inhibitor of KDM6 could facilitate neuroectoderm differentiation from human pluripotent stem cells.

The potential of mechanistic information organised within the AOP framework to increase regulatory uptake of the developmental neurotoxicity (DNT) in vitro battery of assays

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Current in vivo DNT testing for regulatory purposes is not effective.

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In vitro assays anchored to key neurodevelopmental processes are available.

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Development of Adverse Outcome Pathways is required to increase mechanistic understanding of DNT effects.

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DNT Integrated Approaches to Testing and Assessment for various regulatory purposes should be developed.

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The OECD Guidance Document on use of in vitro DNT battery of assays is currently under development.

A major challenge in regulatory developmental neurotoxicity (DNT) assessment is lack of toxicological information for many compounds. Therefore, the Test Guidelines programme of the Organisation for Economic Cooperation and Development (OECD) took the initiative to coordinate an international collaboration between diverse stakeholders to consider integration of alternative approaches towards improving the current chemical DNT testing. During the past few years, a series of workshops was organized during which a consensus was reached that incorporation of a DNT testing battery that relies on in vitro assays anchored to key neurodevelopmental processes should be developed. These key developmental processes include neural progenitor cell proliferation, neuronal and oligodendrocyte differentiation, neural cell migration, neurite outgrowth, synaptogenesis and neuronal network formation, as well key events identified in the existing Adverse Outcome Pathways (AOPs). AOPs deliver mechanistic information on the causal links between molecular initiating event, intermediate key events and an adverse outcome of regulatory concern, providing the biological context to facilitate development of Integrated Approaches to Testing and Assessment (IATA) for various regulatory purposes. Developing IATA case studies, using mechanistic information derived from AOPs, is expected to increase scientific confidence for the use of in vitro methods within an IATA, thereby facilitating regulatory uptake. This manuscript summarizes the current state of international efforts to enhance DNT testing by using an in vitro battery of assays focusing on the role of AOPs in informing the development of IATA for different regulatory purposes, aiming to deliver an OECD guidance document on use of in vitro DNT battery of assays that include in vitro data interpretation.

Variable Outcomes in Neural Differentiation of Human PSCs Arise from Intrinsic Differences in Developmental Signaling Pathways

Directed differentiation of human pluripotent stem cells varies in specificity and efficiency. Stochastic, genetic, intracellular, and environmental factors affect maintenance of pluripotency and differentiation into early embryonic lineages. However, factors affecting variation in in vitro differentiation to defined cell types are not well understood. To address this, we focused on a well-established differentiation process to cerebral cortex neural progenitor cells and their neuronal progeny from human pluripotent stem cells. Analysis of 162 differentiation outcomes of 61 stem cell lines derived from 37 individuals showed that most variation occurs along gene expression axes reflecting dorsoventral and rostrocaudal spatial expression during in vivo brain development. Line-independent and line-dependent variations occur, with the latter driven largely by differences in endogenous Wnt signaling activity. Tuning Wnt signaling during a specific phase early in the differentiation process reduces variability, demonstrating that cell-line/genome-specific differentiation outcome biases can be corrected by controlling extracellular signaling.

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Analysis of 162 iPSC neural differentiation quantifies sources of variability

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Variation in outcomes occurs along developmental brain spatial and regional axes

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Variation depends on iPSC-line-specific differences in Wnt/β-catenin signaling

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Effects of signaling differences can be rescued by exogenous pathway activation

Generation of region-specific and high-purity neurons from human feeder-free iPSCs

Human induced pluripotent stem cells (iPSCs) have great potential to elucidate the molecular pathogenesis of neurological/psychiatric diseases. In particular, neurological/psychiatric diseases often display brain region-specific symptoms, and the technology for generating region-specific neural cells from iPSCs has been established for detailed modeling of neurological/psychiatric disease phenotypes in vitro. On the other hand, recent advances in culturing human iPSCs without feeder cells have enabled highly efficient and reproducible neural induction. However, conventional regional control technologies have mainly been developed based on on-feeder iPSCs, and these methods are difficult to apply to feeder-free (ff) iPSC cultures. In this study, we established a novel culture system to generate region-specific neural cells from human ff-iPSCs. This system is the best optimized approach for feeder-free iPSC culture and generates specific neuronal subtypes with high purity and functionality, including forebrain cortical neurons, forebrain interneurons, midbrain dopaminergic neurons, and spinal motor neurons. In addition, the temporal patterning of cortical neuron layer specification in the forebrain was reproduced in our culture system, which enables the generation of layer-specific cortical neurons. Neuronal activity was demonstrated in the present culture system by using multiple electrode array and calcium imaging. Collectively, our ff-iPSC-based culture system would provide a desirable platform for modeling various types of neurological/psychiatric disease phenotypes.

Modeling neurodevelopment in a dish with pluripotent stem cells

Pluripotent stem cells (PSCs) can differentiate into all cell types in the body, and their differentiation procedures recapitulate the developmental processes of embryogenesis. Focusing on neurodevelopment, we describe here the application of knowledge gained from embryology to the neural induction of PSCs. Furthermore, PSC-based neural modeling provides novel insights into neurodevelopmental processes. In particular, human PSC cultures are a powerful tool for the study of human-specific neurodevelopmental processes and could even enable the elucidation of the mechanisms of human brain evolution. We also discuss challenges and potential future directions in further improving PSC-based neural modeling.

Non-xenogeneic expansion and definitive endoderm differentiation of human pluripotent stem cells in an automated bioreactor

Scalable processes are requisite for the robust biomanufacturing of human pluripotent stem cell (hPSC)-derived therapeutics. Toward this end, we demonstrate the xeno-free expansion and directed differentiation of human embryonic and induced pluripotent stem cells to definitive endoderm (DE) in a controlled stirred suspension bioreactor (SSB). Based on previous work on converting hPSCs to insulin-producing progeny, differentiation of two hPSC lines was optimized in planar cultures yielding up to 87% FOXA2⁺ /SOX17⁺ cells. Next, hPSCs were propagated in an SSB with controlled pH and dissolved oxygen. Cultures displayed a 10- to 12-fold increase in cell number over 5-6 days with the maintenance of pluripotency (>85% OCT4⁺ ) and viability (>85%). For differentiation, SSB cultures yielded up to 89% FOXA2⁺ /SOX17⁺ cells or ~ 8 DE cells per seeded hPSC. Specification to DE cell fate was consistently more efficient in the bioreactor compared to planar cultures. Hence, a tunable strategy is established that is suitable for the xeno-free manufacturing of DE cells from different hPSC lines in scalable SSBs. This study advances bioprocess development for producing a wide gamut of human DE cell-derived therapeutics.

Biphasic Regulation of Mesenchymal Genes Controls Fate Switches During Hematopoietic Differentiation of Human Pluripotent Stem Cells

Epithelial-mesenchymal transition (EMT) or its reverse process mesenchymal-epithelial transition (MET) occurs in multiple physiological and pathological processes. However, whether an entire EMT-MET process exists and the potential function during human hematopoiesis remain largely elusive. Utilizing human pluripotent stem cell (hPSC)-based systems, it is discovered that while EMT occurs at the onset of human hematopoietic differentiation, MET is not detected subsequently during differentiation. Instead, a biphasic activation of mesenchymal genes during hematopoietic differentiation of hPSCs is observed. The expression of mesenchymal genes is upregulated during the fate switch from pluripotency to the mesoderm, sustained at the hemogenic endothelium (HE) stage, and attenuated during hemogenic endothelial cell (HEP) differentiation to hematopoietic progenitor cells (HPCs). A similar expression pattern of mesenchymal genes is also observed during human and murine hematopoietic development in vivo. Wnt signaling and its downstream gene SNAI1 mediate the up-regulation of mesenchymal genes and initiation of mesoderm induction from pluripotency. Inhibition of transforming growth factor-β (TGF-β) signaling and downregulation of HAND1, a downstream gene of TGF-β, are required for the downregulation of mesenchymal genes and the capacity of HEPs to generate HPCs. These results suggest that the biphasic regulation of mesenchymal genes is an essential mechanism during human hematopoiesis.

Cell therapy for spinal cord injury by using human iPSC-derived region-specific neural progenitor cells

The transplantation of neural progenitor cells (NPCs) derived from human induced pluripotent stem cells (iPSCs) has beneficial effects on spinal cord injury (SCI). However, while there are many subtypes of NPCs with different regional identities, the subtype of iPSC-derived NPCs that is most appropriate for cell therapy for SCI has not been identified. Here, we generated forebrain- and spinal cord-type NPCs from human iPSCs and grafted them onto the injured spinal cord in mice. These two types of NPCs retained their regional identities after transplantation and exhibited different graft-host interconnection properties. NPCs with spinal cord regional identity but not those with forebrain identity resulted in functional improvement in SCI mice, especially in those with mild-to-moderate lesions. This study highlights the importance of the regional identity of human iPSC-derived NPCs used in cell therapy for SCI.

Parkinson's disease, aging and adult neurogenesis: Wnt/β-catenin signalling as the key to unlock the mystery of endogenous brain repair

A common hallmark of age-dependent neurodegenerative diseases is an impairment of adult neurogenesis. Wingless-type mouse mammary tumor virus integration site (Wnt)/β-catenin (WβC) signalling is a vital pathway for dopaminergic (DAergic) neurogenesis and an essential signalling system during embryonic development and aging, the most critical risk factor for Parkinson's disease (PD). To date, there is no known cause or cure for PD. Here we focus on the potential to reawaken the impaired neurogenic niches to rejuvenate and repair the aged PD brain. Specifically, we highlight WβC-signalling in the plasticity of the subventricular zone (SVZ), the largest germinal region in the mature brain innervated by nigrostriatal DAergic terminals, and the mesencephalic aqueduct-periventricular region (Aq-PVR) Wnt-sensitive niche, which is in proximity to the SNpc and harbors neural stem progenitor cells (NSCs) with DAergic potential. The hallmark of the WβC pathway is the cytosolic accumulation of β-catenin, which enters the nucleus and associates with T cell factor/lymphoid enhancer binding factor (TCF/LEF) transcription factors, leading to the transcription of Wnt target genes. Here, we underscore the dynamic interplay between DAergic innervation and astroglial-derived factors regulating WβC-dependent transcription of key genes orchestrating NSC proliferation, survival, migration and differentiation. Aging, inflammation and oxidative stress synergize with neurotoxin exposure in "turning off" the WβC neurogenic switch via down-regulation of the nuclear factor erythroid-2-related factor 2/Wnt-regulated signalosome, a key player in the maintenance of antioxidant self-defense mechanisms and NSC homeostasis. Harnessing WβC-signalling in the aged PD brain can thus restore neurogenesis, rejuvenate the microenvironment, and promote neurorescue and regeneration.

Generation of homogeneous midbrain organoids with in vivo-like cellular composition facilitates neurotoxin-based Parkinson's disease modeling

Recent studies have demonstrated the generation of midbrain-like organoids (MOs) from human pluripotent stem cells. However, the low efficiency of MO generation and the relatively immature and heterogeneous structures of the MOs hinder the translation of these organoids from the bench to the clinic. Here we describe the robust generation of MOs with homogeneous distribution of midbrain dopaminergic (mDA) neurons. Our MOs contain not only mDA neurons but also other neuronal subtypes as well as functional glial cells, including astrocytes and oligodendrocytes. Furthermore, our MOs exhibit mDA neuron-specific cell death upon treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, indicating that MOs could be a proper human model system for studying the in vivo pathology of Parkinson's disease (PD). Our optimized conditions for producing homogeneous and mature MOs might provide an advanced patient-specific platform for in vitro disease modeling as well as for drug screening for PD.

Wnt-Notch Signaling Interactions During Neural and Astroglial Patterning of Human Stem Cells

The human brain formation involves complicated processing, which is regulated by a gene regulatory network influenced by different signaling pathways. The cross-regulatory interactions between elements of different pathways affect the process of cell fate assignment during neural and astroglial tissue patterning. In this study, the interactions between Wnt and Notch pathways, the two major pathways that influence neural and astroglial differentiation of human induced pluripotent stem cells (hiPSCs) individually, were investigated. In particular, the synergistic effects of Wnt-Notch pathway on the neural patterning processes along the anterior-posterior or dorsal-ventral axis of hiPSC-derived cortical spheroids were explored. The human cortical spheroids derived from hiPSCs were treated with Wnt activator CHIR99021 (CHIR), Wnt inhibitor IWP4, and Notch inhibitor (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester [DAPT]) individually, or in combinations (CHIR + DAPT, IWP4 + DAPT). The results suggest that CHIR + DAPT can promote Notch signaling, similar or higher than CHIR alone, whereas IWP4 + DAPT reduces Notch activity compared to IWP4 alone. Also, CHIR + DAPT promoted hindbrain marker HOXB4 expression more consistently than CHIR alone, while IWP4 + DAPT promoted Olig2 expression, indicating the synergistic effects distinctly different from that of the individual small molecule. In addition, IWP4 simultaneously promoted dorsal and ventral identity. The patterned neural spheroids can be switched for astroglial differentiation using bone morphogenetic protein 4. This study should advance the derivations of neurons, astroglial cells, and brain region-specific organoids from hiPSCs for disease modeling, drug screening, as well as for hiPSC-based therapies. Impact Statement Wnt signaling plays a central role in neural patterning of human pluripotent stem cells. It can interact with Notch signaling in defining dorsal-ventral and rostral-caudal (or anterior-posterior) axis of brain organoids. This study investigates novel Wnt and Notch interactions (i.e., Wntch) in neural patterning of dorsal forebrain spheroids or organoids derived from human induced pluripotent stem cells. The synergistic effects of Wnt activator or inhibitor with Notch inhibitor were observed. This study should advance the derivations of neurons, astroglial cells, and brain region-specific organoids from human stem cells for disease modeling and drug screening, as well as for stem cell-based therapies. The results can be used to establish better in vitro culture methods for efficiently mimicking in vivo structure of central nervous system.

Genomics Analysis of Metabolic Pathways of Human Stem Cell-Derived Microglia-Like Cells and the Integrated Cortical Spheroids

Brain spheroids or organoids derived from human pluripotent stem cells (hiPSCs) are still not capable of completely recapitulating in vivo human brain tissue, and one of the limitations is lack of microglia. To add built-in immune function, coculture of the dorsal forebrain spheroids with isogenic microglia-like cells (D-MG) was performed in our study. The three-dimensional D-MG spheroids were analyzed for their transcriptome and compared with isogenic microglia-like cells (MG). Cortical spheroids containing microglia-like cells displayed different metabolic programming, which may affect the associated phenotype. The expression of genes related to glycolysis and hypoxia signaling was increased in cocultured D-MG spheroids, indicating the metabolic shift to aerobic glycolysis, which is in favor of M1 polarization of microglia-like cells. In addition, the metabolic pathways and the signaling pathways involved in cell proliferation, cell death, PIK3/AKT/mTOR signaling, eukaryotic initiation factor 2 pathway, and Wnt and Notch pathways were analyzed. The results demonstrate the activation of mTOR and p53 signaling, increased expression of Notch ligands, and the repression of NF-κB and canonical Wnt pathways, as well as the lower expression of cell cycle genes in the cocultured D-MG spheroids. This analysis indicates that physiological 3-D microenvironment may reshape the immunity of in vitro cortical spheroids and better recapitulate in vivo brain tissue function for disease modeling and drug screening.

Typical halogenated flame retardants affect human neural stem cell gene expression during proliferation and differentiation via glycogen synthase kinase 3 beta and T3 signaling

2',2',4,4'-tetrabromo diphenyl ether (BDE-47), one of the most abundant congeners of commercial pentaBDE utilized as flame retardants, has been phased out of production due to its potential neural toxicity and endocrine disrupting activities, and yet still present in the environment. Several alternatives to BDE-47, including tetrabromobisphenol A (TBBPA), tetrabromobisphenol S (TBBPS), tetrachlorobisphenol A (TCBPA) and decabromodiphenyl ether (BDE-209), are presently employed without restrictions and their potential toxic effects on human neural development are still unclear. In this study, we utilized a human neural stem cell (hNSC)-based system to evaluate the potential developmental neurotoxic effects of the above-mentioned five chemicals, at environment and human exposure relevant concentrations. We found that those compounds slightly altered the expression of hNSC identity markers (SOX2, SOX3 and NES), without impairing cell viability or proliferation, in part by either modulating glycogen synthase kinase 3 beta (GSK3β) signaling (TBBPS, TCBPA and BDE-47), and slightly disturbing the NOTCH pathway (TBBPA, TBBPS and TCBPA). Moreover, the five chemicals seemed to alter hNSC differentiation by perturbing triiodothyronine (T3) cellular signaling. Thus, our findings suggest that the five compounds, especially TBBPS, TCBPA, and BDE-47, may affect hNSC self-renewal and differentiation abilities and potentially elicit neural developmental toxicity.

EGFR is required for Wnt9a-Fzd9b signalling specificity in haematopoietic stem cells

Wnt signalling drives a plethora of processes in development, homeostasis, and disease; however, the role and mechanism of individual ligand/receptor (Wnt/Frizzled, Fzd) interactions in specific biological processes remain poorly understood. Wnt9a is specifically required for the amplification of blood progenitor cells during development. Using genetic studies in zebrafish and human embryonic stem cells, paired with in vitro cell biology and biochemistry, we have determined that Wnt9a signals specifically through Fzd9b to elicit β-catenin-dependent Wnt signalling that regulates haematopoietic stem and progenitor cell emergence. We demonstrate that the epidermal growth factor receptor (EGFR) is required as a co-factor for Wnt9a/Fzd9b signalling. EGFR-mediated phosphorylation of one tyrosine residue on the Fzd9b intracellular tail in response to Wnt9a promotes internalization of the Wnt9a/Fzd9b/LRP signalosome and subsequent signal transduction. These findings provide mechanistic insights for specific Wnt/Fzd signals, which will be crucial for specific therapeutic targeting and regenerative medicine.

Differential Effects of Heparin and Hyaluronic Acid on Neural Patterning of Human Induced Pluripotent Stem Cells

A lack of well-established animal models that can efficiently represent human brain pathology has led to the development of human induced pluripotent stem cell (hiPSC)-derived brain tissues. Brain organoids have enhanced our ability to understand the developing human brain and brain disorders (e.g., Schizophrenia, microcephaly), but the organoids still do not accurately recapitulate the anatomical organization of the human brain. Therefore, it is important to evaluate and optimize induction and signaling factors in order to engineer the next generation of brain organoids. In this study, the impact of hyaluronic acid (HA), a major brain extracellular matrix (ECM) component that interacts with cells through ligand-binding receptors, on the patterning of brain organoids from hiPSCs was evaluated. To mediate HA- binding capacity of signaling molecules, heparin was added in addition to HA or conjugated to HA to form hydrogels (with two different moduli). The neural cortical spheroids derived from hiPSCs were treated with either HA or heparin plus HA (Hep- HA) and were analyzed for ECM impacts on neural patterning. The results indicate that Hep-HA has a caudalizing effect on hiPSC-derived neural spheroids, in particular for stiff Hep-HA hydrogels. Wnt and Hippo/Yes-associated protein (YAP) signaling was modulated (using Wnt inhibitor IWP4 or actin disruption agent Cytochalasin D respectively) to understand the underlying mechanism. IWP4 and cytochalasin D promote forebrain identity. The results from this study should enhance the understanding of influence of biomimetic ECM factors for brain organoid generation.

GLIS3 Transcriptionally Activates WNT Genes to Promote Differentiation of Human Embryonic Stem Cells into Posterior Neural Progenitors

Anterior-posterior (A-P) specification of the neural tube involves initial acquisition of anterior fate followed by the induction of posterior characteristics in the primitive anterior neuroectoderm. Several morphogens have been implicated in the regulation of A-P neural patterning; however, our understanding of the upstream regulators of these morphogens remains incomplete. Here, we show that the Krüppel-like zinc finger transcription factor GLI-Similar 3 (GLIS3) can direct differentiation of human embryonic stem cells (hESCs) into posterior neural progenitor cells in lieu of the default anterior pathway. Transcriptomic analyses reveal that this switch in cell fate is due to rapid activation of Wingless/Integrated (WNT) signaling pathway. Mechanistically, through genome-wide RNA-Seq, ChIP-Seq, and functional analyses, we show that GLIS3 binds to and directly regulates the transcription of several WNT genes, including the strong posteriorizing factor WNT3A, and that inhibition of WNT signaling is sufficient to abrogate GLIS3-induced posterior specification. Our findings suggest a potential role for GLIS3 in the regulation of A-P specification through direct transcriptional activation of WNT genes. Stem Cells 2018 Stem Cells 2019;37:202-215.

Wnt/β-Catenin Signaling Pathway Governs a Full Program for Dopaminergic Neuron Survival, Neurorescue and Regeneration in the MPTP Mouse Model of Parkinson's Disease

Wingless-type mouse mammary tumor virus (MMTV) integration site (Wnt) signaling is one of the most critical pathways in developing and adult tissues. In the brain, Wnt signaling contributes to different neurodevelopmental aspects ranging from differentiation to axonal extension, synapse formation, neurogenesis, and neuroprotection. Canonical Wnt signaling is mediated mainly by the multifunctional β-catenin protein which is a potent co-activator of transcription factors such as lymphoid enhancer factor (LEF) and T-cell factor (TCF). Accumulating evidence points to dysregulation of Wnt/β-catenin signaling in major neurodegenerative disorders. This review highlights a Wnt/β-catenin/glial connection in Parkinson's disease (PD), the most common movement disorder characterized by the selective death of midbrain dopaminergic (mDAergic) neuronal cell bodies in the subtantia nigra pars compacta (SNpc) and gliosis. Major findings of the last decade document that Wnt/β-catenin signaling in partnership with glial cells is critically involved in each step and at every level in the regulation of nigrostriatal DAergic neuronal health, protection, and regeneration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, focusing on Wnt/β-catenin signaling to boost a full neurorestorative program in PD.

An integrated biomanufacturing platform for the large-scale expansion and neuronal differentiation of human pluripotent stem cell-derived neural progenitor cells

Human pluripotent stem cell derived neural progenitor cells (hNPCs) have the unique properties of long-term in vitro expansion as well as differentiation into the various neurons and supporting cell types of the central nervous system (CNS). Because of these characteristics, hNPCs have tremendous potential in the modeling and treatment of various CNS diseases and disorders. However, expansion and neuronal differentiation of hNPCs in quantities necessary for these applications is not possible with current two dimensional (2-D) approaches. Here, we used a fully defined peptide substrate as the basis for a microcarrier (MC)-based suspension culture system. Several independently derived hNPC lines were cultured on MCs for multiple passages as well as efficiently differentiated to neurons. Finally, this MC-based system was used in conjunction with a low shear rotating wall vessel (RWV) bioreactor for the integrated, large-scale expansion and neuronal differentiation of hNPCs. Overall, this fully defined and scalable biomanufacturing system will facilitate the generation of hNPCs and their neuronal derivatives in quantities necessary for basic and translational applications.

STATEMENT OF SIGNIFICANCE

In this work, we developed a microcarrier (MC)-based culture system that allows for the expansion and neuronal differentiation of human pluripotent stem cell-derived neural progenitor cells (hNPCs) under defined conditions. In turn, this MC approach was implemented in a rotating wall vessel (RWV) bioreactor for the large-scale expansion and neuronal differentiation of hNPCs. This work is of significance as it overcomes current limitations of conventional two dimensional (2-D) culture systems to enable the generation of hNPCs and their neuronal derivatives in quantities required for downstream applications in disease modeling, drug screening, and regenerative medicine.

Timing of Wnt Inhibition Modulates Directed Differentiation of Medial Ganglionic Eminence Progenitors from Human Pluripotent Stem Cells

In vitro differentiation of human pluripotent stem cell into relevant cell types is a desirable model system that has the human biological context, is a renewable source, and is scalable. GABA interneurons and basal forebrain cholinergic neurons, derivates of the medial ganglionic eminence (MGE), are implicated in diverse neuropsychiatric diseases. Various protocols have been proposed to generate MGE progenitors: the embryoid body- (EB-) based rosette-derived (RD), the adherent (AdD), and the nonadherent (NAdD) approaches. While Wnt inhibition is frequently incorporated into the strategy, the timing varies between protocols and there is a lack of standardized outcome reporting, which precludes direct comparison. Here, we report a head-to-head comparison in three distinct experimental models to establish whether Wnt inhibition during neural stem cell, NSC (stage 1), or neural progenitor cell, NPC (stage 2), formation facilitates MGE differentiation. Wnt inhibition at both stages promotes MGE progenitor differentiation when compared to no inhibition. However, NSC (stage 1) Wnt inhibition markedly reduces the number of MGE progenitors available for downstream applications in the RD and the NAdD protocols due to early inhibition of proliferation. NPC (stage 2) Wnt inhibition in the adherent system is comparable to the EB-based methods offering a techically less challenging alternative.

Wnt signal activation induces midbrain specification through direct binding of the beta-catenin/TCF4 complex to the EN1 promoter in human pluripotent stem cells

The canonical Wnt signal pathway plays a pivotal role in anteroposterior patterning and midbrain specification during early neurogenesis. Activating Wnt signal has been a strategy for differentiating human pluripotent stem cells (PSCs) into midbrain dopaminergic (DA) neurons; however, the underlying molecular mechanism(s) of how the Wnt signal drives posterior fate remained unclear. In this study, we found that activating the canonical Wnt signal significantly upregulated the expression of EN1, a midbrain-specific marker, in a fibroblast growth factor signal-dependent manner in human PSC-derived neural precursor cells (NPCs). The EN1 promoter region contains a putative TCF4-binding site that directly interacts with the β-catenin/TCF complex upon Wnt signal activation. Once differentiated, NPCs treated with a Wnt signal agonist gave rise to functional midbrain neurons including glutamatergic, GABAergic, and DA neurons. Our results provide a potential molecular mechanism that underlies midbrain specification of human PSC-derived NPCs by Wnt activation, as well as a differentiation paradigm for generating human midbrain neurons that may serve as a cellular platform for studying the ontogenesis of midbrain neurons and neurological diseases relevant to the midbrain.

Wnt/Yes-Associated Protein Interactions During Neural Tissue Patterning of Human Induced Pluripotent Stem Cells

Human induced pluripotent stem cells (hiPSCs) have special ability to self-assemble into neural spheroids or mini-brain-like structures. During the self-assembly process, Wnt signaling plays an important role in regional patterning and establishing positional identity of hiPSC-derived neural progenitors. Recently, the role of Wnt signaling in regulating Yes-associated protein (YAP) expression (nuclear or cytoplasmic), the pivotal regulator during organ growth and tissue generation, has attracted increasing interests. However, the interactions between Wnt and YAP expression for neural lineage commitment of hiPSCs remain poorly explored. The objective of this study is to investigate the effects of Wnt signaling and YAP expression on the cellular population in three-dimensional (3D) neural spheroids derived from hiPSCs. In this study, Wnt signaling was activated using CHIR99021 for 3D neural spheroids derived from human iPSK3 cells through embryoid body formation. Our results indicate that Wnt activation induces nuclear localization of YAP and upregulates the expression of HOXB4, the marker for hindbrain/spinal cord. By contrast, the cells exhibit more rostral forebrain neural identity (expression of TBR1) without Wnt activation. Cytochalasin D was then used to induce cytoplasmic YAP and the results showed the decreased HOXB4 expression. In addition, the incorporation of microparticles in the neural spheroids was investigated for the perturbation of neural patterning. This study may indicate the bidirectional interactions of Wnt signaling and YAP expression during neural tissue patterning, which have the significance in neurological disease modeling, drug screening, and neural tissue regeneration.

Function of FEZF1 during early neural differentiation of human embryonic stem cells

The understanding of the mechanism underlying human neural development has been hampered due to lack of a cellular system and complicated ethical issues. Human embryonic stem cells (hESCs) provide an invaluable model for dissecting human development because of unlimited self-renewal and the capacity to differentiate into nearly all cell types in the human body. In this study, using a chemical defined neural induction protocol and molecular profiling, we identified Fez family zinc finger 1 (FEZF1) as a potential regulator of early human neural development. FEZF1 is rapidly up-regulated during neural differentiation in hESCs and expressed before PAX6, a well-established marker of early human neural induction. We generated FEZF1-knockout H1 hESC lines using CRISPR-CAS9 technology and found that depletion of FEZF1 abrogates neural differentiation of hESCs. Moreover, loss of FEZF1 impairs the pluripotency exit of hESCs during neural specification, which partially explains the neural induction defect caused by FEZF1 deletion. However, enforced expression of FEZF1 itself fails to drive neural differentiation in hESCs, suggesting that FEZF1 is necessary but not sufficient for neural differentiation from hESCs. Taken together, our findings identify one of the earliest regulators expressed upon neural induction and provide insight into early neural development in human.

Altered Differentiation Potential of Gaucher's Disease iPSC Neuronal Progenitors due to Wnt/β-Catenin Downregulation

Gaucher’s disease (GD) is an autosomal recessive disorder caused by mutations in the GBA1 gene, which encodes acid β-glucocerebrosidase (GCase). Severe GBA1 mutations cause neuropathology that manifests soon after birth, suggesting that GCase deficiency interferes with neuronal development. We found that neuronopathic GD induced pluripotent stem cell (iPSC)-derived neuronal progenitor cells (NPCs) exhibit developmental defects due to downregulation of canonical Wnt/β-catenin signaling and that GD iPSCs’ ability to differentiate to dopaminergic (DA) neurons was strikingly reduced due to early loss of DA progenitors. Incubation of the mutant cells with the Wnt activator CHIR99021 (CHIR) or with recombinant GCase restored Wnt/β-catenin signaling and rescued DA differentiation. We also found that GD NPCs exhibit lysosomal dysfunction, which may be involved in Wnt downregulation by mutant GCase. We conclude that neuronopathic mutations in GCase lead to neurodevelopmental abnormalities due to a critical requirement of this enzyme for canonical Wnt/β-catenin signaling at early stages of neurogenesis.

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Neuronopathic GBA1 mutations attenuate canonical Wnt signaling in iPSC-derived NPCs

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GD NPC differentiation to DA neurons impaired due to early loss of DA progenitors

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GBA1-mediated lysosomal alterations may be involved in Wnt signal downregulation

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The Wnt pathway may be a potential new therapeutic target for neuronopathic GD

The WNT target SP5 negatively regulates WNT transcriptional programs in human pluripotent stem cells

The WNT/β-catenin signaling pathway is a prominent player in many developmental processes, including gastrulation, anterior-posterior axis specification, organ and tissue development, and homeostasis. Here, we use human pluripotent stem cells (hPSCs) to study the dynamics of the transcriptional response to exogenous activation of the WNT pathway. We describe a mechanism involving the WNT target gene SP5 that leads to termination of the transcriptional program initiated by WNT signaling. Integration of gene expression profiles of wild-type and SP5 mutant cells with genome-wide SP5 binding events reveals that SP5 acts to diminish expression of genes previously activated by the WNT pathway. Furthermore, we show that activation of SP5 by WNT signaling is most robust in cells with developmental potential, such as stem cells. These findings indicate a mechanism by which the developmental WNT signaling pathway reins in expression of transcriptional programs.

Single-cell gene expression analysis reveals regulators of distinct cell subpopulations among developing human neurons

The stochastic dynamics and regulatory mechanisms that govern differentiation of individual human neural precursor cells (NPC) into mature neurons are currently not fully understood. Here, we used single-cell RNA-sequencing (scRNA-seq) of developing neurons to dissect/identify NPC subtypes and critical developmental stages of alternative lineage specifications. This study comprises an unsupervised, high-resolution strategy for identifying cell developmental bifurcations, tracking the stochastic transcript kinetics of the subpopulations, elucidating regulatory networks, and finding key regulators. Our data revealed the bifurcation and developmental tracks of the two NPC subpopulations, and we captured an early (24 h) transition phase that leads to alternative neuronal specifications. The consequent up-regulation and down-regulation of stage- and subpopulation-specific gene groups during the course of maturation revealed biological insights with regard to key regulatory transcription factors and lincRNAs that control cellular programs in the identified neuronal subpopulations.

Derivation of Cortical Spheroids from Human Induced Pluripotent Stem Cells in a Suspension Bioreactor

Human induced pluripotent stem cells (hiPSCs) emerge as a promising source to construct human brain-like tissues, spheroids, or organoids in vitro for disease modeling and drug screening. A suspension bioreactor can be used to generate large size of brain organoids from hiPSCs through enhanced diffusion, but the influence of a dynamic bioreactor culture environment on neural tissue patterning from hiPSCs has not been well understood. The objective of this study is to assess the influence of a suspension bioreactor culture on cortical spheroid (i.e., forebrain-like aggregates) formation from hiPSCs. Single undifferentiated hiPSK3 cells or preformed embryoid bodies were inoculated into the bioreactor. Aggregate size distribution, neural marker expression (e.g., Nestin, PAX6, β-tubulin III, and MAP-2), and cortical tissue patterning markers (e.g., TBR1, BRN2, SATB2, and vGlut1) were evaluated with static control. Bioreactor culture was found to promote the expression of TBR1, a deep cortical layer VI marker, and temporally affect SATB2, a superficial cortical layer II-IV marker that appears later according to inside-out cortical tissue development. Prolonged culture after 70 days showed layer-specific cortical structure in the spheroids. Differential expression of matrix metalloproteinase-2 and -3 was also observed for bioreactor and static culture. The altered expression of cortical markers by a suspension bioreactor indicates the importance of culture environment on cortical tissue development from hiPSCs.

Wnt/β-catenin signaling during early vertebrate neural development

The vertebrate central nervous system (CNS) is comprised of vast number of distinct cell types arranged in a highly organized manner. This high degree of complexity is achieved by cellular communication, including direct cell-cell contact, cell-matrix interactions, and cell-growth factor signaling. Among the several developmental signals controlling the development of the CNS, Wnt proteins have emerged as particularly critical and, hence, have captivated the attention of many researchers. With Wnts' evolutionarily conserved function as primordial symmetry breaking signals, these proteins and their downstream effects are responsible for simultaneously establishing cellular diversity and tissue organization. With their expansive repertoire of secreted agonists and antagonists, cell surface receptors, signaling cascades and downstream biological effects, Wnts are ideally suited to control the complex processes underlying vertebrate neural development. In this review, we will describe the mechanisms by which Wnts exert their potent effects on cells and tissues and highlight the many roles of Wnt signaling during neural development, starting from the initial induction of the neural plate, the subsequent patterning along the embryonic axes, to the intricately organized structure of the CNS. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1239-1259, 2017.

Recent Zika Virus Isolates Induce Premature Differentiation of Neural Progenitors in Human Brain Organoids

The recent Zika virus (ZIKV) epidemic is associated with microcephaly in newborns. Although the connection between ZIKV and neurodevelopmental defects is widely recognized, the underlying mechanisms are poorly understood. Here we show that two recently isolated strains of ZIKV, an American strain from an infected fetal brain (FB-GWUH-2016) and a closely-related Asian strain (H/PF/2013), productively infect human iPSC-derived brain organoids. Both of these strains readily target to and replicate in proliferating ventricular zone (VZ) apical progenitors. The main phenotypic effect was premature differentiation of neural progenitors associated with centrosome perturbation, even during early stages of infection, leading to progenitor depletion, disruption of the VZ, impaired neurogenesis, and cortical thinning. The infection pattern and cellular outcome differ from those seen with the extensively passaged ZIKV strain MR766. The structural changes we see after infection with these more recently isolated viral strains closely resemble those seen in ZIKV-associated microcephaly.

A robust vitronectin-derived peptide for the scalable long-term expansion and neuronal differentiation of human pluripotent stem cell (hPSC)-derived neural progenitor cells (hNPCs)

Despite therapeutic advances, neurodegenerative diseases and disorders remain some of the leading causes of mortality and morbidity in the United States. Therefore, cell-based therapies to replace lost or damaged neurons and supporting cells of the central nervous system (CNS) are of great therapeutic interest. To that end, human pluripotent stem cell (hPSC) derived neural progenitor cells (hNPCs) and their neuronal derivatives could provide the cellular 'raw material' needed for regenerative medicine therapies for a variety of CNS disorders. In addition, hNPCs derived from patient-specific hPSCs could be used to elucidate the underlying mechanisms of neurodegenerative diseases and identify potential drug candidates. However, the scientific and clinical application of hNPCs requires the development of robust, defined, and scalable substrates for their long-term expansion and neuronal differentiation. In this study, we rationally designed a vitronectin-derived peptide (VDP) that served as an adhesive growth substrate for the long-term expansion of several hNPC lines. Moreover, VDP-coated surfaces allowed for the directed neuronal differentiation of hNPC at levels similar to cells differentiated on traditional extracellular matrix protein-based substrates. Overall, the ability of VDP to support the long-term expansion and directed neuronal differentiation of hNPCs will significantly advance the future translational application of these cells in treating injuries, disorders, and diseases of the CNS.

Wnts Are Expressed in the Ependymal Region of the Adult Spinal Cord

The Wnt family of proteins plays key roles during central nervous system development and in several physiological processes during adulthood. Recently, experimental evidence has linked Wnt-related genes to regulation and maintenance of stem cells in the adult neurogenic niches. In the spinal cord, the ependymal cells surrounding the central canal form one of those niches, but little is known about their Wnt expression patterns. Using microdissection followed by TaqMan® low-density arrays, we show here that the ependymal regions of young, mature rats and adult humans express several Wnt-related genes, including ligands, conventional and non-conventional receptors, co-receptors, and soluble inhibitors. We found 13 genes shared between rats and humans, 4 exclusively expressed in rats and 9 expressed only in humans. Also, we observed a reduction with age on spontaneous proliferation of ependymal cells in rats paralleled by a decrease in the expression of Fzd1, Fzd8, and Fzd9. Our results suggest a role for Wnts in the regulation of the adult spinal cord neurogenic niche and provide new data on the specific differences in this region between humans and rodents.

Neural patterning of human induced pluripotent stem cells in 3-D cultures for studying biomolecule-directed differential cellular responses

INTRODUCTION

Appropriate neural patterning of human induced pluripotent stem cells (hiPSCs) is critical to generate specific neural cells/tissues and even mini-brains that are physiologically relevant to model neurological diseases. However, the capacity of signaling factors that regulate 3-D neural tissue patterning in vitro and differential responses of the resulting neural populations to various biomolecules have not yet been fully understood.

METHODS

By tuning neural patterning of hiPSCs with small molecules targeting sonic hedgehog (SHH) signaling, this study generated different 3-D neuronal cultures that were mainly comprised of either cortical glutamatergic neurons or motor neurons.

RESULTS

Abundant glutamatergic neurons were observed following the treatment with an antagonist of SHH signaling, cyclopamine, while Islet-1 and HB9-expressing motor neurons were enriched by an SHH agonist, purmorphamine. In neurons derived with different neural patterning factors, whole-cell patch clamp recordings showed similar voltage-gated Na(+)/K(+) currents, depolarization-evoked action potentials and spontaneous excitatory post-synaptic currents. Moreover, these different neuronal populations exhibited differential responses to three classes of biomolecules, including (1) matrix metalloproteinase inhibitors that affect extracellular matrix remodeling; (2) N-methyl-d-aspartate that induces general neurotoxicity; and (3) amyloid β (1-42) oligomers that cause neuronal subtype-specific neurotoxicity.

CONCLUSIONS

This study should advance our understanding of hiPSC self-organization and neural tissue development and provide a transformative approach to establish 3-D models for neurological disease modeling and drug discovery.

STATEMENT OF SIGNIFICANCE

Appropriate neural patterning of human induced pluripotent stem cells (hiPSCs) is critical to generate specific neural cells, tissues and even mini-brains that are physiologically relevant to model neurological diseases. However, the capability of sonic hedgehog-related small molecules to tune different neuronal subtypes in 3-D differentiation from hiPSCs and the differential cellular responses of region-specific neuronal subtypes to various biomolecules have not been fully investigated. By tuning neural patterning of hiPSCs with small molecules targeting sonic hedgehog signaling, this study provides knowledge on the differential susceptibility of region-specific neuronal subtypes derived from hiPSCs to different biomolecules in extracellular matrix remodeling and neurotoxicity. The findings are significant for understanding 3-D neural patterning of hiPSCs for the applications in brain organoid formation, neurological disease modeling, and drug discovery.

How Wnt Signaling Builds the Brain: Bridging Development and Disease

Wnt/β-catenin signaling plays a crucial role throughout all stages of brain development and remains important in the adult brain. Accordingly, many neurological disorders have been linked to Wnt signaling. Defects in Wnt signaling during neural development can give rise to birth defects or lead to neurological dysfunction later in life. Developmental signaling events can also be hijacked in the adult and result in disease. Moreover, knowledge about the physiological role of Wnt signaling in the brain might lead to new therapeutic strategies for neurological diseases. Especially, the important role for Wnt signaling in neural differentiation of pluripotent stem cells has received much attention as this might provide a cure for neurodegenerative disorders. In this review, we summarize the versatile role of Wnt/β-catenin signaling during neural development and discuss some recent studies linking Wnt signaling to neurological disorders.

Establishment of Human Neural Progenitor Cells from Human Induced Pluripotent Stem Cells with Diverse Tissue Origins

Human neural progenitor cells (hNPCs) have previously been generated from limited numbers of human induced pluripotent stem cell (hiPSC) clones. Here, 21 hiPSC clones derived from human dermal fibroblasts, cord blood cells, and peripheral blood mononuclear cells were differentiated using two neural induction methods, an embryoid body (EB) formation-based method and an EB formation method using dual SMAD inhibitors (dSMADi). Our results showed that expandable hNPCs could be generated from hiPSC clones with diverse somatic tissue origins. The established hNPCs exhibited a mid/hindbrain-type neural identity and uniform expression of neural progenitor genes.

Wnt-YAP interactions in the neural fate of human pluripotent stem cells and the implications for neural organoid formation

Human pluripotent stem cells (hPSCs) have shown the ability to self-organize into different types of neural organoids (e.g., whole brain organoids, cortical spheroids, midbrain organoids etc.) recently. The extrinsic and intrinsic signaling elicited by Wnt pathway, Hippo/Yes-associated protein (YAP) pathway, and extracellular microenvironment plays a critical role in brain tissue morphogenesis. This article highlights recent advances in neural tissue patterning from hPSCs, in particular the role of Wnt pathway and YAP activity in this process. Understanding the Wnt-YAP interactions should provide us the guidance to predict and modulate brain-like tissue structure through the regulation of extracellular microenvironment of hPSCs.

Comprehensive profiling reveals mechanisms of SOX2-mediated cell fate specification in human ESCs and NPCs

SOX2 is a key regulator of multiple types of stem cells, especially embryonic stem cells (ESCs) and neural progenitor cells (NPCs). Understanding the mechanism underlying the function of SOX2 is of great importance for realizing the full potential of ESCs and NPCs. Here, through genome-wide comparative studies, we show that SOX2 executes its distinct functions in human ESCs (hESCs) and hESC-derived NPCs (hNPCs) through cell type- and stage-dependent transcription programs. Importantly, SOX2 suppresses non-neural lineages in hESCs and regulates neurogenesis from hNPCs by inhibiting canonical Wnt signaling. In hESCs, SOX2 achieves such inhibition by direct transcriptional regulation of important Wnt signaling modulators, WLS and SFRP2. Moreover, SOX2 ensures pluripotent epigenetic landscapes via interacting with histone variant H2A.Z and recruiting polycomb repressor complex 2 to poise developmental genes in hESCs. Together, our results advance our understanding of the mechanism by which cell type-specific transcription factors control lineage-specific gene expression programs and specify cell fate.

Generation of Regionally Specific Neural Progenitor Cells (NPCs) and Neurons from Human Pluripotent Stem Cells (hPSCs)

Neural progenitor cells (NPCs) derived from human pluripotent stem cells (hPSCs) are a multipotent cell population capable of long-term expansion and differentiation into a variety of neuronal subtypes. As such, NPCs have tremendous potential for disease modeling, drug screening, and regenerative medicine. Current methods for the generation of NPCs results in cell populations homogenous for pan-neural markers such as SOX1 and SOX2 but heterogeneous with respect to regional identity. In order to use NPCs and their neuronal derivatives to investigate mechanisms of neurological disorders and develop more physiologically relevant disease models, methods for generation of regionally specific NPCs and neurons are needed. Here, we describe a protocol in which exogenous manipulation of WNT signaling, through either activation or inhibition, during neural differentiation of hPSCs, promotes the formation of regionally homogenous NPCs and neuronal cultures. In addition, we provide methods to monitor and characterize the efficiency of hPSC differentiation to these regionally specific cell identities.

Methods to Manipulate and Monitor Wnt Signaling in Human Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) may revolutionize medical practice by providing: (a) a renewable source of cells for tissue replacement therapies, (b) a powerful system to model human diseases in a dish, and (c) a platform for examining efficacy and safety of novel drugs. Furthermore, these cells offer a unique opportunity to study early human development in vitro, in particular, the process by which a seemingly uniform cell population interacts to give rise to the three main embryonic lineages: ectoderm, endoderm. and mesoderm. This process of lineage allocation is regulated by a number of inductive signals that are mediated by growth factors, including FGF, TGFβ, and Wnt. In this book chapter, we introduce a set of tools, methods, and protocols to specifically manipulate the Wnt signaling pathway with the intention of altering the cell fate outcome of hPSCs.