Functional cortical neurons and astrocytes from human pluripotent stem cells in 3D culture

Differential Effects of Extracellular Vesicles of Lineage-Specific Human Pluripotent Stem Cells on the Cellular Behaviors of Isogenic Cortical Spheroids

Extracellular vesicles (EVs) contribute to a variety of signaling processes and the overall physiological and pathological states of stem cells and tissues. Human induced pluripotent stem cells (hiPSCs) have unique characteristics that can mimic embryonic tissue development. There is growing interest in the use of EVs derived from hiPSCs as therapeutics, biomarkers, and drug delivery vehicles. However, little is known about the characteristics of EVs secreted by hiPSCs and paracrine signaling during tissue morphogenesis and lineage specification. Methods: In this study, the physical and biological properties of EVs isolated from hiPSC-derived neural progenitors (ectoderm), hiPSC-derived cardiac cells (mesoderm), and the undifferentiated hiPSCs (healthy iPSK3 and Alzheimer’s-associated SY-UBH lines) were analyzed. Results: Nanoparticle tracking analysis and electron microscopy results indicate that hiPSC-derived EVs have an average size of 100–250 nm. Immunoblot analyses confirmed the enrichment of exosomal markers Alix, CD63, TSG101, and Hsc70 in the purified EV preparations. MicroRNAs including miR-133, miR-155, miR-221, and miR-34a were differently expressed in the EVs isolated from distinct hiPSC lineages. Treatment of cortical spheroids with hiPSC-EVs in vitro resulted in enhanced cell proliferation (indicated by BrdU+ cells) and axonal growth (indicated by β-tubulin III staining). Furthermore, hiPSC-derived EVs exhibited neural protective abilities in Aβ42 oligomer-treated cultures, enhancing cell viability and reducing oxidative stress. Our results demonstrate that the paracrine signaling provided by tissue context-dependent EVs derived from hiPSCs elicit distinct responses to impact the physiological state of cortical spheroids. Overall, this study advances our understanding of cell‒cell communication in the stem cell microenvironment and provides possible therapeutic options for treating neural degeneration.

Old but Gold: Tracking the New Guise of Histone Deacetylase 6 (HDAC6) Enzyme as a Biomarker and Therapeutic Target in Rare Diseases

Epigenetic regulation orchestrates many cellular processes and greatly influences key disease mechanisms. Histone deacetylase (HDAC) enzymes play a crucial role either as biomarkers or therapeutic targets owing to their involvement in specific pathophysiological pathways. Beyond their well-characterized role as histone modifiers, HDACs also interact with several nonhistone substrates and their increased expression has been highlighted in specific diseases. The HDAC6 isoform, due to its unique cytoplasmic localization, modulates the acetylation status of tubulin, HSP90, TGF-β, and peroxiredoxins. HDAC6 also exerts noncatalytic activities through its interaction with ubiquitin. Both catalytic and noncatalytic functions of HDACs are being actively studied in the field of specific rare disorders beyond the well-established role in carcinogenesis. This Perspective outlines the application of HDAC(6) inhibitors in rare diseases, such as Rett syndrome, inherited retinal disorders, idiopathic pulmonary fibrosis, and Charcot-Marie-Tooth disease, highlighting their therapeutic potential as innovative and targeted disease-modifying agents.

MicroRNA Signature in Human Normal and Tumoral Neural Stem Cells

MicroRNAs, also called miRNAs or simply miR-, represent a unique class of non-coding RNAs that have gained exponential interest during recent years because of their determinant involvement in regulating the expression of several genes. Despite the increasing number of mature miRNAs recognized in the human species, only a limited proportion is engaged in the ontogeny of the central nervous system (CNS). miRNAs also play a pivotal role during the transition of normal neural stem cells (NSCs) into tumor-forming NSCs. More specifically, extensive studies have identified some shared miRNAs between NSCs and neural cancer stem cells (CSCs), namely miR-7, -124, -125, -181 and miR-9, -10, -130. In the context of NSCs, miRNAs are intercalated from embryonic stages throughout the differentiation pathway in order to achieve mature neuronal lineages. Within CSCs, under a different cellular context, miRNAs perform tumor suppressive or oncogenic functions that govern the homeostasis of brain tumors. This review will draw attention to the most characterizing studies dealing with miRNAs engaged in neurogenesis and in the tumoral neural stem cell context, offering the reader insight into the power of next generation miRNA-targeted therapies against brain malignances.

Stem cell models of human synapse development and degeneration

Many brain disorders exhibit altered synapse formation in development or synapse loss with age. To understand the complexities of human synapse development and degeneration, scientists now engineer neurons and brain organoids from human-induced pluripotent stem cells (hIPSC). These hIPSC-derived brain models develop both excitatory and inhibitory synapses and functional synaptic activity. In this review, we address the ability of hIPSC-derived brain models to recapitulate synapse development and insights gained into the molecular mechanisms underlying synaptic alterations in neuronal disorders. We also discuss the potential for more accurate human brain models to advance our understanding of synapse development, degeneration, and therapeutic responses.

Cystatin B Involvement in Synapse Physiology of Rodent Brains and Human Cerebral Organoids

Cystatin B (CSTB) is a ubiquitous protein belonging to a superfamily of protease inhibitors. CSTB may play a critical role in brain physiology because its mutations cause progressive myoclonic epilepsy-1A (EPM1A), the most common form of progressive myoclonic epilepsy. However, the molecular mechanisms underlying the role of CSTB in the central nervous system (CNS) are largely unknown. To investigate the possible involvement of CSTB in the synaptic plasticity, we analyzed its expression in synaptosomes as a model system in studying the physiology of the synaptic regions of the CNS. We found that CSTB is not only present in the synaptosomes isolated from rat and mouse brain cortex, but also secreted into the medium in a depolarization-controlled manner. In addition, using biorthogonal noncanonical amino acid tagging (BONCAT) procedure, we demonstrated, for the first time, that CSTB is locally synthesized in the synaptosomes. The synaptic localization of CSTB was confirmed in a human 3D model of cortical development, namely cerebral organoids. Altogether, these results suggest that CSTB may play a role in the brain plasticity and open a new perspective in studying the involvement of CSTB deregulation in neurodegenerative and neuropsychiatric diseases.

Cyborg Organoids: Implantation of Nanoelectronics via Organogenesis for Tissue-Wide Electrophysiology

Tissue-wide electrophysiology with single-cell and millisecond spatiotemporal resolution is critical for heart and brain studies. Issues arise, however, from the invasive, localized implantation of electronics that destroys well-connected cellular networks within matured organs. Here, we report the creation of cyborg organoids: the three-dimensional (3D) assembly of soft, stretchable mesh nanoelectronics across the entire organoid by the cell-cell attraction forces from 2D-to-3D tissue reconfiguration during organogenesis. We demonstrate that stretchable mesh nanoelectronics can migrate with and grow into the initial 2D cell layers to form the 3D organoid structure with minimal impact on tissue growth and differentiation. The intimate contact between the dispersed nanoelectronics and cells enables us to chronically and systematically observe the evolution, propagation, and synchronization of the bursting dynamics in human cardiac organoids through their entire organogenesis.

Pluripotent Stem Cells for Brain Repair: Protocols and Preclinical Applications in Cortical and Hippocampal Pathologies

Brain injuries causing chronic sensory or motor deficit, such as stroke, are among the leading causes of disability worldwide, according to the World Health Organization; furthermore, they carry heavy social and economic burdens due to decreased quality of life and need of assistance. Given the limited effectiveness of rehabilitation, novel therapeutic strategies are required to enhance functional recovery. Since cell-based approaches have emerged as an intriguing and promising strategy to promote brain repair, many efforts have been made to study the functional integration of neurons derived from pluripotent stem cells (PSCs), or fetal neurons, after grafting into the damaged host tissue. PSCs hold great promises for their clinical applications, such as cellular replacement of damaged neural tissues with autologous neurons. They also offer the possibility to create in vitro models to assess the efficacy of drugs and therapies. Notwithstanding these potential applications, PSC-derived transplanted neurons have to match the precise sub-type, positional and functional identity of the lesioned neural tissue. Thus, the requirement of highly specific and efficient differentiation protocols of PSCs in neurons with appropriate neural identity constitutes the main challenge limiting the clinical use of stem cells in the near future. In this Review, we discuss the recent advances in the derivation of telencephalic (cortical and hippocampal) neurons from PSCs, assessing specificity and efficiency of the differentiation protocols, with particular emphasis on the genetic and molecular characterization of PSC-derived neurons. Second, we address the remaining challenges for cellular replacement therapies in cortical brain injuries, focusing on electrophysiological properties, functional integration and therapeutic effects of the transplanted neurons.

Neural layer self-assembly in geometrically confined rat and human 3D cultures

Neurological disorders affect millions of Americans and this number is expected to rise with the aging population. Development of drugs to treat these disorders may be facilitated by improved in vitro models that faithfully reproduce salient features of the relevant brain regions. Current 3D culture methods face challenges with reliably reproducing microarchitectural features of brain morphology such as cortical or hippocampal layers. In this work, polydimethylsiloxane (PDMS) mini-wells were used to create low aspect ratio, adherent 3D constructs where neurons self-assemble into layers. Layer self-assembly was determined to depend on the size of the PDMS mini-well. Layer formation occurred in cultures composed of primary rat cortical neurons or human induced pluripotent stem cell-derived neurons and astrocytes and was robust and reproducible. Layered 3D constructs were found to have spontaneous neural activity characterized by long bursts similar to activity in the developing cortex. The responses of layered 3D cultures to drugs were more similar to in vivo data than those of 2D cultures. 3D constructs created with this method may be thus suitable as in vitro models for drug discovery for neurological disorders.

Stem cell models of schizophrenia, what have we learned and what is the potential?

Schizophrenia is a complex disorder with clinical manifestations in early adulthood. However, it may start with disruption of brain development caused by genetic or environmental factors, or both. Early deteriorating effects of genetic/environmental factors on neural development might be key to described disease causing mechanisms. Establishing cellular models with cells from affected individual using the induced pluripotent stem cells (iPSC) technology could be used to mimic early neurodevelopment alterations caused by risk genes or environmental stressors. Indeed, cellular models have allowed identification and further study of risk factors and the biological pathways in which they are involved. New advancements in differentiation methods such as defined and robust monolayer protocols and cerebral 3D organoids have made it possible to faithfully mimic neural development and neuronal functionality while CRISPR-editing tools assist to engineer isogenic cell lines to precisely explore genetic variation in polygenic diseases such as schizophrenia. Here we review the current field of iPSC models of schizophrenia and how risk factors can be modelled as well as discussing the common biological pathways involved.

Distinct Vulnerability and Resilience of Human Neuroprogenitor Subtypes in Cerebral Organoid Model of Prenatal Hypoxic Injury

Prenatal hypoxic injury (HI) is a leading cause of neurological disability. The immediate and long-term effects of hypoxia on progenitor homeostasis and developmental progression during early human brain development remain unclear. This gap is due to difficulty to access human fetal brain tissues and inadequate animal models to study human corticogenesis. Recent optimizations of cerebral organoid models derived from human embryonic stem (ES) cells present new opportunities to investigate pathophysiology of prenatal HI. Here, we implemented a transient HI model using human cerebral organoids with dorsal forebrain specification. We demonstrated that transient hypoxia resulted in immediate and prolonged apoptosis in cerebral organoids, with outer radial glia (oRG), a progenitor population more prominent in primates, and differentiating neuroblasts/immature neurons suffering larger losses. In contrast, neural stem cells in ventricular zone displayed relative resilience to HI and exhibited a shift of cleavage plane angle favoring symmetric division, thereby providing a mechanism to replenish the stem cell pool. Furthermore, we defined the vulnerable window and neurodifferentiation stages that are particularly sensitive to HI. Understanding cell type-specific and stage-dependent effects of prenatal HI on survival and mitotic behavior of human neuroprogenitor subtypes during early human corticogenesis helps elucidate the etiology of neurodevelopmental disorders, and provides a therapeutic starting point to protect the vulnerable populations at critical timeframes.

Functionalization of Brain Region-specific Spheroids with Isogenic Microglia-like Cells

Current brain spheroids or organoids derived from human induced pluripotent stem cells (hiPSCs) still lack a microglia component, the resident immune cells in the brain. The objective of this study is to engineer brain region-specific organoids from hiPSCs incorporated with isogenic microglia-like cells in order to enhance immune function. In this study, microglia-like cells were derived from hiPSCs using a simplified protocol with stage-wise growth factor induction, which expressed several phenotypic markers, including CD11b, IBA-1, CX3CR1, and P2RY12, and phagocytosed micron-size super-paramagnetic iron oxides. The derived cells were able to upregulate pro-inflammatory gene (TNF-α) and secrete anti-inflammatory cytokines (i.e., VEGF, TGF-β1, and PGE2) when stimulated with amyloid β42 oligomers, lipopolysaccharides, or dexamethasone. The derived isogenic dorsal cortical (higher expression of TBR1 and PAX6) and ventral (higher expression of NKX2.1 and PROX1) spheroids/organoids displayed action potentials and synaptic activities. Co-culturing the microglia-like cells (MG) with the dorsal (D) or ventral (V) organoids showed differential migration ability, intracellular Ca2+ signaling, and the response to pro-inflammatory stimuli (V-MG group had higher TNF-α and TREM2 expression). Transcriptome analysis exhibited 37 microglia-related genes that were differentially expressed in MG and D-MG groups. In addition, the hybrid D-MG spheroids exhibited higher levels of immunoreceptor genes in activating members, but the MG group contained higher levels for most of genes in inhibitory members (except SIGLEC5 and CD200). This study should advance our understanding of the microglia function in brain-like tissue and establish a transformative approach to modulate cellular microenvironment toward the goal of treating various neurological disorders.

Disease modelling in human organoids

The past decade has seen an explosion in the field of in vitro disease modelling, in particular the development of organoids. These self-organizing tissues derived from stem cells provide a unique system to examine mechanisms ranging from organ development to homeostasis and disease. Because organoids develop according to intrinsic developmental programmes, the resultant tissue morphology recapitulates organ architecture with remarkable fidelity. Furthermore, the fact that these tissues can be derived from human progenitors allows for the study of uniquely human processes and disorders. This article and accompanying poster highlight the currently available methods, particularly those aimed at modelling human biology, and provide an overview of their capabilities and limitations. We also speculate on possible future technological advances that have the potential for great strides in both disease modelling and future regenerative strategies.

Summary: Human organoids are important tools for modelling disease. This At a Glance article summarises the current organoid models of several human diseases, and discusses future prospects for these technologies.

"Necessity Is the Mother of Invention" or Inexpensive, Reliable, and Reproducible Protocol for Generating Organoids

Organoids are three-dimensional (3D) cell cultures that replicate some of the key features of morphology, spatial architecture, and functions of a particular organ. Organoids can be generated from both adult and pluripotent stem cells (PSCs), and complex organoids can also be obtained by combining different types of cells, including differentiated cells. The ability of pluripotent cells to self-organize into organotypic structures containing several cell subtypes specific for a particular organ was used for creating organoids of the brain, eye, kidney, intestine, and other organs. Despite the advantages of using PSCs for obtaining organoids, an essential shortcoming that prevents their widespread use has been a low yield when they are obtained from a PSC monolayer culture and a large variation in size. This leads to great heterogeneity on further differentiation. In this article, we describe our own protocol for generating standardized organoids, with emphasis on a method for generating brain organoids, which allows scaling-up experiments and makes their cultivation less expensive and easier.

Alzheimer's in a dish - induced pluripotent stem cell-based disease modeling

Background

Since the discovery of the induced pluripotent stem cell (iPSC) technique more than a decade ago, extensive progress has been made to develop clinically relevant cell culture systems. Alzheimer’s disease (AD) is the most common neurodegenerative disease, accounting for approximately two thirds of all cases of dementia. The massively increasing number of affected individuals explains the major interest of research in this disease as well as the strong need for better understanding of disease mechanisms.

Main body

IPSC-derived neural cells have been widely used to recapitulating key aspects of AD. In this Review we highlight the progress made in studying AD pathophysiology and address the currently available techniques, such as specific differentiation techniques for AD-relevant cell types as well as 2D and 3D cultures. Finally, we critically discuss the key challenges and future directions of this field and how some of the major limitations of the iPSC technique may be overcome.

Conclusion

Stem cell-based disease models have the potential to induce a paradigm shift in biomedical research. In particular, the combination of the iPSC technology with recent advances in gene editing or 3D cell cultures represents a breakthrough for in vitro disease modeling and provides a platform for a better understanding of disease mechanisms in human cells and the discovery of novel therapeutics.

In vitro and in silico Models to Study Mosquito-Borne Flavivirus Neuropathogenesis, Prevention, and Treatment

Mosquito-borne flaviviruses can cause disease in the nervous system, resulting in a significant burden of morbidity and mortality. Disease models are necessary to understand neuropathogenesis and identify potential therapeutics and vaccines. Non-human primates have been used extensively but present major challenges. Advances have also been made toward the development of humanized mouse models, but these models still do not fully represent human pathophysiology. Recent developments in stem cell technology and cell culture techniques have allowed the development of more physiologically relevant human cell-based models. In silico modeling has also allowed researchers to identify and predict transmission patterns and discover potential vaccine and therapeutic candidates. This review summarizes the research on in vitro and in silico models used to study three mosquito-borne flaviviruses that cause neurological disease in humans: West Nile, Dengue, and Zika. We also propose a roadmap for 21st century research on mosquito-borne flavivirus neuropathogenesis, prevention, and treatment.

The Emergence of Stem Cell-Based Brain Organoids: Trends and Challenges

Recent developments in 3D cultures exploiting the self-organization ability of pluripotent stem cells have enabled the generation of powerful in vitro systems termed brain organoids. These 3D tissues recapitulate many aspects of human brain development and disorders occurring in vivo. When combined with improved differentiation methods, these in vitro systems allow the generation of more complex "assembloids," which are able to reveal cell diversities, microcircuits, and cell-cell interactions within their 3D organization. Here, the ways in which human brain organoids have contributed to demystifying the complexities of brain development and modeling of developmental disorders is reviewed and discussed. Furthermore, challenging questions that are yet to be addressed by emerging brain organoid research are discussed.

An atlas of nano-enabled neural interfaces

Advances in microscopy and molecular strategies have allowed researchers to gain insight into the intricate organization of the mammalian brain and the roles that neurons play in processing information. Despite vast progress, therapeutic strategies for neurological disorders remain limited, owing to a lack of biomaterials for sensing and modulating neuronal signalling in vivo. Therefore, there is a pressing need for developing material-based tools that can form seamless biointerfaces and interrogate the brain with unprecedented resolution. In this Review, we discuss important considerations in material design and implementation, highlight recent breakthroughs in neural sensing and modulation, and propose future directions in neurotechnology research. Our goal is to create an atlas for nano-enabled neural interfaces and to demonstrate how emerging nanotechnologies can interrogate neural systems spanning multiple biological length scales.

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.

Engineering a 3D functional human peripheral nerve in vitro using the Nerve-on-a-Chip platform

Development of “organ-on-a-chip” systems for neuroscience applications are lagging due in part to the structural complexity of the nervous system and limited access of human neuronal & glial cells. In addition, rates for animal models in translating to human success are significantly lower for neurodegenerative diseases. Thus, a preclinical in vitro human cell-based model capable of providing critical clinical metrics such as nerve conduction velocity and histomorphometry are necessary to improve prediction and translation of in vitro data to successful clinical trials. To answer this challenge, we present an in vitro biomimetic model of all-human peripheral nerve tissue capable of showing robust neurite outgrowth (~5 mm), myelination of hNs by primary human Schwann cells (~5%), and evaluation of nerve conduction velocity (0.13–0.28 m/sec), previously unrealized for any human cell-based in vitro system. To the best of our knowledge, this Human Nerve-on-a-chip (HNoaC) system is the first biomimetic microphysiological system of myelinated human peripheral nerve which can be used for evaluating electrophysiological and histological metrics, the gold-standard assessment techniques previously only possible with in vivo studies.

Effects of astrocyte on neuronal outgrowth in a layered 3D structure

Background

Human brain models and pharmacological models of brain diseases are in high demand for drug screening because animal models have been found to be less than ideal for fully representing the human brain and are likely to fail during drug screening and testing; therefore, the construction of brain-like tissues is necessary. Due to the complexity of cortical tissue, the in vitro construction of brain-like tissue models has been restricted to mostly two-dimensional (2D) models and, on a limited scale, three-dimensional (3D) models.

Methods

In this study, 3D tissue blocks encapsulating neurons and astrocytes were constructed and cultured in vitro to mimic the cortex of the brain and to investigate the effects of astrocytes on the growth of neurons in a 3D culture.

Results

The results indicated that such methodology can provide a 3D culture environment suitable for neurons and astrocytes to live and function. When both cells were evenly mixed and cultured in a 3D manner, the astrocytes, which showed better outgrowth and a higher proliferation rate, benefited more than the neurons. On the other hand, the neurons benefited, showing longer axons and a denser network of dendrites, when they were accompanied by astrocytes at a certain distance.

Conclusion

In conclusion, astrocytes stimulated the outgrowth of neurons in a 3D culture environment in vitro. Regardless, the spatial relationship between both types of cells should be controlled. Thus, culturing cells in a 3D manner is necessary to investigate the correlations between them. This study provides a foundation for biofabricating 3D neurons’ cultures to allow for a deeper insight into the relationship between astrocytes or other glial cells and neurons in a 3D culture that is similar to the natural environment of the brain.

Engineering microsystems to recapitulate brain physiology on a chip

The structural and functional organization of the human brain consists of 52 regions with distinct cellular organization. In vitro models for normal and pathological states using isolated brain-region-specific 3D engineered tissues fail to recapitulate information integration and/or transfer that arises from connectivity among neuroanatomical structures. Therefore, development of brain-on-a-chip microsystems must shift to multiple region neuron network designs to be relevant in brain functionality and deficit modeling. However, in vitro formation of multiregional networks on microdevices presents several challenges that we illustrate using a few neurological disorders; and we offer guidance, depending on objectives (HTS, disease modeling, etc.) for rational design of microfluidic systems and better emulation of in vivo conditions.

Use and application of 3D-organoid technology

The capacity of the 3D-organoid cultures to resemble a near-physiological tissue organization and to mimic - to a certain degree - organ functionality, make organoids an excellent model for applications spanning from basic developmental/stem cell research to personalized medicine. Here, we review key findings achieved through organoid technology, and we discuss applications such as disease - and tumour modelling, correction of genetic mutations and understanding gene - and cell functions. Finally, we discuss future developments in the field.

Putting Two Heads Together to Build a Better Brain

3D organoids enable in vitro human brain development models, but they have not yet recapitulated some essential features of brain circuit formation. Recently, several studies appearing in Nature, Nature Methods, and Cell Stem Cell generated fused organoid models of inhibitory and excitatory neuron development, which can now achieve functional circuit integration.

Drug screening for human genetic diseases using iPSC models

Induced pluripotent stem cells (iPSCs) enable the generation of previously unattainable, scalable quantities of disease-relevant tissues from patients suffering from essentially any genetic disorder. This cellular material has proven instrumental for drug screening efforts on these disorders, and has facilitated the identification of novel therapeutics for patients. Here we will review the foundational technologies that have enabled iPSCs, the power and limitations of iPSC-based compound screens along with screening guidelines, and recent examples of screening efforts. Additionally we will provide a brief commentary on the future scientific roadmap using pluripotent- and 3D organoid-based, combinatorial approaches.

Stem Cells to Inform the Neurobiology of Mental Illness

The inception of human-induced pluripotent stem cell (hiPSCs) technology has provided an exciting platform upon which the modelling and treatment of human neurodevelopmental and neuropsychiatric disorders may be expedited. Although the genetic architecture of these disorders is far more complex than previously imagined, many key loci have at last been identified. This has allowed in vivo and in vitro technologies to be refined to model specific high-penetrant genetic loci involved in both disorders. Animal models of neurodevelopmental disorders, such as schizophrenia and autism spectrum disorders, show limitations in recapitulating the full complexity and heterogeneity of human neurodevelopmental disease states. Indeed, patient-derived hiPSCs offer distinct advantages over classical animal models in the study of human neuropathologies. Here we have discussed the current, relative translational merit of hiPSCs in investigating human neurodevelopmental and neuropsychiatric disorders with a specific emphasis on the utility of such systems to aid in the identification of biomarkers. We have highlighted the promises and pitfalls of reprogramming cell fate for the study of these disorders and provide recommendations for future directions in this field in order to overcome current limitations. Ultimately, this will aid in the development of effective clinical strategies for diverse patient populations affected by these disorders with the aim of also leading to biomarker identification.

Suitability of 3D human brain spheroid models to distinguish toxic effects of gold and poly-lactic acid nanoparticles to assess biocompatibility for brain drug delivery

Background

The blood brain barrier (BBB) is the bottleneck of brain-targeted drug development. Due to their physico-chemical properties, nanoparticles (NP) can cross the BBB and accumulate in different areas of the central nervous system (CNS), thus are potential tools to carry drugs and treat brain disorders. In vitro systems and animal models have demonstrated that some NP types promote neurotoxic effects such as neuroinflammation and neurodegeneration in the CNS. Thus, risk assessment of the NP is required, but current 2D cell cultures fail to mimic complex in vivo cellular interactions, while animal models do not necessarily reflect human effects due to physiological and species differences.

Results

We evaluated the suitability of in vitro models that mimic the human CNS physiology, studying the effects of metallic gold NP (AuNP) functionalized with sodium citrate (Au-SC), or polyethylene glycol (Au-PEG), and polymeric polylactic acid NP (PLA-NP). Two different 3D neural models were used (i) human dopaminergic neurons differentiated from the LUHMES cell line (3D LUHMES) and (ii) human iPSC-derived brain spheroids (BrainSpheres). We evaluated NP uptake, mitochondrial membrane potential, viability, morphology, secretion of cytokines, chemokines and growth factors, and expression of genes related to ROS regulation after 24 and 72 h exposures. NP were efficiently taken up by spheroids, especially when PEGylated and in presence of glia. AuNP, especially PEGylated AuNP, effected mitochondria and anti-oxidative defense. PLA-NP were slightly cytotoxic to 3D LUHMES with no effects to BrainSpheres.

Conclusions

3D brain models, both monocellular and multicellular are useful in studying NP neurotoxicity and can help identify how specific cell types of CNS are affected by NP.

Electronic supplementary material

The online version of this article (10.1186/s12989-019-0307-3) contains supplementary material, which is available to authorized users.

The moral status of cerebral organoids

Organoids are 3D biological structures constructed from stem cells in vitro. They partially mimic the function of real organs. Although the number of articles detailing this technology has increased in recent years, papers debating their ethical issues are few. In addition, many of such articles outline a mere summary of potential ethical concerns associated with organoids, although some have focused on consciousness assessment or organoid use in cystic fibrosis treatment. This article seeks to evaluate the moral status of cerebral organoids and to determine under which conditions their use should be allowed from a bioethical standpoint. We will present an overview of recent steps in developing highly advanced cerebral organoids, followed by an analysis of their ethics based on three factors: human origin, a specific biological threshold (which, once crossed, grants an entity moral status), and the potential to generate human beings. We will also make practical recommendations for researchers working in this biological field.

Brain organoids and insights on human evolution

Human brain organoids, generated from pluripotent stem cells, have emerged as a promising technique for modeling early stages of human neurodevelopment in controlled laboratory conditions. Although the applications for disease modeling in a dish have become routine, the use of these brain organoids as evolutionary tools is only now getting momentum. Here, we will review the current state of the art on the use of brain organoids from different species and the molecular and cellular insights generated from these studies. Besides, we will discuss how this model might be beneficial for human health and the limitations and future perspectives of this technology.

Modeling neuropsychiatric disorders using human induced pluripotent stem cells

Neuropsychiatric disorders are complex disorders characterized by heterogeneous genetic variations, variable symptoms, and widespread changes in anatomical pathology. In the context of neuropsychiatric disorders, limited access to relevant tissue types presents challenges for understanding disease etiology and developing effective treatments. Induced pluripotent stem cells (iPSCs) reprogrammed from patient somatic cells offer an opportunity to recapitulate disease development in relevant cell types, and they provide novel approaches for understanding disease mechanisms and for development of effective treatments. Here we review recent progress and challenges in differentiation paradigms for generating disease-relevant cells and recent studies of neuropsychiatric disorders using human pluripotent stem cell (hPSC) models where cellular phenotypes linked to disease have been reported. The use of iPSC-based disease models holds great promise for understanding disease mechanisms and supporting discovery of effective treatments.

OLIG2 Drives Abnormal Neurodevelopmental Phenotypes in Human iPSC-Based Organoid and Chimeric Mouse Models of Down Syndrome

Down syndrome (DS) is a common neurodevelopmental disorder, and cognitive defects in DS patients may arise from imbalances in excitatory and inhibitory neurotransmission. Understanding the mechanisms underlying such imbalances may provide opportunities for therapeutic intervention. Here, we show that human induced pluripotent stem cells (hiPSCs) derived from DS patients overproduce OLIG2⁺ ventral forebrain neural progenitors. As a result, DS hiPSC-derived cerebral organoids excessively produce specific subclasses of GABAergic interneurons and cause impaired recognition memory in neuronal chimeric mice. Increased OLIG2 expression in DS cells directly upregulates interneuron lineage-determining transcription factors. shRNA-mediated knockdown of OLIG2 largely reverses abnormal gene expression in early-stage DS neural progenitors, reduces interneuron production in DS organoids and chimeric mouse brains, and improves behavioral deficits in DS chimeric mice. Thus, altered OLIG2 expression may underlie neurodevelopmental abnormalities and cognitive defects in DS patients.

New frontiers in modeling tuberous sclerosis with human stem cell-derived neurons and brain organoids

Recent advances in human stem cell and genome engineering have enabled the generation of genetically defined human cellular models for brain disorders. These models can be established from a patient's own cells and can be genetically engineered to generate isogenic, controlled systems for mechanistic studies. Given the challenges of obtaining and working with primary human brain tissue, these models fill a critical gap in our understanding of normal and abnormal human brain development and provide an important complement to animal models. Recently, there has been major progress in modeling the neuropathophysiology of the canonical "mTORopathy" tuberous sclerosis complex (TSC) with such approaches. Studies using two- and three-dimensional cultures of human neurons and glia have provided new insights into how mutations in the TSC1 and TSC2 genes impact human neural development and function. Here we discuss recent progress in human stem cell-based modeling of TSC and highlight challenges and opportunities for further efforts in this area.

Human-Derived Brain Models: Windows into Neuropsychiatric Disorders and Drug Therapies

Human-derived neurons and brain organoids have revolutionized our ability to model brain development in a dish. In this review, we discuss the potential for human brain models to advance drug discovery for complex neuropsychiatric disorders. First, we address the advantages of human brain models to screen for new drugs capable of altering CNS activity. Next, we propose an experimental pipeline for using human-derived neurons and brain organoids to rapidly assess drug impact on key events in brain development, including neurite extension, synapse formation, and neural activity. The experimental pipeline begins with automated high content imaging for analysis of neurites, synapses, and neuronal viability. Following morphological examination, multi-well microelectrode array technology examines neural activity in response to drug treatment. These techniques can be combined with high throughput sequencing and mass spectrometry to assess associated transcriptional and proteomic changes. These combined technologies provide a foundation for neuropsychiatric drug discovery and future clinical assessment of patient-specific drug responses.

Modern approaches for modelling dystonia and Huntington's disease in vitro and in vivo

Dystonia associated with Huntington's disease, Parkinson's disease or other neurodegenerative diseases substantially affects patients' quality of life and is a major health problem worldwide. The above-mentioned diseases are characterized by neurodegeneration accompanied by motor and cognitive impairment and often have complex aetiology. A frequent feature of these conditions is the abnormal accumulation of protein aggregates within specific neuronal populations in the affected brain regions. Familial neurodegenerative diseases are associated with a number of genetic mutations. Identification of these mutations allowed creation of modern model systems for studying neurodegeneration, either in cultured cells or in model animals. Animal models, especially mouse models, have contributed considerably to improving our understanding of the pathophysiology of neurodegenerative diseases. These models have allowed study of the pathogenic mechanisms and development of new disease-modifying strategies and therapeutic approaches. However, due to the complex nature of these pathologies and the irreversible damage that they cause to the neural tissue, effective therapies against neurodegeneration remain to be elaborated. In this review, we provide an overview of cellular and animal models developed for studying neurodegenerative diseases, including Huntington's disease and dystonia of different origins.

Using brain organoids to study human neurodevelopment, evolution and disease

The brain is one of the most complex organs, responsible for the advanced intellectual and cognitive ability of humans. Although primates are to some extent capable of performing cognitive tasks, their abilities are less evolved. One of the reasons for this is the vast differences in the brain of humans compared to other mammals, in terms of shape, size and complexity. Such differences make the study of human brain development fascinating. Interestingly, the cerebral cortex is by far the most complex brain region resulting from its selective evolution within mammals over millions of years. Unraveling the molecular and cellular mechanisms regulating brain development, as well as the evolutionary differences seen across species and the need to understand human brain disorders, are some of the reasons why scientists are interested in improving their current knowledge on human corticogenesis. Toward this end, several animal models including primates have been used, however, these models are limited in their extent to recapitulate human-specific features. Recent technological achievements in the field of stem cell research, which have enabled the generation of human models of corticogenesis, called brain or cerebral organoids, are of great importance. This review focuses on the main cellular and molecular features of human corticogenesis and the use of brain organoids to study it. We will discuss the key differences between cortical development in human and nonhuman mammals, the technological applications of brain organoids and the different aspects of cortical development in normal and pathological conditions, which can be modeled using brain organoids. This article is categorized under: Comparative Development and Evolution > Regulation of Organ Diversity Nervous System Development > Vertebrates: General Principles.

Organoid technology in cancer precision medicine

Organoid technology has been remarkably improved over the last decade. Various organoids have been derived from different types of tissues and recapitulate their organ-specific gene expression signatures, particular tissue spatial structures and functions of their original tissue. The patient-derived organoids (PDOs) have been used to elucidate crucial scientific questions, including the relationships between genetic/epigenetic alterations and drug responses, cell plasticity during disease progressions, and mechanisms of drug resistances. With the great expectations, PDOs will be widely used to facilitate the personalized medical decisions, which have the potential to profoundly improve patient outcomes. In this review, we will discuss the developmental details, current achievements, applications and challenges of organoid technology in precision cancer medicine.

Human 3D cellular model of hypoxic brain injury of prematurity

Owing to recent medical and technological advances in neonatal care, infants born extremely premature have increased survival rates1,2. After birth, these infants are at high risk of hypoxic episodes because of lung immaturity, hypotension and lack of cerebral-flow regulation, and can develop a severe condition called encephalopathy of prematurity3. Over 80% of infants born before post-conception week 25 have moderate-to-severe long-term neurodevelopmental impairments4. The susceptible cell types in the cerebral cortex and the molecular mechanisms underlying associated gray-matter defects in premature infants remain unknown. Here we used human three-dimensional brain-region-specific organoids to study the effect of oxygen deprivation on corticogenesis. We identified specific defects in intermediate progenitors, a cortical cell type associated with the expansion of the human cerebral cortex, and showed that these are related to the unfolded protein response and changes. Moreover, we verified these findings in human primary cortical tissue and demonstrated that a small-molecule modulator of the unfolded protein response pathway can prevent the reduction in intermediate progenitors following hypoxia. We anticipate that this human cellular platform will be valuable for studying the environmental and genetic factors underlying injury in the developing human brain.

Cell diversity in the human cerebral cortex: from the embryo to brain organoids

The development and wiring of the central nervous system is a remarkable biological process that starts with the generation of and interaction between a large diversity of cell types. Our understanding of the developmental logic that drives cellular diversification in the mammalian brain comes, to a large extent, from studies in rodents. However, identifying the unique cellular processes underlying primate corticogenesis has been slow, due to the challenges associated with directly observing and manipulating brain tissue from these species. Recent technological advances in two areas hold promise to accelerate discovery of the mechanisms that govern human brain development, evolution, and pathophysiology of disease. Molecular profiling of large numbers of single cells can now capture cell identity and cell states within a complex tissue. Furthermore, modeling aspects of human organogenesis in vitro, even for tissues as complex as the brain, has been advanced by the use of three-dimensional organoid systems. Here, we describe how these approaches have been applied to date and how they promise to uncover the principles of cell diversification in the developing human brain.

Design Principles for Pluripotent Stem Cell-Derived Organoid Engineering

Human morphogenesis is a complex process involving distinct microenvironmental and physical signals that are manipulated in space and time to give rise to complex tissues and organs. Advances in pluripotent stem cell (PSC) technology have promoted the in vitro recreation of processes involved in human morphogenesis. The development of organoids from human PSCs represents one reliable source for modeling a large spectrum of human disorders, as well as a promising approach for drug screening and toxicological tests. Based on the "self-organization" capacity of stem cells, different PSC-derived organoids have been created; however, considerable differences between in vitro-generated PSC-derived organoids and their in vivo counterparts have been reported. Advances in the bioengineering field have allowed the manipulation of different components, including cellular and noncellular factors, to better mimic the in vivo microenvironment. In this review, we focus on different examples of bioengineering approaches used to promote the self-organization of stem cells, including assembly, patterning, and morphogenesis in vitro, contributing to tissue-like structure formation.

Brain organoids: advances, applications and challenges

Brain organoids are self-assembled three-dimensional aggregates generated from pluripotent stem cells with cell types and cytoarchitectures that resemble the embryonic human brain. As such, they have emerged as novel model systems that can be used to investigate human brain development and disorders. Although brain organoids mimic many key features of early human brain development at molecular, cellular, structural and functional levels, some aspects of brain development, such as the formation of distinct cortical neuronal layers, gyrification, and the establishment of complex neuronal circuitry, are not fully recapitulated. Here, we summarize recent advances in the development of brain organoid methodologies and discuss their applications in disease modeling. In addition, we compare current organoid systems to the embryonic human brain, highlighting features that currently can and cannot be recapitulated, and discuss perspectives for advancing current brain organoid technologies to expand their applications.

Generation and assembly of human brain region-specific three-dimensional cultures

The ability to generate region-specific three-dimensional (3D) models to study human brain development offers great promise for understanding the nervous system in both healthy individuals and patients. In this protocol, we describe how to generate and assemble subdomain-specific forebrain spheroids, also known as brain region-specific organoids, from human pluripotent stem cells (hPSCs). We describe how to pattern the neural spheroids toward either a dorsal forebrain or a ventral forebrain fate, establishing human cortical spheroids (hCSs) and human subpallial spheroids (hSSs), respectively. We also describe how to combine the neural spheroids in vitro to assemble forebrain assembloids that recapitulate the interactions of glutamatergic and GABAergic neurons seen in vivo. Astrocytes are also present in the human forebrain-specific spheroids, and these undergo maturation when the forebrain spheroids are cultured long term. The initial generation of neural spheroids from hPSCs occurs in <1 week, with regional patterning occurring over the subsequent 5 weeks. After the maturation stage, brain region-specific spheroids are amenable to a variety of assays, including live-cell imaging, calcium dynamics, electrophysiology, cell purification, single-cell transcriptomics, and immunohistochemistry studies. Once generated, forebrain spheroids can also be matured for >24 months in culture.

Therapeutic Potential of Patient iPSC-Derived iMelanocytes in Autologous Transplantation

Induced pluripotent stem cells (iPSCs) are a promising melanocyte source as they propagate indefinitely and can be established from patients. However, the in vivo functions of human iPSC-derived melanocytes (hiMels) remain unknown. Here, we generated hiMels from vitiligo patients using a three-dimensional system with enhanced differentiation efficiency, which showed characteristics of human epidermal melanocytes with high sequence similarity and involved in multiple vitiligo-associated signaling pathways. A modified hair follicle reconstitution assay in vivo showed that MITF+PAX3+TYRP1+ hiMels were localized in the mouse hair bulb and epidermis and produced melanin up to 7 weeks after transplantation, whereas MITF+PAX3+TYRP1- hiMelanocyte stem cells integrated into the bulge-subbulge regions. Overall, these data demonstrate the long-term functions of hiMels in vivo to reconstitute pigmented hair follicles and to integrate into normal regions for both mature melanocytes and melanocyte stem cells, providing an alternative source of personalized cellular therapy for depigmentation.

Organs to Cells and Cells to Organoids: The Evolution of in vitro Central Nervous System Modelling

With 100 billion neurons and 100 trillion synapses, the human brain is not just the most complex organ in the human body, but has also been described as "the most complex thing in the universe." The limited availability of human living brain tissue for the study of neurogenesis, neural processes and neurological disorders has resulted in more than a century-long strive from researchers worldwide to model the central nervous system (CNS) and dissect both its striking physiology and enigmatic pathophysiology. The invaluable knowledge gained with the use of animal models and post mortem human tissue remains limited to cross-species similarities and structural features, respectively. The advent of human induced pluripotent stem cell (hiPSC) and 3-D organoid technologies has revolutionised the approach to the study of human brain and CNS in vitro, presenting great potential for disease modelling and translational adoption in drug screening and regenerative medicine, also contributing beneficially to clinical research. We have surveyed more than 100 years of research in CNS modelling and provide in this review an historical excursus of its evolution, from early neural tissue explants and organotypic cultures, to 2-D patient-derived cell monolayers, to the latest development of 3-D cerebral organoids. We have generated a comprehensive summary of CNS modelling techniques and approaches, protocol refinements throughout the course of decades and developments in the study of specific neuropathologies. Current limitations and caveats such as clonal variation, developmental stage, validation of pluripotency and chromosomal stability, functional assessment, reproducibility, accuracy and scalability of these models are also discussed.

Generation of human midbrain organoids from induced pluripotent stem cells

The development of brain organoids represents a major technological advance in the stem cell field, a novel bridge between traditional 2D cultures and in vivo animal models. In particular, the development of midbrain organoids containing functional dopaminergic neurons producing neuromelanin granules, a by-product of dopamine synthesis, represents a potential new model for Parkinson’s disease. To generate human midbrain organoids, we introduce specific inductive cues, at defined timepoints, during the 3D culture process to drive the stem cells towards a midbrain fate. In this method paper, we describe a standardized protocol to generate human midbrain organoids (hMOs) from induced pluripotent stem cells (iPSCs). This protocol was developed to demonstrate how human iPSCs can be successfully differentiated into numerous, high quality midbrain organoids in one batch. We also describe adaptations for cryosectioning of fixed organoids for subsequent histological analysis.

Studying Heterotypic Cell⁻Cell Interactions in the Human Brain Using Pluripotent Stem Cell Models for Neurodegeneration

Human cerebral organoids derived from induced pluripotent stem cells (iPSCs) provide novel tools for recapitulating the cytoarchitecture of the human brain and for studying biological mechanisms of neurological disorders. However, the heterotypic interactions of neurovascular units, composed of neurons, pericytes (i.e., the tissue resident mesenchymal stromal cells), astrocytes, and brain microvascular endothelial cells, in brain-like tissues are less investigated. In addition, most cortical organoids lack a microglia component, the resident immune cells in the brain. Impairment of the blood-brain barrier caused by improper crosstalk between neural cells and vascular cells is associated with many neurodegenerative disorders. Mesenchymal stem cells (MSCs), with a phenotype overlapping with pericytes, have promotion effects on neurogenesis and angiogenesis, which are mainly attributed to secreted growth factors and extracellular matrices. As the innate macrophages of the central nervous system, microglia regulate neuronal activities and promote neuronal differentiation by secreting neurotrophic factors and pro-/anti-inflammatory molecules. Neuronal-microglia interactions mediated by chemokines signaling can be modulated in vitro for recapitulating microglial activities during neurodegenerative disease progression. In this review, we discussed the cellular interactions and the physiological roles of neural cells with other cell types including endothelial cells and microglia based on iPSC models. The therapeutic roles of MSCs in treating neural degeneration and pathological roles of microglia in neurodegenerative disease progression were also discussed.

Functional Cortical Axon Tracts Generated from Human Stem Cell-Derived Neurons

IMPACT STATEMENT

Axon regeneration is negligible in the adult mammalian brain, and thus, white matter damage often leads to permanent neurological deficits. A novel approach for axon repair is the generation of axon tracts in the laboratory setting followed by transplantation of these constructs. This article details a human substrate for this repair strategy. Using the technique of axon stretch growth, functional cortical axon tracts are generated from human pluripotent stem cells at rates of up to 1 mm/day. These results form the basis of a potential patient-specific protocol for cerebral axon transplantation after injury.

Reduced variability of neural progenitor cells and improved purity of neuronal cultures using magnetic activated cell sorting

Genetic and epigenetic variability between iPSC-derived neural progenitor cells (NPCs) combined with differences in investigator technique and selection protocols contributes to variability between NPC lines, which subsequently impacts the quality of differentiated neuronal cultures. We therefore sought to develop an efficient method to reduce this variability in order to improve the purity of NPC and neuronal cultures. Here, we describe a magnetic activated cell sorting (MACS) method for enriching NPC cultures for CD271-/CD133+ cells at both early (<2–3) and late (>10) passage. MACS results in a similar sorting efficiency to fluorescence activated cell sorting (FACS), while achieving an increased yield of live cells and reduced cellular stress. Furthermore, neurons derived from MACS NPCs showed greater homogeneity between cell lines compared to those derived from unsorted NPCs. We conclude that MACS is a cheap technique for incorporation into standard NPC differentiation and maintenance protocols in order to improve culture homogeneity and consistency.

Synaptic dysfunction in neurodegenerative and neurodevelopmental diseases: an overview of induced pluripotent stem-cell-based disease models

Synaptic dysfunction in CNS disorders is the outcome of perturbations in physiological synapse structure and function, and can be either the cause or the consequence in specific pathologies. Accumulating data in the field of neuropsychiatric disorders, including autism spectrum disorders, schizophrenia and bipolar disorder, point to a neurodevelopmental origin of these pathologies. Due to a relatively early onset of behavioural and cognitive symptoms, it is generally acknowledged that mental illness initiates at the synapse level. On the other hand, synaptic dysfunction has been considered as an endpoint incident in neurodegenerative diseases, such as Alzheimer's, Parkinson's and Huntington's, mainly due to the considerably later onset of clinical symptoms and progressive appearance of cognitive deficits. This dichotomy has recently been challenged, particularly since the discovery of cell reprogramming technologies and the generation of induced pluripotent stem cells from patient somatic cells. The creation of 'disease-in-a-dish' models for multiple CNS pathologies has revealed unexpected commonalities in the molecular and cellular mechanisms operating in both developmental and degenerative conditions, most of which meet at the synapse level. In this review we discuss synaptic dysfunction in prototype neurodevelopmental and neurodegenerative diseases, emphasizing overlapping features of synaptopathy that have been suggested by studies using induced pluripotent stem-cell-based systems. These valuable disease models have highlighted a potential neurodevelopmental component in classical neurodegenerative diseases that is worth pursuing and investigating further. Moving from demonstration of correlation to understanding mechanistic causality forms the basis for developing novel therapeutics.