Organoids required! A new path to understanding human brain development and disease

Advances in current in vitro models on neurodegenerative diseases

Many neurodegenerative diseases are identified but their causes and cure are far from being well-known. The problem resides in the complexity of the neural tissue and its location which hinders its easy evaluation. Although necessary in the drug discovery process, in vivo animal models need to be reduced and show relevant differences with the human tissues that guide scientists to inquire about other possible options which lead to in vitro models being explored. From organoids to organ-on-a-chips, 3D models are considered the cutting-edge technology in cell culture. Cell choice is a big parameter to take into consideration when planning an in vitro model and cells capable of mimicking both healthy and diseased tissue, such as induced pluripotent stem cells (iPSC), are recognized as good candidates. Hence, we present a critical review of the latest models used to study neurodegenerative disease, how these models have evolved introducing microfluidics platforms, 3D cell cultures, and the use of induced pluripotent cells to better mimic the neural tissue environment in pathological conditions.

Humanized cerebral organoids-based ischemic stroke model for discovering of potential anti-stroke agents

Establishing a stoke experimental model, which is better in line with the physiology and function of human brain, is the bottleneck for the development of effective anti-stroke drugs. A three-dimensional cerebral organoids (COs) from human pluripotent stem cells can mimic cell composition, cortical structure, brain neural connectivity and epigenetic genomics of in-vivo human brain, which provides a promising application in establishing humanized ischemic stroke model. COs have been used for modeling low oxygen condition-induced hypoxic injury, but there is no report on the changes of COs in response to in vitro oxygen-glucose deprivation (OGD)-induced damage of ischemic stroke as well as its application in testing anti-stroke drugs. In this study we compared the cell composition of COs at different culture time and explored the cell types, cell ratios and volume size of COs at 85 days (85 d-CO). The 85 d-CO with diameter more than 2 mm was chosen for establishing humanized ischemic stroke model of OGD. By determining the time-injury relationship of the model, we observed aggravated ischemic injury of COs with OGD exposure time, obtaining first-hand evidence for the damage degree of COs under different OGD condition. The sensitivity of the model to ischemic injury and related treatment was validated by the proven pan-Caspase inhibitor Z-VAD-FMK (20 μM) and Bcl-2 inhibitor navitoclax (0.5 μM). Neuroprotective agents edaravone, butylphthalide, P7C3-A20 and ZL006 (10 μM for each) exerted similar beneficial effects in this model. Taken together, this study establishes a humanized ischemic stroke model based on COs, and provides evidence as a new research platform for anti-stroke drug development.

Human-specific ARHGAP11B ensures human-like basal progenitor levels in hominid cerebral organoids

The human-specific gene ARHGAP11B has been implicated in human neocortex expansion. However, the extent of ARHGAP11B's contribution to this expansion during hominid evolution is unknown. Here we address this issue by genetic manipulation of ARHGAP11B levels and function in chimpanzee and human cerebral organoids. ARHGAP11B expression in chimpanzee cerebral organoids doubles basal progenitor levels, the class of cortical progenitors with a key role in neocortex expansion. Conversely, interference with ARHGAP11B's function in human cerebral organoids decreases basal progenitors down to the chimpanzee level. Moreover, ARHGAP11A or ARHGAP11B rescue experiments in ARHGAP11A plus ARHGAP11B double-knockout human forebrain organoids indicate that lack of ARHGAP11B, but not of ARHGAP11A, decreases the abundance of basal radial glia-the basal progenitor type thought to be of particular relevance for neocortex expansion. Taken together, our findings demonstrate that ARHGAP11B is necessary and sufficient to ensure the elevated basal progenitor levels that characterize the fetal human neocortex, suggesting that this human-specific gene was a major contributor to neocortex expansion during human evolution.

Human-specific ARHGAP11B ensures human-like basal progenitor levels in hominid cerebral organoids

The human-specific gene ARHGAP11B has been implicated in human neocortex expansion. However, the extent of ARHGAP11B's contribution to this expansion during hominid evolution is unknown. Here we address this issue by genetic manipulation of ARHGAP11B levels and function in chimpanzee and human cerebral organoids. ARHGAP11B expression in chimpanzee cerebral organoids doubles basal progenitor levels, the class of cortical progenitors with a key role in neocortex expansion. Conversely, interference with ARHGAP11B's function in human cerebral organoids decreases basal progenitors down to the chimpanzee level. Moreover, ARHGAP11A or ARHGAP11B rescue experiments in ARHGAP11A plus ARHGAP11B double-knockout human forebrain organoids indicate that lack of ARHGAP11B, but not of ARHGAP11A, decreases the abundance of basal radial glia - the basal progenitor type thought to be of particular relevance for neocortex expansion. Taken together, our findings demonstrate that ARHGAP11B is necessary and sufficient to ensure the elevated basal progenitor levels that characterize the fetal human neocortex, suggesting that this human-specific gene was a major contributor to neocortex expansion during human evolution.

Organoids: a systematic review of ethical issues

Organoids are 3D structures grown from pluripotent stem cells derived from human tissue and serve as in vitro miniature models of human organs. Organoids are expected to revolutionize biomedical research and clinical care. However, organoids are not seen as morally neutral. For instance, tissue donors may perceive enduring personal connections with their organoids, setting higher bars for informed consent and patient participation. Also, several organoid sub-types, e.g., brain organoids and human-animal chimeric organoids, have raised controversy. This systematic review provides an overview of ethical discussions as conducted in the scientific literature on organoids. The review covers both research and clinical applications of organoid technology and discusses the topics informed consent, commercialization, personalized medicine, transplantation, brain organoids, chimeras, and gastruloids. It shows that further ethical research is needed especially on organoid transplantation, to help ensure the responsible development and clinical implementation of this technology in this field.

Expression of the transcription factor PU.1 induces the generation of microglia-like cells in human cortical organoids

Microglia play a role in the emergence and preservation of a healthy brain microenvironment. Dysfunction of microglia has been associated with neurodevelopmental and neurodegenerative disorders. Investigating the function of human microglia in health and disease has been challenging due to the limited models of the human brain available. Here, we develop a method to generate functional microglia in human cortical organoids (hCOs) from human embryonic stem cells (hESCs). We apply this system to study the role of microglia during inflammation induced by amyloid-β (Aβ). The overexpression of the myeloid-specific transcription factor PU.1 generates microglia-like cells in hCOs, producing mhCOs (microglia-containing hCOs), that we engraft in the mouse brain. Single-cell transcriptomics reveals that mhCOs acquire a microglia cell cluster with an intact complement and chemokine system. Functionally, microglia in mhCOs protect parenchyma from cellular and molecular damage caused by Aβ. Furthermore, in mhCOs, we observed reduced expression of Aβ-induced expression of genes associated with apoptosis, ferroptosis, and Alzheimer's disease (AD) stage III. Finally, we assess the function of AD-associated genes highly expressed in microglia in response to Aβ using pooled CRISPRi coupled with single-cell RNA sequencing in mhCOs. In summary, we provide a protocol to generate mhCOs that can be used in fundamental and translational studies as a model to investigate the role of microglia in neurodevelopmental and neurodegenerative disorders.

Editorial for the Special Issue on Microfluidic Brain-on-a-Chip

A little longer than a decade of Organ-on-Chip (OoC) developments has passed [...].

Synthetic modified Fezf2 mRNA (modRNA) with concurrent small molecule SIRT1 inhibition enhances refinement of cortical subcerebral/corticospinal neuron identity from mouse embryonic stem cells

During late embryonic development of the cerebral cortex, the major class of cortical output neurons termed subcerebral projection neurons (SCPN; including the predominant population of corticospinal neurons, CSN) and the class of interhemispheric callosal projection neurons (CPN) initially express overlapping molecular controls that later undergo subtype-specific refinements. Such molecular refinements are largely absent in heterogeneous, maturation-stalled, neocortical-like neurons (termed "cortical" here) spontaneously generated by established embryonic stem cell (ES) and induced pluripotent stem cell (iPSC) differentiation. Building on recently identified central molecular controls over SCPN development, we used a combination of synthetic modified mRNA (modRNA) for Fezf2, the central transcription factor controlling SCPN specification, and small molecule screening to investigate whether distinct chromatin modifiers might complement Fezf2 functions to promote SCPN-specific differentiation by mouse ES (mES)-derived cortical-like neurons. We find that the inhibition of a specific histone deacetylase, Sirtuin 1 (SIRT1), enhances refinement of SCPN subtype molecular identity by both mES-derived cortical-like neurons and primary dissociated E12.5 mouse cortical neurons. In vivo, we identify that SIRT1 is specifically expressed by CPN, but not SCPN, during late embryonic and postnatal differentiation. Together, these data indicate that SIRT1 has neuronal subtype-specific expression in the mouse cortex in vivo, and that its inhibition enhances subtype-specific differentiation of highly clinically relevant SCPN / CSN cortical neurons in vitro.

Cerebral organoids and their potential for studies of brain diseases in domestic animals

The brain is a complex organ and any model for studying it in its normal and pathological aspects becomes a tool of choice for neuroscientists. The mastering and dissemination of protocols allowing brain organoids development have paved the way for a whole range of new studies in the field of brain development, modeling of neurodegenerative or neurodevelopmental diseases, understanding tumors as well as infectious diseases that affect the brain. While studies are so far limited to the use of human cerebral organoids, there is a growing interest in having similar models in other species. This review presents what is currently developed in this field, with a particular focus on the potential of cerebral organoids for studying neuro-infectious diseases in human and domestic animals.

Resolving organoid brain region identities by mapping single-cell genomic data to reference atlases

Self-organizing tissues resembling brain structures generated from human stem cells offer exciting possibilities to study human brain development, disease, and evolution. These 3D models are complex and can contain cells at various stages of differentiation from different brain regions. Single-cell genomic methods provide powerful approaches to explore cell composition, differentiation trajectories, and genetic perturbations in brain organoid systems. However, it remains a major challenge to understand the heterogeneity observed within and between individual organoids. Here, we develop a set of computational tools (VoxHunt) to assess brain organoid patterning, developmental state, and cell identity through comparisons to spatial and single-cell transcriptome reference datasets. We use VoxHunt to characterize and visualize cell compositions using single-cell and bulk genomic data from multiple organoid protocols modeling different brain structures. VoxHunt will be useful to assess organoid engineering protocols and to annotate cell fates that emerge in organoids during genetic and environmental perturbation experiments.

Three-dimensional in vitro tissue culture models of brain organoids

Brain organoids are three-dimensional self-assembled structures that are derived from human induced pluripotent stem cells (hiPSCs). They can recapitulate the spatiotemporal organization and function of the brain, presenting a robust system for in vitro modeling of brain development, evolution, and diseases. Significant advances in biomaterials, microscale technologies, gene editing technologies, and stem cell biology have enabled the construction of human specific brain structures in vitro. However, the limitations of long-term culture, necrosis, and hypoxic cores in different culture models obstruct brain organoid growth and survival. The in vitro models should facilitate oxygen and nutrient absorption, which is essential to generate complex organoids and provides a biomimetic microenvironment for modeling human brain organogenesis and human diseases. This review aims to highlight the progress in the culture devices of brain organoids, including dish, bioreactor, and organ-on-a-chip models. With the modulation of bioactive molecules and biomaterials, the generated organoids recapitulate the key features of the human brain in a more reproducible and hyperoxic fashion. Furthermore, an outlook for future preclinical studies and the genetic modifications of brain organoids is presented.

Neurodevelopmental signatures of narcotic and neuropsychiatric risk factors in 3D human-derived forebrain organoids

It is widely accepted that narcotic use during pregnancy and specific environmental factors (e.g., maternal immune activation and chronic stress) may increase risk of neuropsychiatric illness in offspring. However, little progress has been made in defining human-specific in utero neurodevelopmental pathology due to ethical and technical challenges associated with accessing human prenatal brain tissue. Here we utilized human induced pluripotent stem cells (hiPSCs) to generate reproducible organoids that recapitulate dorsal forebrain development including early corticogenesis. We systemically exposed organoid samples to chemically defined "enviromimetic" compounds to examine the developmental effects of various narcotic and neuropsychiatric-related risk factors within tissue of human origin. In tandem experiments conducted in parallel, we modeled exposure to opiates (μ-opioid agonist endomorphin), cannabinoids (WIN 55,212-2), alcohol (ethanol), smoking (nicotine), chronic stress (human cortisol), and maternal immune activation (human Interleukin-17a; IL17a). Human-derived dorsal forebrain organoids were consequently analyzed via an array of unbiased and high-throughput analytical approaches, including state-of-the-art TMT-16plex liquid chromatography/mass-spectrometry (LC/MS) proteomics, hybrid MS metabolomics, and flow cytometry panels to determine cell-cycle dynamics and rates of cell death. This pipeline subsequently revealed both common and unique proteome, reactome, and metabolome alterations as a consequence of enviromimetic modeling of narcotic use and neuropsychiatric-related risk factors in tissue of human origin. However, of our 6 treatment groups, human-derived organoids treated with the cannabinoid agonist WIN 55,212-2 exhibited the least convergence of all groups. Single-cell analysis revealed that WIN 55,212-2 increased DNA fragmentation, an indicator of apoptosis, in human-derived dorsal forebrain organoids. We subsequently confirmed induction of DNA damage and apoptosis by WIN 55,212-2 within 3D human-derived dorsal forebrain organoids. Lastly, in a BrdU pulse-chase neocortical neurogenesis paradigm, we identified that WIN 55,212-2 was the only enviromimetic treatment to disrupt newborn neuron numbers within human-derived dorsal forebrain organoids. Cumulatively this study serves as both a resource and foundation from which human 3D biologics can be used to resolve the non-genomic effects of neuropsychiatric risk factors under controlled laboratory conditions. While synthetic cannabinoids can differ from naturally occurring compounds in their effects, our data nonetheless suggests that exposure to WIN 55,212-2 elicits neurotoxicity within human-derived developing forebrain tissue. These human-derived data therefore support the long-standing belief that maternal use of cannabinoids may require caution so to avoid any potential neurodevelopmental effects upon developing offspring in utero.

High-Definition Single-Cell Printing: Cell-by-Cell Fabrication of Biological Structures

Bioprinting is a powerful technology with the potential to transform medical device manufacturing, organ replacement, and the treatment of diseases and physiologic malformations. However, current bioprinters are unable to reliably print the fundamental unit of all living things, single cells. A high-definition single-cell printing, a novel microfluidic technology, is presented here that can accurately print single cells from a mixture of multiple candidates. The bioprinter employs a highly miniaturized microfluidic sorter to deterministically select single cells of interest for printing, achieving an accuracy of ≈10 µm and speed of ≈100 Hz. This approach is demonstrated by fabricating intricate cell patterns with pre-defined features through selective single-cell printing. The approach is used to synthesize well-defined spheroids with controlled composition and morphology. The speed, accuracy, and flexibility of the approach will advance bioprinting to enable new studies in organoid science, tissue engineering, and spatially targeted cell therapies.

Surface-Functionalized Self-Standing Microdevices Exhibit Predictive Localization and Seamless Integration in 3D Neural Spheroids

Brain organoids is an exciting technology proposed to advance studies on human brain development, diseases, and possible therapies. Establishing and applying such models, however, is hindered by the lack of technologies to chronically monitor neural activity. A promising new approach comprising self-standing biosensing microdevices capable of achieving seamless tissue integration during cell aggregation and culture. To date, there is little information on how to control the aggregation of such bioartificial 3D neural assemblies. Here, the growth of hybrid neurospheroids obtained by the aggregation of silicon sham microchips (100 × 100 × 50 μm³ ) with primary cortical cells is investigated. Results obtained via protein-binding microchips with different molecules reveal that surface functionalization can tune the integration and final 3D location of self-standing microdevices into neurospheroids. Morphological and functional characterization suggests that the presence of an integrated microdevice does not alter spheroid growth, cellular composition, nor functional development. Ultimately, cells and microdevices constituting such hybrid neurospheroids can be disaggregated for further single-cell analysis, and quantifications confirm an unaltered ratio of neurons and glia. These results uncover the potential of surface-engineered self-standing microdevices to grow untethered 3D brain tissue models with inbuilt bioelectronic sensors at predefined sites.

Brain Organoids: Tiny Mirrors of Human Neurodevelopment and Neurological Disorders

Unravelling the complexity of the human brain is a challenging task. Nowadays, modern neurobiologists have developed 3D model systems called "brain organoids" to overcome the technical challenges in understanding human brain development and the limitations of animal models to study neurological diseases. Certainly like most model systems in neuroscience, brain organoids too have limitations, as these minuscule brains lack the complex neuronal circuitry required to begin the operational tasks of human brain. However, researchers are hopeful that future endeavors with these 3D brain tissues could provide mechanistic insights into the generation of circuit complexity as well as reproducible creation of different regions of the human brain. Herein, we have presented the contemporary state of brain organoids with special emphasis on their mode of generation and their utility in modelling neurological disorders, drug discovery, and clinical trials.

Cerebral organoids transplantation improves neurological motor function in rat brain injury

BACKGROUND AND PURPOSE

Cerebral organoids (COs) have been used for studying brain development, neural disorders, and species-specific drug pharmacology and toxicology, but the potential of COs transplantation therapy for brain injury remains to be answered.

METHODS

With preparation of traumatic brain injury (TBI) model of motor dysfunction, COs at 55 and 85 days (55 and 85 d-CO) were transplanted into damaged motor cortex separately to identify better transplantation donor for brain injury. Further, the feasibility, effectiveness, and underlying mechanism of COs transplantation therapy for brain injury were explored.

RESULTS

55 d-CO was demonstrated as better transplantation donor than 85 d-CO, evidenced by more neurogenesis and higher cell survival rate without aggravating apoptosis and inflammation after transplantation into damaged motor cortex. Cells from transplanted COs had the potential of multilinage differentiation to mimic in-vivo brain cortical development, support region-specific reconstruction of damaged motor cortex, form neurotransmitter-related neurons, and migrate into different brain regions along corpus callosum. Moreover, COs transplantation upregulated hippocampal neural connection proteins and neurotrophic factors. Notably, COs transplantation improved neurological motor function and reduced brain damage.

CONCLUSIONS

This study revealed 55 d-CO as better transplantation donor and demonstrated the feasibility and efficacy of COs transplantation in TBI, hoping to provide first-hand preclinical evidence of COs transplantation for brain injury.

Negative Symptoms of Schizophrenia and Dopaminergic Transmission: Translational Models and Perspectives Opened by iPSC Techniques

Negative symptoms (NS) represent a heterogeneous dimension of schizophrenia (SCZ), associated with a poor functional outcome. A dysregulated dopamine (DA) system, including a reduced D1 receptor activation in the prefrontal cortex, DA hypoactivity in the caudate and alterations in D3 receptor activity, seems to contribute to the pathogenesis of NS. However, failure to take into account the NS heterogeneity has slowed down progress in research on their neurobiological correlates and discoveries of new effective treatments. A better neurobiological characterization of NS is needed, and this requires objective quantification of their features that can be applied in translational models, such as animal models and human inducible pluripotent stem cells (iPSC). In this review we summarize the evidence for dopaminergic alterations relevant to NS in translational animal models focusing on dysfunctional motivation, a core aspect of NS. Among others, experiments on mutant rodents with an overexpression of DA D2 or D3 receptors and the dopamine deficient mice are discussed. In the second part we summarize the findings from recent studies using iPSC to model the pathogenesis of SCZ. By retaining the genetic background of risk genetic variants, iPSC offer the possibility to study the effect of de novo mutations or inherited polymorphisms from subgroups of patients and their response to drugs, adding an important tool for personalized psychiatry. Given the key role of DA in NS, we focus on findings of iPSC-derived DA neurons. Since implementation of iPSC-derived neurons to study the neurobiology of SCZ is a relatively recent acquisition, the available data are limited. We highlight some methodological aspects of relevance in the interpretation of in vitro testing results, including limitations and strengths, offering a critical viewpoint for the implementation of future pharmacological studies aimed to the discovery and characterization of novel treatments for NS.

Reproducible generation of human midbrain organoids for in vitro modeling of Parkinson's disease

The study of human midbrain development and midbrain related diseases, like Parkinson's disease (PD), is limited by deficiencies in the currently available and validated laboratory models. Three dimensional midbrain organoids represent an innovative strategy to recapitulate some aspects of the complexity and physiology of the human midbrain. Nevertheless, also these novel organoid models exhibit some inherent weaknesses, including the presence of a necrotic core and batch-to-batch variability. Here we describe an optimized approach for the standardized generation of midbrain organoids that addresses these limitations, while maintaining key features of midbrain development like dopaminergic neuron and astrocyte differentiation. Moreover, we have established a novel time-efficient, fit for purpose analysis pipeline and provided proof of concept for its usage by investigating toxin induced PD.

Midbrain Organoids: A New Tool to Investigate Parkinson's Disease

The study of human 3D cell culture models not only bridges the gap between traditional 2D in vitro experiments and in vivo animal models, it also addresses processes that cannot be recapitulated by either of these traditional models. Therefore, it offers an opportunity to better understand complex biology including brain development. The brain organoid technology provides a physiologically relevant context, which holds great potential for its application in modeling neurological diseases. Here, we compare different methods to obtain highly specialized structures that resemble specific features of the human midbrain. Regionally patterned neural stem cells (NSCs) were utilized to derive such human midbrain-specific organoids (hMO). The resulting neural tissue exhibited abundant neurons with midbrain dopaminergic neuron identity, as well as astroglia and oligodendrocyte differentiation. Within the midbrain organoids, neurite myelination, and the formation of synaptic connections were observed. Regular neuronal fire patterning and neural network synchronicity were determined by multielectrode array recordings. In addition to electrophysiologically functional neurons producing and secreting dopamine, responsive neuronal subtypes, such as GABAergic and glutamatergic neurons were also detected. In order to model disorders like Parkinson's disease (PD) in vitro, midbrain organoids carrying a disease specific mutation were derived and compared to healthy control organoids to investigate relevant neurodegenerative pathophysiology. In this way midbrain-specific organoids constitute a powerful tool for human-specific in vitro modeling of neurological disorders with a great potential to be utilized in advanced therapy development.

Urine Sample-Derived Cerebral Organoids Suitable for Studying Neurodevelopment and Pharmacological Responses

Cerebral organoids (COs) developed from human induced pluripotent stem cells (hiPSCs) have been noticed for their potential in research and clinical applications. While skin fibroblast-derived hiPSCs are proficient at forming COs, the cellular and molecular features of COs developed using hiPSCs generated from other somatic cells have not been systematically examined. Urinary epithelial cells (UECs) isolated from human urine samples are somatic cells that can be non-invasively collected from most individuals. In this work, we streamlined the production of COs using hiPSCs reprogrammed from urine sample-derived UECs. UEC-derived hiPSC-developed COs presented a robust capacity for neurogenesis and astrogliogenesis. Although UEC-derived hiPSCs required specific protocol optimization to properly form COs, the cellular and transcriptomic features of COs developed from UEC-derived hiPSCs were comparable to those of COs developed from embryonic stem cells. UEC-derived hiPSC-developed COs that were initially committed to forebrain development showed cellular plasticity to transition between prosencephalic and rhombencephalic fates in vitro and in vivo, indicating their potential to develop into the cell components of various brain regions. The opposite regulation of AKT activity and neural differentiation was found in these COs treated with AKT and PTEN inhibitors. Overall, our data reveal the suitability, advantage, and possible limitations of human urine sample-derived COs for studying neurodevelopment and pharmacological responses.

Loss of the neural-specific BAF subunit ACTL6B relieves repression of early response genes and causes recessive autism

Synaptic activity in neurons leads to the rapid activation of genes involved in mammalian behavior. ATP-dependent chromatin remodelers such as the BAF complex contribute to these responses and are generally thought to activate transcription. However, the mechanisms keeping such "early activation" genes silent have been a mystery. In the course of investigating Mendelian recessive autism, we identified six families with segregating loss-of-function mutations in the neuronal BAF (nBAF) subunit ACTL6B (originally named BAF53b). Accordingly, ACTL6B was the most significantly mutated gene in the Simons Recessive Autism Cohort. At least 14 subunits of the nBAF complex are mutated in autism, collectively making it a major contributor to autism spectrum disorder (ASD). Patient mutations destabilized ACTL6B protein in neurons and rerouted dendrites to the wrong glomerulus in the fly olfactory system. Humans and mice lacking ACTL6B showed corpus callosum hypoplasia, indicating a conserved role for ACTL6B in facilitating neural connectivity. Actl6b knockout mice on two genetic backgrounds exhibited ASD-related behaviors, including social and memory impairments, repetitive behaviors, and hyperactivity. Surprisingly, mutation of Actl6b relieved repression of early response genes including AP1 transcription factors (Fos, Fosl2, Fosb, and Junb), increased chromatin accessibility at AP1 binding sites, and transcriptional changes in late response genes associated with early response transcription factor activity. ACTL6B loss is thus an important cause of recessive ASD, with impaired neuron-specific chromatin repression indicated as a potential mechanism.

Ethical issues related to brain organoid research

This review provides a snapshot of the current ethical issues related to research with human brain organoids. The issues fall into the following main themes: research oversight; human biomaterials procurement and donor consent; translational delivery; animal research; and organoid consciousness and moral status. Each of these areas poses challenges for researchers, bioethicists, regulators, research institutions, and tissue banks. However, progress can be made if these parties build on past experiences with stem cell research, ethics, and policy, but adapted accordingly to new aspects of brain organoid research.

Cerebral Organoids Repair Ischemic Stroke Brain Injury

Stroke is the second leading cause of death and main cause of disability worldwide, but with few effective therapies. Although stem cell-based therapy has been proposed as an exciting regenerative medicine strategy for brain injury, there are limitations. The developed cerebral organoids (COs) represent a promising transplantation source for stroke that remains to be answered. Here, we transplanted COs at 55 days and explored the feasibility in the rat middle cerebral artery occlusion (MCAO) model of stroke. COs transplantation at 6 h or even 24 h after MCAO significantly reduces brain infarct volume and improves neurological motor function. Transplanted COs show the potential of multilineage differentiation to mimic in vivo cortical development, support motor cortex region-specific reconstruction, form neurotransmitter-related neurons, and achieve synaptic connection with host brain via in situ differentiation and cell replacement in stroke. Cells from transplanted COs show extensive migration into different brain regions along corpus callosum. The mechanisms underlying COs transplantation therapy are also associated with enhanced neurogenesis, synaptic reconstruction, axonal regeneration and angiogenesis, and decreased neural apoptosis with more survival neurons after stroke. Moreover, COs transplantation promotes predominantly exogenous neurogenesis in the transplantation periphery of ipsilateral cortex and predominantly endogenous neurogenesis in the hippocampus and subventricular zone. Together, we demonstrate the efficacy and underlying mechanisms of COs transplantation in stroke. This preliminary but promising study provides first-hand preclinical evidence for COs transplantation as a potential and effective intervention for stroke treatment.

From Schizophrenia Genetics to Disease Biology: Harnessing New Concepts and Technologies

Schizophrenia (SZ) is a severe mental disorder afflicting around 1% of the population. It is highly heritable but with complex genetics. Recent research has unraveled a plethora of risk loci for SZ. Accordingly, our conceptual understanding of SZ genetics has been rapidly evolving, from oligogenic models towards polygenic or even omnigenic models. A pressing challenge to the field, however, is the translation of the many genetic findings of SZ into disease biology insights leading to more effective treatments. Bridging this gap requires the integration of genetic findings and functional genomics using appropriate cellular models. Harnessing new technologies, such as the development of human induced pluripotent stem cells (hiPSC) and the CRISPR/Cas-based genome/epigenome editing approach are expected to change our understanding of SZ disease biology to a fundamentally higher level. Here, we discuss some new developments.

The Convergence of Stem Cell Technologies and Phenotypic Drug Discovery

Recent advances in induced pluripotent stem cell technologies and phenotypic screening shape the future of bioactive small-molecule discovery. In this review we analyze the impact of small-molecule phenotypic screens on drug discovery as well as on the investigation of human development and disease biology. We further examine the role of 3D spheroid/organoid structures, microfluidic systems, and miniaturized on-a-chip systems for future discovery strategies. In highlighting representative examples, we analyze how recent achievements can translate into future therapies. Finally, we discuss remaining challenges that need to be overcome for the adaptation of the next generation of screening approaches.

Cortical Dysmaturation in Congenital Heart Disease

Congenital heart disease (CHD) is among the most common birth defects. Children with CHD frequently display long-term intellectual and behavioral disability. Emerging evidence indicates that cardiac anomalies lead to a reduction in cerebral oxygenation, which appears to profoundly impact on the maturation of cerebral regions responsible for higher-order cognitive functions. In this review we focus on the potential mechanisms by which dysregulation of cortical neuronal development during early life may lead to the significant cognitive impairments that commonly occur in children with CHD. Further understanding of the mechanisms underlying cortical dysmaturation due to CHD will be necessary to identify strategies for neonatal neuroprotection and for mitigating developmental delays in this patient population.

Applications of Human Brain Organoids to Clinical Problems

Brain organoids are an exciting new technology with the potential to significantly change how diseases of the brain are understood and treated. These three-dimensional neural tissues are derived from the self-organization of pluripotent stem cells, and they recapitulate the developmental process of the human brain, including progenitor zones and rudimentary cortical layers. Brain organoids have been valuable in investigating different aspects of developmental neurobiology and comparative biology. Several characteristics of organoids also make them attractive as models of brain disorders. Data generated from human organoids are more generalizable to patients because of the match in species background. Personalized organoids also can be generated from patient-derived induced pluripotent stem cells. Furthermore, the three-dimensionality of brain organoids supports cellular, mechanical, and topographical cues that are lacking in planar systems. In this review, we discuss the translational potential of brain organoids, using the examples of Zika virus, autism-spectrum disorder, and glioblastoma multiforme to consider how they could contribute to disease modeling, personalized medicine, and testing of therapeutics. We then discuss areas of improvement in organoid technology that will enhance the translational potential of brain organoids, as well as the possibility of their use as substrates for repairing cerebral circuitry after injury. Developmental Dynamics 248:53-64, 2019. © 2018 Wiley Periodicals, Inc.

Childhood-Onset Schizophrenia: Insights from Induced Pluripotent Stem Cells

Childhood-onset schizophrenia (COS) is a rare psychiatric disorder characterized by earlier onset, more severe course, and poorer outcome relative to adult-onset schizophrenia (AOS). Even though, clinical, neuroimaging, and genetic studies support that COS is continuous to AOS. Early neurodevelopmental deviations in COS are thought to be significantly mediated through poorly understood genetic risk factors that may also predispose to long-term outcome. In this review, we discuss findings from induced pluripotent stem cells (iPSCs) that allow the generation of disease-relevant cell types from early brain development. Because iPSCs capture each donor's genotype, case/control studies can uncover molecular and cellular underpinnings of COS. Indeed, recent studies identified alterations in neural progenitor and neuronal cell function, comprising dendrites, synapses, electrical activity, glutamate signaling, and miRNA expression. Interestingly, transcriptional signatures of iPSC-derived cells from patients with COS showed concordance with postmortem brain samples from SCZ, indicating that changes in vitro may recapitulate changes from the diseased brain. Considering this progress, we discuss also current caveats from the field of iPSC-based disease modeling and how to proceed from basic studies to improved diagnosis and treatment of COS.

Human brain development and its in vitro recapitulation

Humans have a large and gyrencephalic brain. The higher intellectual ability of humans is dependent on the proper development of the brain. Brain malformation is often associated with cognitive dysfunction. It is thus important to know how our brain grows during development. Several animal species have been used as models to understand the mechanisms of brain development, and have provided us with basic information in this regard. It has been revealed that mammalian brain development basically proceeds through a similar process by common mechanisms, including neural stem cell proliferation and neurogenesis. However, humans also display species-specific features in these processes. These differences seem to be important for building the proper human brain structure. Analysis of these human-specific features requires human brain samples, which are difficult to obtain due to both ethical and practical reasons. Nevertheless, brain organoids derived from human pluripotent stem cells can be used as models to study human brain development and pathology because such organoids can partly recapitulate human fetal developmental processes. In this review, we will review some human-specific features during brain development and discuss brain organoid technology as a model system. We will especially focusing on neocortical development.

Induction of myelinating oligodendrocytes in human cortical spheroids

Organoid technologies provide an accessible system to examine cellular composition, interactions, and organization in the developing human brain, but previously have lacked oligodendrocytes, the myelinating glia of the central nervous system. Here we reproducibly generate oligodendrocytes and myelin in human pluripotent stem cell-derived “oligocortical spheroids”. Transcriptional, immunohistochemical, and electron microscopy analyses demonstrate molecular features consistent with maturing oligodendrocytes by 20 weeks in culture, including expression of MYRF, PLP1, and MBP proteins and initial myelin wrapping of axons, with maturation to longitudinal wrapping and compact myelin by 30 weeks. Promyelinating drugs enhance the rate and extent of oligodendrocyte generation and myelination, while oligocortical spheroids generated from patients with a genetic myelin disorder recapitulate human disease phenotypes. Oligocortical spheroids provide a versatile platform to observe and dissect the complex interactions required for myelination of the developing central nervous system and offer new opportunities for disease modeling and therapeutic development in human tissue.

Brain organoids as models to study human neocortex development and evolution

Since their recent development, organoids that emulate human brain tissue have allowed in vitro neural development studies to go beyond the limits of monolayer culture systems, such as neural rosettes. We present here a review of organoid studies that focuses on cortical wall development, starting with a technical comparison between pre-patterning and self-patterning brain organoid protocols. We then follow neocortex development in space and time and list those aspects where organoids have succeeded in emulating in vivo development, as well as those aspects that continue to be pending tasks. Finally, we present a summary of medical and evolutionary insight made possible by organoid technology.

Development of the Thalamocortical Interactions: Past, Present and Future

For the past two decades, we have advanced in our understanding of the mechanisms implicated in the formation of brain circuits. The connection between the cortex and thalamus has deserved much attention, as thalamocortical connectivity is crucial for sensory processing and motor learning. Classical dye tracing studies in wild-type and knockout mice initially helped to characterize the developmental progression of this connectivity and revealed key transcription factors involved. With the recent advances in technical tools to specifically label subsets of projecting neurons, knock-down genes individually and/or modify their activity, the field has gained further understanding on the rules operating in thalamocortical circuit formation and plasticity. In this review, I will summarize the most relevant discoveries that have been made in this field, from development to early plasticity processes covering three major aspects: axon guidance, thalamic influence on sensory cortical specification, and the role of spontaneous thalamic activity. I will emphasize how the implementation of new tools has helped the field to progress and what I consider to be open questions and the perspective for the future.

Somatic mutations in neurons during aging and neurodegeneration

The nervous system is composed of a large variety of neurons with a diverse array of morphological and functional properties. This heterogeneity is essential for the construction and maintenance of a distinct set of neural networks with unique characteristics. Accumulating evidence now indicates that neurons do not only differ at a functional level, but also at the genomic level. These genomic discrepancies seem to be the result of somatic mutations that emerge in nervous tissue during development and aging. Ultimately, these mutations bring about a genetically heterogeneous population of neurons, a phenomenon that is commonly referred to as "somatic brain mosaicism". Improved understanding of the development and consequences of somatic brain mosaicism is crucial to understand the impact of somatic mutations on neuronal function in human aging and disease. Here, we highlight a number of topics related to somatic brain mosaicism, including some early experimental evidence for somatic mutations in post-mitotic neurons of the hypothalamo-neurohypophyseal system. We propose that age-related somatic mutations are particularly interesting, because aging is a major risk factor for a variety of neuronal diseases, including Alzheimer's disease. We highlight potential links between somatic mutations and the development of these diseases and argue that recent advances in single-cell genomics and in vivo physiology have now finally made it possible to dissect the origins and consequences of neuronal mutations in unprecedented detail.

Self-organizing cortex generated from human iPSCs with combination of FGF2 and ambient oxygen

Human brain development has generally been studied through the analysis of postmortem tissues because of limited access to fetal brain tissues. This approach, however, only provides information from the perspective of long-term development. To investigate the pathophysiology of neurodevelopmental disorders, it is necessary to understand the detailed mechanisms of human brain development. Recent advances in pluripotent stem cell (PSC) technologies enable us to establish in vitro brain models from human induced PSCs (hiPSCs), which can be used to examine the pathophysiological mechanisms of neurodevelopmental disorders. We previously demonstrated that self-organized cerebral tissues can be generated from human PSCs in a three-dimensional (3D) culture system. Here, we describe the cerebral tissues differentiated from hiPSCs in a further-optimized 3D culture. We found that treatment with FGF2 is helpful to form iPSC aggregates with efficiency. Neuroepithelial structures spontaneously formed with apico-basal polarity in the aggregates expressing forebrain marker FOXG1. The neuroepithelium self-forms a multilayered structure including progenitor zones (ventricular and subventricular zones) and neuronal zone (cortical plate). Furthermore, with the same level of oxygen (O₂) as in ambient air (20% O₂), we found that self-formation of cortical structures lasted for 70 days in culture. Thus, our optimized 3D culture for the generation of cortical structure from hiPSCs is a simple yet effective method.