Efficient specification of interneurons from human pluripotent stem cells by dorsoventral and rostrocaudal modulation

Neuroimmune mechanisms in autism etiology - untangling a complex problem using human cellular models

Autism spectrum disorder (ASD) affects 1 in 36 people and is more often diagnosed in males than in females. Core features of ASD are impaired social interactions, repetitive behaviors and deficits in verbal communication. ASD is a highly heterogeneous and heritable disorder, yet its underlying genetic causes account only for up to 80% of the cases. Hence, a subset of ASD cases could be influenced by environmental risk factors. Maternal immune activation (MIA) is a response to inflammation during pregnancy, which can lead to increased inflammatory signals to the fetus. Inflammatory signals can cross the placenta and blood brain barriers affecting fetal brain development. Epidemiological and animal studies suggest that MIA could contribute to ASD etiology. However, human mechanistic studies have been hindered by a lack of experimental systems that could replicate the impact of MIA during fetal development. Therefore, mechanisms altered by inflammation during human pre-natal brain development, and that could underlie ASD pathogenesis have been largely understudied. The advent of human cellular models with induced pluripotent stem cell (iPSC) and organoid technology is closing this gap in knowledge by providing both access to molecular manipulations and culturing capability of tissue that would be otherwise inaccessible. We present an overview of multiple levels of evidence from clinical, epidemiological, and cellular studies that provide a potential link between higher ASD risk and inflammation. More importantly, we discuss how stem cell-derived models may constitute an ideal experimental system to mechanistically interrogate the effect of inflammation during the early stages of brain development.

Interneuron Transplantation for Drug-Resistant Epilepsy

Current epilepsy surgical techniques, such as brain resection, laser ablation, and neurostimulation, target seizure networks macroscopically, and they may yield an unfavorable balance between seizure reduction, procedural invasiveness, and neurologic morbidity. The transplantation of GABAergic interneurons is a regenerative technique for altering neural inhibition in cortical circuits, with potential as an alternative and minimally invasive approach to epilepsy treatment. This article (1) reviews some of the preclinical evidence supporting interneuron transplantation as an epilepsy therapy, (2) describes a first-in-human study of interneuron transplantation for epilepsy, and (3) considers knowledge gaps that stand before the effective clinical application of this novel treatment.

From cells to insights: the power of human pluripotent stem cell-derived cortical interneurons in psychiatric disorder modeling

Psychiatric disorders, such as schizophrenia (SCZ) and autism spectrum disorders (ASD), represent a global health challenge with their poorly understood and complex etiologies. Cortical interneurons (cINs) are the primary inhibitory neurons in the cortex and their subtypes, especially those that are generated from the medial ganglionic emission (MGE) region, have been shown to play an important role in the pathogenesis of these psychiatric disorders. Recent advances in induced pluripotent stem cell (iPSC) technologies provide exciting opportunities to model and study these disorders using human iPSC-derived cINs. In this review, we present a comprehensive overview of various methods employed to generate MGE-type cINs from human iPSCs, which are mainly categorized into induction by signaling molecules vs. direct genetic manipulation. We discuss their advantages, limitations, and potential applications in psychiatric disorder modeling to aid researchers in choosing the appropriate methods based on their research goals. We also provide examples of how these methods have been applied to study the pathogenesis of psychiatric disorders. In addition, we discuss ongoing challenges and future directions in the field. Overall, iPSC-derived cINs provide a powerful tool to model the developmental pathogenesis of psychiatric disorders, thus aiding in uncovering disease mechanisms and potential therapeutic targets. This review article will provide valuable resources for researchers seeking to navigate the complexities of cIN generation methods and their applications in the study of psychiatric disorders.

Human pallial MGE-type GABAergic interneuron cell therapy for chronic focal epilepsy

Mesial temporal lobe epilepsy (MTLE) is the most common focal epilepsy. One-third of patients have drug-refractory seizures and are left with suboptimal therapeutic options such as brain tissue-destructive surgery. Here, we report the development and characterization of a cell therapy alternative for drug-resistant MTLE, which is derived from a human embryonic stem cell line and comprises cryopreserved, post-mitotic, medial ganglionic eminence (MGE) pallial-type GABAergic interneurons. Single-dose intrahippocampal delivery of the interneurons in a mouse model of chronic MTLE resulted in consistent mesiotemporal seizure suppression, with most animals becoming seizure-free and surviving longer. The grafted interneurons dispersed locally, functionally integrated, persisted long term, and significantly reduced dentate granule cell dispersion, a pathological hallmark of MTLE. These disease-modifying effects were dose-dependent, with a broad therapeutic range. No adverse effects were observed. These findings support an ongoing phase 1/2 clinical trial (NCT05135091) for drug-resistant MTLE.

YBX1-Mediated DNA Methylation-Dependent SHANK3 Expression in PBMCs and Developing Cortical Interneurons in Schizophrenia

Schizophrenia (SCZ) is a severe psychiatric and neurodevelopmental disorder. The pathological process of SCZ starts early during development, way before the first onset of psychotic symptoms. DNA methylation plays an important role in regulating gene expression and dysregulated DNA methylation is involved in the pathogenesis of various diseases. The methylated DNA immunoprecipitation-chip (MeDIP-chip) is performed to investigate genome-wide DNA methylation dysregulation in peripheral blood mononuclear cells (PBMCs) of patients with first-episode SCZ (FES). Results show that the SHANK3 promoter is hypermethylated, and this hypermethylation (HyperM) is negatively correlated with the cortical surface area in the left inferior temporal cortex and positively correlated with the negative symptom subscores in FES. The transcription factor YBX1 is further found to bind to the HyperM region of SHANK3 promoter in induced pluripotent stem cells (iPSCs)-derived cortical interneurons (cINs) but not glutamatergic neurons. Furthermore, a direct and positive regulatory effect of YBX1 on the expression of SHANK3 is confirmed in cINs using shRNAs. In summary, the dysregulated SHANK3 expression in cINs suggests the potential role of DNA methylation in the neuropathological mechanism underlying SCZ. The results also suggest that HyperM of SHANK3 in PBMCs can serve as a potential peripheral biomarker of SCZ.

Neural Differentiation of Induced Pluripotent Stem Cells for a Xenogeneic Material-Free 3D Neurological Disease Model Neurulation from Pluripotent Cells Using a Human Hydrogel

Alzheimer's Disease (AD) is characterized by synapse and neuronal loss and the accumulation of neurofibrillary tangles and Amyloid β plaques. Despite significant research efforts to understand the late stages of the disease, its etiology remains largely unknown. This is in part because of the imprecise AD models in current use. In addition, little attention has been paid to neural stem cells (NSC), which are the cells responsible for the development and maintenance of brain tissue during an individual's lifespan. Thus, an in vitro 3D human brain tissue model using induced pluripotent stem (iPS) cell-derived neural cells in human physiological conditions may be an excellent alternative to standard models to investigate AD pathology. Following the differentiation process mimicking development, iPS cells can be turned into NSCs and, ultimately, neural cells. During differentiation, the traditionally used xenogeneic products may alter the cells' physiology and prevent accurate disease pathology modeling. Hence, establishing a xenogeneic material-free cell culture and differentiation protocol is essential. This study investigated the differentiation of iPS cells to neural cells using a novel extracellular matrix derived from human platelet lysates (PL Matrix). We compared the stemness properties and differentiation efficacies of iPS cells in a PL matrix against those in a conventional 3D scaffold made of an oncogenic murine-matrix. Using well-defined conditions without xenogeneic material, we successfully expanded and differentiated iPS cells into NSCs via dual-SMAD inhibition, which regulates the BMP and TGF signaling cascades in a manner closer to human conditions. This in vitro, 3D, xenogeneic-free scaffold will enhance the quality of disease modeling for neurodegenerative disease research, and the knowledge produced could be used in developing more effective translational medicine.

Single-Cell Transcriptomics Reveals Conserved Regulatory Networks in Human and Mouse Interneuron Development

Inhibitory GABAergic interneurons originate in the embryonic medial ganglionic eminence (MGE) and control network activity in the neocortex. Dysfunction of these cells is believed to lead to runaway excitation underlying seizure-based neurological disorders such as epilepsy, autism, and schizophrenia. Despite their importance in heath and disease, our knowledge about the development of this diverse neuronal population remains incomplete. Here we conducted single-cell RNA sequencing (scRNA-seq) of human foetal MGE from 10 to 15 weeks post conception. These MGE tissues are composed of largely cycling progenitors and immature post-mitotic interneurons with characteristic regional marker expression. Analysis of integrated human and mouse MGE data revealed species-conserved transcriptomic profiles and regulatory programs. Moreover, we identified novel candidate transcription regulators for human interneuron differentiation. These findings provide a framework for in vitro modelling of interneuron development and a strategy for potentially enhancing interneuron production from human pluripotent stem cells.

Human cortical interneurons optimized for grafting specifically integrate, abort seizures, and display prolonged efficacy without over-inhibition

Previously, we demonstrated the efficacy of human pluripotent stem cell (hPSC)-derived GABAergic cortical interneuron (cIN) grafts in ameliorating seizures. However, a safe and reliable clinical translation requires a mechanistic understanding of graft function, as well as the assurance of long-term efficacy and safety. By employing hPSC-derived chemically matured migratory cINs in two models of epilepsy, we demonstrate lasting efficacy in treating seizures and comorbid deficits, as well as safety without uncontrolled growth. Host inhibition does not increase with increasing grafted cIN densities, assuring their safety without the risk of over-inhibition. Furthermore, their closed-loop optogenetic activation aborted seizure activity, revealing mechanisms of graft-mediated seizure control and allowing graft modulation for optimal translation. Monosynaptic tracing shows their extensive and specific synaptic connections with host neurons, resembling developmental connection specificity. These results offer confidence in stem cell-based therapy for epilepsy as a safe and reliable treatment for patients suffering from intractable epilepsy.

Mutations in the transcriptional regulator MeCP2 severely impact key cellular and molecular signatures of human astrocytes during maturation

Mutations in the MECP2 gene underlie a spectrum of neurodevelopmental disorders, most commonly Rett syndrome (RTT). We ask whether MECP2 mutations interfere with human astrocyte developmental maturation, thereby affecting their ability to support neurons. Using human-based models, we show that RTT-causing MECP2 mutations greatly impact the key role of astrocytes in regulating overall brain bioenergetics and that these metabolic aberrations are likely mediated by dysfunctional mitochondria. During post-natal maturation, astrocytes rely on neurons to induce their complex stellate morphology and transcriptional changes. While MECP2 mutations cause cell-intrinsic aberrations in the astrocyte transcriptional landscape, surprisingly, they do not affect the neuron-induced astrocyte gene expression. Notably, however, astrocytes are unable to develop complex mature morphology due to cell- and non-cell-autonomous aberrations caused by MECP2 mutations. Thus, MECP2 mutations critically impact key cellular and molecular features of human astrocytes and, hence, their ability to interact and support the structural and functional maturation of neurons.

Generation of Homogeneous Populations of Cortical Interneurons from Human Pluripotent Stem Cells

Cortical interneurons (cINs), especially those that are derived from the medial ganglionic eminence (MGE) during early development, are associated with various neuropsychiatric disorders. Human pluripotent stem cell (hPSC)-derived cINs can provide unlimited cell sources for studying disease mechanisms and developing novel therapeutics. Here, we describe an optimized method to generate homogeneous cIN populations based on three-dimensional (3D) cIN sphere generation. This optimized differentiation system could sustain generated cINs relatively long term without compromising their survival or phenotypes.

Impact of schizophrenia GWAS loci converge onto distinct pathways in cortical interneurons vs glutamatergic neurons during development

Remarkable advances have been made in schizophrenia (SCZ) GWAS, but gleaning biological insight from these loci is challenging. Genetic influences on gene expression (e.g., eQTLs) are cell type-specific, but most studies that attempt to clarify GWAS loci's influence on gene expression have employed tissues with mixed cell compositions that can obscure cell-specific effects. Furthermore, enriched SCZ heritability in the fetal brain underscores the need to study the impact of SCZ risk loci in specific developing neurons. MGE-derived cortical interneurons (cINs) are consistently affected in SCZ brains and show enriched SCZ heritability in human fetal brains. We identified SCZ GWAS risk genes that are dysregulated in iPSC-derived homogeneous populations of developing SCZ cINs. These SCZ GWAS loci differential expression (DE) genes converge on the PKC pathway. Their disruption results in PKC hyperactivity in developing cINs, leading to arborization deficits. We show that the fine-mapped GWAS locus in the ATP2A2 gene of the PKC pathway harbors enhancer marks by ATACseq and ChIPseq, and regulates ATP2A2 expression. We also generated developing glutamatergic neurons (GNs), another population with enriched SCZ heritability, and confirmed their functionality after transplantation into the mouse brain. Then, we identified SCZ GWAS risk genes that are dysregulated in developing SCZ GNs. GN-specific SCZ GWAS loci DE genes converge on the ion transporter pathway, distinct from those for cINs. Disruption of the pathway gene CACNA1D resulted in deficits of Ca2+ currents in developing GNs, suggesting compromised neuronal function by GWAS loci pathway deficits during development. This study allows us to identify cell type-specific and developmental stage-specific mechanisms of SCZ risk gene function, and may aid in identifying mechanism-based novel therapeutic targets.

Probing the molecular and cellular pathological mechanisms of schizophrenia using human induced pluripotent stem cell models

With recent advancements in psychiatric genomics, as a field, "stem cell-based disease modelers" were given the exciting yet daunting task of translating the extensive list of disease-associated risks into biologically and clinically relevant information in order to deliver therapeutically meaningful leads and insights. Despite their limitations, human induced pluripotent stem cell (iPSCs) based models have greatly aided our understanding of the molecular and cellular mechanisms underlying the complex etiology of brain disorders including schizophrenia (SCZ). In this review, we summarize the major findings from studies in the past decade which utilized iPSC models to investigate cell type-specific phenotypes relevant to idiopathic SCZ and disease penetrant alleles. Across cell type differences, several biological themes emerged, serving as potential neurodevelopmental mechanisms of SCZ, including oxidative stress and mitochondrial dysfunction, depletion of progenitor pools and insufficient differentiation potential of these progenitors, and structural and functional deficits of neurons and other supporting cells. Here, we discuss both the recent progress as well as challenges and improvements needed for future studies utilizing iPSCs as a model for SCZ and other neuropsychiatric disorders.

Altered patterning of trisomy 21 interneuron progenitors

Individuals with Down syndrome (DS; Ts21), the most common genetic cause of intellectual disability, have smaller brains that reflect fewer neurons at pre- and post-natal stages, implicating impaired neurogenesis during development. Our stereological analysis of adult DS cortex indicates a reduction of calretinin-expressing interneurons. Using Ts21 human induced pluripotent stem cells (iPSCs) and isogenic controls, we find that Ts21 progenitors generate fewer COUP-TFII+ progenitors with reduced proliferation. Single-cell RNA sequencing of Ts21 progenitors confirms the altered specification of progenitor subpopulations and identifies reduced WNT signaling. Activation of WNT signaling partially restores the COUP-TFII+ progenitor population in Ts21, suggesting that altered WNT signaling contributes to the defective development of cortical interneurons in DS.

Quality criteria for in vitro human pluripotent stem cell-derived models of tissue-based cells

The advent of the technology to isolate or generate human pluripotent stem cells provided the potential to develop a wide range of human models that could enhance understanding of mechanisms underlying human development and disease. These systems are now beginning to mature and provide the basis for the development of in vitro assays suitable to understand the biological processes involved in the multi-organ systems of the human body, and will improve strategies for diagnosis, prevention, therapies and precision medicine. Induced pluripotent stem cell lines are prone to phenotypic and genotypic changes and donor/clone dependent variability, which means that it is important to identify the most appropriate characterization markers and quality control measures when sourcing new cell lines and assessing differentiated cell and tissue culture preparations for experimental work. This paper considers those core quality control measures for human pluripotent stem cell lines and evaluates the state of play in the development of key functional markers for their differentiated cell derivatives to promote assurance of reproducibility of scientific data derived from pluripotent stem cell-based systems.

A Novel LHX6 Reporter Cell Line for Tracking Human iPSC-Derived Cortical Interneurons

GABAergic interneurons control the neural circuitry and network activity in the brain. The dysfunction of cortical interneurons, especially those derived from the medial ganglionic eminence, contributes to neurological disease states. Pluripotent stem cell-derived interneurons provide a powerful tool for understanding the etiology of neuropsychiatric disorders, as well as having the potential to be used as medicine in cell therapy for neurological conditions such as epilepsy. Although large numbers of interneuron progenitors can be readily induced in vitro, the generation of defined interneuron subtypes remains inefficient. Using CRISPR/Cas9-assisted homologous recombination in hPSCs, we inserted the coding sequence of mEmerald and mCherry fluorescence protein, respectively, downstream that of the LHX6, a gene required for, and a marker of medial ganglionic eminence (MGE)-derived cortical interneurons. Upon differentiation of the LHX6-mEmerald and LHX6-mCherry hPSCs towards the MGE fate, both reporters exhibited restricted expression in LHX6⁺ MGE derivatives of hPSCs. Moreover, the reporter expression responded to changes of interneuron inductive cues. Thus, the LHX6-reporter lines represent a valuable tool to identify molecules controlling human interneuron development and design better interneuron differentiation protocols as well as for studying risk genes associated with interneuronopathies.

Innovations and advances in modelling and measuring pain in animals

Best practices in preclinical algesiometry (pain behaviour testing) have shifted over the past decade as a result of technological advancements, the continued dearth of translational progress and the emphasis that funding institutions and journals have placed on rigour and reproducibility. Here we describe the changing trends in research methods by analysing the methods reported in preclinical pain publications from the past 40 years, with a focus on the last 5 years. We also discuss how the status quo may be hampering translational success. This discussion is centred on four fundamental decisions that apply to every pain behaviour experiment: choice of subject (model organism), choice of assay (pain-inducing injury), laboratory environment and choice of outcome measures. Finally, we discuss how human tissues, which are increasingly accessible, can be used to validate the translatability of targets and mechanisms identified in animal pain models.

MGE-Like Neural Progenitor Cell Survival and Expression of Parvalbumin and Proenkephalin in a Jaundiced Rat Model of Kernicterus

Kernicterus is a permanent condition caused by brain damage from bilirubin toxicity. Dystonia is one of the most debilitating symptoms of kernicterus and results from damage to the globus pallidus (GP). One potential therapeutic strategy to treat dystonia in kernicterus is to replace lost GP neurons and restore basal ganglia circuits through stem cell transplantation. Toward this end, we differentiated human embryonic stem cells (hESCs) into medial ganglion eminence (MGE; the embryological origin of most of the GP neurons)-like neural precursor cells (NPCs). We determined neurochemical phenotype in cell culture and after transplanting into the GP of jaundiced Gunn rats. We also determined grafted cell survival as well as migration, distribution, and morphology after transplantation. As in the GP, most cultured MGE-like NPCs expressed γ-aminobutyric acid (GABA), with some co-expressing markers for parvalbumin (PV) and others expressing markers for pro-enkephalin (PENK). MGE-like NPCs survived in brains at least 7 weeks after transplantation, with most aggregating near the injection site. Grafted cells expressed GABA and PV or PENK as in the normal GP. Although survival was low and the maturity of grafted cells varied, many cells produced neurite outgrowth. While promising, our results suggest the need to further optimize the differentiation protocol for MGE-like NPC for potential use in treating dystonia in kernicterus.

Identification of TGFβ signaling as a regulator of interneuron neurogenesis in a human pluripotent stem cell model

Cortical interneurons are GABAergic inhibitory cells that connect locally in the neocortex and play a pivotal role in shaping cortical network activities. Dysfunction of these cells is believed to lead to runaway excitation underlying seizure-based diseases, such as epilepsy, autism and schizophrenia. There is a growing interest in using cortical interneurons derived from human pluripotent stem cells for understanding their complex development and for modeling neuropsychiatric diseases. Here, we report the identification of a novel role of transforming growth factor β (TGFβ) signaling in modulating interneuron progenitor maintenance and neuronal differentiation. TGFβ signaling inhibition suppresses terminal differentiation of interneuron progenitors, while exogenous TGFβ3 accelerates the transition of progenitors into postmitotic neurons. We provide evidence that TGFb signaling exerts this function via regulating cell cycle length of the NKX2.1+ neural progenitors. Together, the present study represents a useful platform for studying human interneuron development and interneuron-associated neurological diseases with human pluripotent stem cells.

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.

Interneuron Dysfunction and Inhibitory Deficits in Autism and Fragile X Syndrome

The alteration of excitatory–inhibitory (E–I) balance has been implicated in various neurological and psychiatric diseases, including autism spectrum disorder (ASD). Fragile X syndrome (FXS) is a single-gene disorder that is the most common known cause of ASD. Understanding the molecular and physiological features of FXS is thought to enhance our knowledge of the pathophysiology of ASD. Accumulated evidence implicates deficits in the inhibitory circuits in FXS that tips E–I balance toward excitation. Deficits in interneurons, the main source of an inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), have been reported in FXS, including a reduced number of cells, reduction in intrinsic cellular excitability, or weaker synaptic connectivity. Manipulating the interneuron activity ameliorated the symptoms in the FXS mouse model, which makes it reasonable to conceptualize FXS as an interneuronopathy. While it is still poorly understood how the developmental profiles of the inhibitory circuit go awry in FXS, recent works have uncovered several developmental alterations in the functional properties of interneurons. Correcting disrupted E–I balance by potentiating the inhibitory circuit by targeting interneurons may have a therapeutic potential in FXS. I will review the recent evidence about the inhibitory alterations and interneuron dysfunction in ASD and FXS and will discuss the future directions of this field.

[Directed differentiation of porcine induced pluripotent stem cells into forebrain GABAergic neuron progenitors]

OBJECTIVE

To establish an efficient protocol for directed differentiation of miniature-swine induced pluripotent stem cells (iPSCs) into GABAergic progenitors in a chemically defined system.

OBJECTIVE

We adopted a two-stage protocol for inducing the differentiation of porcine iPSCs. In the first stage, embryoid bodies (EBs) derived from porcine iPSCs after 3 days of suspension culture were induced in neural induction medium (containing SB431542, DMH1 and FGF2) till day 12 to differentiate into primitive neuroepithelia cells (NECs). In the second stage, the primitive NECs were induced in neural induction medium (containing Pur and B27) to obtain neural rosettes, which further differentiated into GABAergic neuron progenitors on day 21. After labeling with CM-DiI, the progenitor cells were stereotactically transplanted into the substantia nigra (SN) of 6-OHDA-lesioned PD model rats, and the cell survival, migration and differentiation in vivo were observed.

OBJECTIVE

Porcine iPSCs could be passaged stably on the feeder cell layer and expressed the pluripotent stem cell markers OCT4, Nanog, SSEA1and TRA-160. Karyotype analysis demonstrated the absence of contamination by cells from other species. On day 12 of induced differentiation, the cells formed adherent colonies containing NECs in the form of neural rosettes, which expressed the neuroepithelial markers PAX6, SOX2 and Nestin and the neurite marker beta Ⅲ Tubulin (Tuj1). After induction for 21 days, the NECs differentiated into GABAergic neural progenitors highly expressing NKX2.1 and FOXG1. Eight weeks after transplantation, the iPSCs-iGABA progeniters survived in the striatum of the PD rats, where they differentiate into GABAergic neurons and TH+ neurons and significantly improved dyskinesia of the rats.

OBJECTIVE

The miniature-swine iPSCsderived GABA progenitors may serve as promising donor cells for neural grafting for treatment of neurodegenerative diseases.

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.

Migratory cortical interneuron-specific transcriptome abnormalities in schizophrenia

Cortical interneurons (cINs) are substantially affected in Schizophrenia (SCZ) and enriched for SCZ heritability during development. To understand SCZ-specific changes in these cells during development, we isolated migratory cINs from cIN spheres derived from 5 healthy control (HC) and 5 SCZ induced pluripotent stem cell lines (iPSCs). Transcriptome analyses show dysregulation in extracellular matrix pathways as the major disturbances in SCZ migratory cINs, whereas sphere cINs show dysregulation in immune pathways. This result suggests the importance of using homogeneous cell populations to identify stage-specific abnormalities and provides a platform to further study the biology of schizophrenia pathogenesis during early development.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α’s roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington’s Disease, Parkinson’s Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α's roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington's Disease, Parkinson's Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Human forebrain endothelial cell therapy for psychiatric disorders

Abnormalities of or reductions in GABAergic interneurons are implicated in the pathology of severe neuropsychiatric disorders, for which effective treatments are still elusive. Transplantation of human stem cell-derived interneurons is a promising cell-based therapy for treatment of these disorders. In mouse xenograft studies, human stem cell-derived-interneuron precursors could differentiate in vivo, but required a prolonged time of four to seven months to migrate from the graft site and integrate with the host tissue. This poses a serious roadblock for clinical translation of this approach. For transplantation to be effective, grafted neurons should migrate to affected areas at a faster rate. We have previously shown that endothelial cells of the periventricular vascular network are the natural substrates for GABAergic interneurons in the developing mouse forebrain, and provide valuable guidance cues for their long-distance migration. In addition, periventricular endothelial cells house a GABA signaling pathway with direct implications for psychiatric disease origin. In this study we translated this discovery into human, with significant therapeutic implications. We generated human periventricular endothelial cells, using human pluripotent stem cell technology, and extensively characterized its molecular, cellular, and functional properties. Co-culture of human periventricular endothelial cells with human interneurons significantly accelerated interneuron migration in vitro and led to faster migration and wider distribution of grafted interneurons in vivo, compared to neuron-only transplants. Furthermore, the co-transplantation strategy was able to rescue abnormal behavioral symptoms in a pre-clinical model of psychiatric disorder, within 1 month after transplantation. We anticipate this strategy to open new doors and facilitate exciting advances in angiogenesis-mediated treatment of psychiatric disorders.

iPSC-derived homogeneous populations of developing schizophrenia cortical interneurons have compromised mitochondrial function

Schizophrenia (SCZ) is a neurodevelopmental disorder. Thus, studying pathogenetic mechanisms underlying SCZ requires studying the development of brain cells. Cortical interneurons (cINs) are consistently observed to be abnormal in SCZ postmortem brains. These abnormalities may explain altered gamma oscillation and cognitive function in patients with SCZ. Of note, currently used antipsychotic drugs ameliorate psychosis, but they are not very effective in reversing cognitive deficits. Characterizing mechanisms of SCZ pathogenesis, especially related to cognitive deficits, may lead to improved treatments. We generated homogeneous populations of developing cINs from 15 healthy control (HC) iPSC lines and 15 SCZ iPSC lines. SCZ cINs, but not SCZ glutamatergic neurons, show dysregulated Oxidative Phosphorylation (OxPhos) related gene expression, accompanied by compromised mitochondrial function. The OxPhos deficit in cINs could be reversed by Alpha Lipoic Acid/Acetyl-L-Carnitine (ALA/ALC) but not by other chemicals previously identified as increasing mitochondrial function. The restoration of mitochondrial function by ALA/ALC was accompanied by a reversal of arborization deficits in SCZ cINs. OxPhos abnormality, even in the absence of any circuit environment with other neuronal subtypes, appears to be an intrinsic deficit in SCZ cINs.

Activated microglia cause metabolic disruptions in developmental cortical interneurons that persist in interneurons from individuals with schizophrenia

The mechanisms by which prenatal immune activation increase risk for neuropsychiatric disorders are unclear. Here, we generated developmental cortical interneurons (cINs), known to be affected in schizophrenia (SCZ) when matured, from induced pluripotent stem cells (iPSCs) from healthy controls (HC) and SCZ patients, and cocultured them with or without activated microglia. Coculture with activated microglia disturbed metabolic pathways, as indicated by unbiased transcriptome analysis, and impaired mitochondrial function, arborization, synapse formation and synaptic GABA release. Deficits in mitochondrial function and arborization were reversed by Alpha Lipoic Acid/Acetyl-L-Carnitine (ALA/ALC) treatments that boost mitochondrial function. Notably, activated microglia-conditioned medium altered metabolism in cINs and HC-derived iPSCs but not in SCZ-patient-derived iPSCs or in glutamatergic neurons. After removal of activated microglia-conditioned medium, SCZ cINs but not HC cINs showed prolonged metabolic deficits, suggesting an interaction between SCZ genetic backgrounds and environmental risk factors.

Generation of cerebral cortical GABAergic interneurons from pluripotent stem cells

The cerebral cortex functions by the complex interactions of intrinsic and extrinsic neuronal activities, glial actions, and the effects of humoral factors. The intrinsic neuronal influences are mediated by two major subclasses: excitatory glutamatergic neurons that generally have axonal projections extending beyond the neuron's locality and inhibitory GABAergic neurons that generally project locally. These interneurons can be grouped based on morphological, neurochemical, electrophysiological, axonal targeting, and circuit influence characteristics. Cortical interneurons (CIns) can also be grouped based on their origins within the subcortical telencephalon. Interneuron subtypes, of which a dozen or more are thought to exist, are characterized by combinations of these subgrouping features. Due to their well-documented relevance to the causes of and treatments for neuropsychiatric disorders, and to their remarkable capacity to migrate extensively following transplantation, there has been tremendous interest in generating cortical GABAergic interneurons from human pluripotent stem cells. In this concise review, we discuss recent progress in understanding how interneuron subtypes are generated in vivo, and how that progress is being applied to the generation of rodent and human CIns in vitro. In addition, we will discuss approaches for the rigorous designation of interneuron subgroups or subtypes in transplantation studies, and challenges to this field, including the protracted maturation of human interneurons.

Stem cell therapy for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss and cognitive impairment. It is caused by synaptic failure and excessive accumulation of misfolded proteins. To date, almost all advanced clinical trials on specific AD-related pathways have failed mostly due to a large number of neurons lost in the brain of patients with AD. Also, currently available drug candidates intervene too late. Stem cells have improved characteristics of self-renewal, proliferation, differentiation, and recombination with the advent of stem cell technology and the transformation of these cells into different types of central nervous system neurons and glial cells. Stem cell treatment has been successful in AD animal models. Recent preclinical studies on stem cell therapy for AD have proved to be promising. Cell replacement therapies, such as human embryonic stem cells or induced pluripotent stem cell-derived neural cells, have the potential to treat patients with AD, and human clinical trials are ongoing in this regard. However, many steps still need to be taken before stem cell therapy becomes a clinically feasible treatment for human AD and related diseases. This paper reviews the pathophysiology of AD and the application prospects of related stem cells based on cell type.

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.

All-trans retinoic acid induces reprogramming of canine dedifferentiated cells into neuron-like cells

The specification of cell identity depends on the exposure of cells to sequences of bioactive ligands. All-trans retinoic acid (ATRA) affects neuronal development in the early stage, and it is involved in neuronal lineage reprogramming. We previously established a fibroblast-like dedifferentiated fat cells (DFATs) derived from highly homogeneous mature adipocytes, which are more suitable for the study of cellular reprogramming. Canine cognitive dysfunction is similar to human cognitive dysfunction, suggesting that dogs could be a pathological and pharmacological model for human neuronal diseases. However, the effect of ATRA on neuronal reprogramming in dogs has remained unclear. Therefore, in this study, we investigated the effect of ATRA on the neuronal reprogramming of canine DFATs. ATRA induced the expression of neuronal marker mRNA/protein. The neuron-like cells showed Ca²⁺ influx with depolarization (50 mM KCl; 84.75 ± 4.05%) and Na⁺ channel activation (50 μM veratridine; 96.02 ± 2.02%). Optical imaging of presynaptic terminal activity and detection of neurotransmitter release showed that the neuron-like cells exhibited the GABAergic neuronal property. Genome-wide RNA-sequencing analysis shows that the transcriptome profile of canine DFATs is effectively reprogrammed towards that of cortical interneuron lineage. Collectively, ATRA can produce functional GABAergic cortical interneuron-like cells from canine DFATs, exhibiting neuronal function with > 80% efficiency. We further demonstrated the contribution of JNK3 to ATRA-induced neuronal reprogramming in canine DFATs. In conclusion, the neuron-like cells from canine DFATs could be a powerful tool for translational research in cell transplantation therapy, in vitro disease modeling, and drug screening for neuronal diseases.

ASCL1- and DLX2-induced GABAergic neurons from hiPSC-derived NPCs

BACKGROUND

Somatic cell reprogramming is routinely used to generate donor-specific human induced pluripotent stem cells (hiPSCs) to facilitate studies of disease in a human context. The directed differentiation of hiPSCs can generate large quantities of patient-derived cells; however, such methodologies frequently yield heterogeneous populations of neurons and glia that require extended timelines to achieve electrophysiological maturity. More recently, transcription factor-based induction protocols have been show to rapidly generate defined neuronal populations from hiPSCs.

NEW METHOD

In a manner similar to our previous adaption of NGN2-glutamatergic neuronal induction from hiPSC-derived neural progenitor cells (NPCs), we now adapt an established protocol of lentiviral overexpression of ASCL1 and DLX2 to hiPSC-NPCs.

RESULTS

We demonstrate induction of a robust and highly pure population of functional GABAergic neurons (iGANs). Importantly, we successfully applied this technique to hiPSC-NPCs derived from ten donors across two independent laboratories, finding it to be an efficient and highly reproducible approach to generate induced GABAergic neurons. Our results show that, like hiPSC-iGANs, NPC-iGANs exhibit increased GABAergic marker expression, electrophysiological maturity, and have distinct transcriptional profiles that distinguish them from other cell-types of the brain. Nonetheless, until donor-matched hiPSCs-iGANs and NPC-iGANs are directly compared, we cannot rule out the possibility that subtle differences in patterning or maturity may exist between these populations; one should always control for cell source in all iGAN experiments.

CONCLUSIONS

This methodology, relying upon an easily cultured starting population of hiPSC-NPCs, makes possible the generation of large-scale defined co-cultures of induced glutamatergic and GABAergic neurons for hiPSC-based disease models and precision drug screening.

Vascular Integrity and Signaling Determining Brain Development, Network Excitability, and Epileptogenesis

Our understanding of the etiological mechanisms leading up to epilepsy has undergone radical changes over time due to more insights into the complexity of the disease. The traditional hypothesis emphasized network hyperexcitability and an imbalance of inhibition and excitation, eventually leading to seizures. In this context, the contribution of the vascular system, and particularly the interactions between blood vessels and neuronal tissue, came into focus only recently. Thus, one highly exciting causative or contributing factor of epileptogenesis is the disruption of the blood-brain barrier (BBB) in the context of not only posttraumatic epilepsy, but also other etiologies. This hypothesis is now recognized as a synergistic mechanism that can give rise to epilepsy, and BBB repair for restoration of cerebrovascular integrity is considered a therapeutic alternative. Endothelial cells lining the inner surface of blood vessels are an integral component of the BBB system. Sealed by tight junctions, they are crucial in maintaining homeostatic activities of the brain, as well as acting as an interface in the neurovascular unit. Additional potential vascular mechanisms such as inflammation, altered neurovascular coupling, or changes in blood flow that can modulate neuronal circuit activity have been implicated in epilepsy. Our own work has shown how intrinsic defects within endothelial cells from the earliest developmental time points, which preclude neuronal changes, can lead to vascular abnormalities and autonomously support the development of hyperexcitability and epileptiform activity. In this article, we review the importance of vascular integrity and signaling for network excitability and epilepsy by highlighting complementary basic and clinical research studies and by outlining possible novel therapeutic strategies.

All Together Now: Modeling the Interaction of Neural With Non-neural Systems Using Organoid Models

The complex development of the human nervous system has been traditionally studied using a combination of animal models, human post-mortem brain tissue, and human genetics studies. However, there has been a lack of experimental human cellular models that would allow for a more precise elucidation of the intricate dynamics of early human brain development. The development of stem cell technologies, both embryonic and induced pluripotent stem cells (iPSCs), has given neuroscientists access to the previously inaccessible early stages of human brain development. In particular, the recent development of three-dimensional culturing methodologies provides a platform to study the differentiation of stem cells in both normal development and disease states in a more in vivo like context. Three-dimensional neural models or cerebral organoids possess an innate advantage over two-dimensional neural cultures as they can recapitulate tissue organization and cell type diversity that resemble the developing brain. Brain organoids also provide the exciting opportunity to model the integration of different brain regions in vitro. Furthermore, recent advances in the differentiation of non-neuronal tissue from stem cells provides the opportunity to study the interaction between the developing nervous system and other non-neuronal systems that impact neuronal function. In this review, we discuss the potential and limitations of the organoid system to study in vitro neurological diseases that arise in the neuroendocrine and the enteric nervous system or from interactions with the immune system.

Large-Scale Generation and Characterization of Homogeneous Populations of Migratory Cortical Interneurons from Human Pluripotent Stem Cells

During development, cortical interneurons (cINs) are generated from the ventral telencephalon, robustly migrate to the dorsal telencephalon, make local synaptic connections, and critically regulate brain circuitry by inhibiting other neurons. Thus, their abnormality is associated with various brain disorders. Human pluripotent stem cell (hPSC)-derived cINs can provide unlimited sources with which to study the pathogenesis mechanism of these disorders as well as provide a platform to develop novel therapeutics. By employing spinner culture, we could obtain a >10-fold higher yield of cIN progenitors compared to conventional culture without affecting their phenotype. Generated cIN spheres can be maintained feeder-free up to 10 months and are optimized for passaging and cryopreservation. In addition, we identified a combination of chemicals that synchronously matures generated progenitors into SOX6⁺KI67⁻ migratory cINs and extensively characterized their maturation in terms of metabolism, migration, arborization, and electrophysiology. When transplanted into mouse brains, chemically matured migratory cINs generated grafts that efficiently disperse and integrate into the host circuitry without uncontrolled growth, making them an optimal cell population for cell therapy. Efficient large-scale generation of homogeneous migratory cINs without the need of feeder cells will play a critical role in the full realization of hPSC-derived cINs for development of novel therapeutics.

Concise Review: Human-Animal Neurological Chimeras: Humanized Animals or Human Cells in an Animal?

Blastocyst complementation is an emerging methodology in which human stem cells are transferred into genetically engineered preimplantation animal embryos eventually giving rise to fully developed human tissues and organs within the animal host for use in regenerative medicine. The ethical issues surrounding this method have caused the National Institutes of Health to issue a moratorium on funding for blastocyst complementation citing the potential for human cells to substantially contribute to the brain of the chimeric animal. To address this concern, we performed an in-depth review of the neural transplantation literature to determine how the integration of human cells into the nonhuman neural circuitry has altered the behavior of the host. Despite reports of widespread integration of human cell transplants, our review of 150 transplantation studies found no evidence suggestive of humanization of the animal host, and we thus conclude that, at present, concerns over humanization should not prevent research on blastocyst complementation to continue. We suggest proceeding in a controlled and transparent manner, however, and include recommendations for future research with careful consideration for how human cells may contribute to the animal host nervous system. Stem Cells 2019;37:444-452.

Dysregulated protocadherin-pathway activity as an intrinsic defect in induced pluripotent stem cell-derived cortical interneurons from subjects with schizophrenia

We generated cortical interneurons (cINs) from iPSCs derived from14 healthy controls (HC cINs) and 14 patients with schizophrenia (SCZ cINs). Both HC cINs and SCZ cINs were authentic, fired spontaneously, received functional excitatory inputs from host neurons, and induced GABA-mediated inhibition in host neurons in vivo. However, SCZ cINs had dysregulated expression of protocadherin genes, which lie within documented SCZ loci. Mice lacking protocadherin α showed defective arborization and synaptic density of prefrontal cortex cINs and behavioral abnormalities. SCZ cINs similarly showed defects in synaptic density and arborization, which were reversed by inhibitors of Protein Kinase C, a downstream kinase in the protocadherin pathway. These findings reveal an intrinsic abnormality in SCZ cINs in the absence of any circuit-driven pathology. They also demonstrate the utility of homogenous and functional populations of a relevant neuronal subtype for probing pathogenesis mechanisms during development.

Human Pluripotent Stem Cell-Derived Striatal Interneurons: Differentiation and Maturation In Vitro and in the Rat Brain

Striatal interneurons are born in the medial and caudal ganglionic eminences (MGE and CGE) and play an important role in human striatal function and dysfunction in Huntington's disease and dystonia. MGE/CGE-like neural progenitors have been generated from human pluripotent stem cells (hPSCs) for studying cortical interneuron development and cell therapy for epilepsy and other neurodevelopmental disorders. Here, we report the capacity of hPSC-derived MGE/CGE-like progenitors to differentiate into functional striatal interneurons. In vitro, these hPSC neuronal derivatives expressed cortical and striatal interneuron markers at the mRNA and protein level and displayed maturing electrophysiological properties. Following transplantation into neonatal rat striatum, progenitors differentiated into striatal interneuron subtypes and were consistently found in the nearby septum and hippocampus. These findings highlight the potential for hPSC-derived striatal interneurons as an invaluable tool in modeling striatal development and function in vitro or as a source of cells for regenerative medicine.

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hPSCs differentiate into cortical and striatal interneuron-like cells in vitro

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They present mature electrophysiological and morphological properties in vitro

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They express striatal interneuron subtype markers upon transplantation in rat brain

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hPSC-interneuron-like cells adopt region-specific morphologies in vivo

Cortical interneuron function in autism spectrum condition

Cortical interneurons (INs) are a diverse group of neurons that project locally and shape the function of neural networks throughout the brain. Multiple lines of evidence suggest that a proper balance of glutamate and GABA signaling is essential for both the proper function and development of the brain. Dysregulation of this system may lead to neurodevelopmental disorders, including autism spectrum condition (ASC). We evaluate the development and function of INs in rodent and human models and examine how neurodevelopmental dysfunction may produce core symptoms of ASC. Finding common physiological mechanisms that underlie neurodevelopmental disorders may lead to novel pharmacological targets and candidates that could improve the cognitive and emotional symptoms associated with ASC.

Human Models Are Needed for Studying Human Neurodevelopmental Disorders

The analysis of animal models of neurological disease has been instrumental in furthering our understanding of neurodevelopment and brain diseases. However, animal models are limited in revealing some of the most fundamental aspects of development, genetics, pathology, and disease mechanisms that are unique to humans. These shortcomings are exaggerated in disorders that affect the brain, where the most significant differences between humans and animal models exist, and could underscore failures in targeted therapeutic interventions in affected individuals. Human pluripotent stem cells have emerged as a much-needed model system for investigating human-specific biology and disease mechanisms. However, questions remain regarding whether these cell-culture-based models are sufficient or even necessary. In this review, we summarize human-specific features of neurodevelopment and the most common neurodevelopmental disorders, present discrepancies between animal models and human diseases, demonstrate how human stem cell models can provide meaningful information, and discuss the challenges that exist in our pursuit to understand distinctively human aspects of neurodevelopment and brain disease. This information argues for a more thoughtful approach to disease modeling through consideration of the valuable features and limitations of each model system, be they human or animal, to mimic disease characteristics.

Induced pluripotent stem cells (iPSCs) as model to study inherited defects of neurotransmission in inborn errors of metabolism

The ability to reprogram somatic cells to induced pluripotent stem cells (iPSCs) has revolutionized the way of modeling human disease. Especially for the modeling of rare human monogenetic diseases with limited numbers of patients available worldwide and limited access to the mostly affected tissues, iPSCs have become an invaluable tool. To study rare diseases affecting neurotransmitter biosynthesis and neurotransmission, stem cell models carrying patient-specific mutations have become highly important as most of the cell types present in the human brain and the central nervous system (CNS), including motoneurons, neurons, oligodendrocytes, astrocytes, and microglia, can be differentiated from iPSCs following distinct developmental programs. Differentiation can be performed using classical 2D differentiation protocols, thereby generating specific subtypes of neurons or glial cells in a dish. On the other side, 3D differentiation into "organoids" opened new ways to study misregulated developmental processes associated with rare neurological and neurometabolic diseases. For the analysis of defects in neurotransmission associated with rare neurometabolic diseases, different types of brain organoids have been made available during the last years including forebrain, midbrain and cerebral organoids. In this review, we illustrate reprogramming of somatic cells to iPSCs, differentiation in 2D and 3D, as well as already available disease-specific iPSC models, and discuss current and future applications of these techniques.

Induction of human somatostatin and parvalbumin neurons by expressing a single transcription factor LIM homeobox 6

Human GABAergic interneurons (GIN) are implicated in normal brain function and in numerous mental disorders. However, the generation of functional human GIN subtypes from human pluripotent stem cells (hPSCs) has not been established. By expressing LHX6, a transcriptional factor that is critical for GIN development, we induced hPSCs to form GINs, including somatostatin (SST, 29%) and parvalbumin (PV, 21%) neurons. Our RNAseq results also confirmed the alteration of GIN identity with the overexpression of LHX6. Five months after transplantation into the mouse brain, the human GABA precursors generated increased population of SST and PV neurons by overexpressing LHX6. Importantly, the grafted human GINs exhibited functional electrophysiological properties and even fast-spiking-like action potentials. Thus, expression of the single transcription factor LHX6 under our GIN differentiation condition is sufficient to robustly induce human PV and SST subtypes.

In vitro Models for Seizure-Liability Testing Using Induced Pluripotent Stem Cells

The brain is the most complex organ in the body, controlling our highest functions, as well as regulating myriad processes which incorporate the entire physiological system. The effects of prospective therapeutic entities on the brain and central nervous system (CNS) may potentially cause significant injury, hence, CNS toxicity testing forms part of the “core battery” of safety pharmacology studies. Drug-induced seizure is a major reason for compound attrition during drug development. Currently, the rat ex vivo hippocampal slice assay is the standard option for seizure-liability studies, followed by primary rodent cultures. These models can respond to diverse agents and predict seizure outcome, yet controversy over the relevance, efficacy, and cost of these animal-based methods has led to interest in the development of human-derived models. Existing platforms often utilize rodents, and so lack human receptors and other drug targets, which may produce misleading data, with difficulties in inter-species extrapolation. Current electrophysiological approaches are typically used in a low-throughput capacity and network function may be overlooked. Human-derived induced pluripotent stem cells (iPSCs) are a promising avenue for neurotoxicity testing, increasingly utilized in drug screening and disease modeling. Furthermore, the combination of iPSC-derived models with functional techniques such as multi-electrode array (MEA) analysis can provide information on neuronal network function, with increased sensitivity to neurotoxic effects which disrupt different pathways. The use of an in vitro human iPSC-derived neural model for neurotoxicity studies, combined with high-throughput techniques such as MEA recordings, could be a suitable addition to existing pre-clinical seizure-liability testing strategies.