Genetically engineered human cortical spheroid models of tuberous sclerosis

Amplification of human interneuron progenitors promotes brain tumors and neurological defects

Evolutionary development of the human brain is characterized by the expansion of various brain regions. Here, we show that developmental processes specific to humans are responsible for malformations of cortical development (MCDs), which result in developmental delay and epilepsy in children. We generated a human cerebral organoid model for tuberous sclerosis complex (TSC) and identified a specific neural stem cell type, caudal late interneuron progenitor (CLIP) cells. In TSC, CLIP cells over-proliferate, generating excessive interneurons, brain tumors, and cortical malformations. Epidermal growth factor receptor inhibition reduces tumor burden, identifying potential treatment options for TSC and related disorders. The identification of CLIP cells reveals the extended interneuron generation in the human brain as a vulnerability for disease. In addition, this work demonstrates that analyzing MCDs can reveal fundamental insights into human-specific aspects of brain development.

Modeling tuberous sclerosis with organoids

[Figure: see text].

Modeling human neurodevelopmental diseases with brain organoids

Studying the etiology of human neurodevelopmental diseases has long been a challenging task due to the brain's complexity and its limited accessibility. Human pluripotent stem cells (hPSCs)-derived brain organoids are capable of recapitulating various features and functionalities of the human brain, allowing the investigation of intricate pathogenesis of developmental abnormalities. Over the past years, brain organoids have facilitated identifying disease-associated phenotypes and underlying mechanisms for human neurodevelopmental diseases. Integrating with more cutting-edge technologies, particularly gene editing, brain organoids further empower human disease modeling. Here, we review the latest progress in modeling human neurodevelopmental disorders with brain organoids.

Modeling Somatic Mutations Associated With Neurodevelopmental Disorders in Human Brain Organoids

Neurodevelopmental disorders (NDDs) are a collection of diseases with early life onset that often present with developmental delay, cognitive deficits, and behavioral conditions. In some cases, severe outcomes such as brain malformations and intractable epilepsy can occur. The mutations underlying NDDs may be inherited or de novo, can be gain- or loss-of-function, and can affect one or more genes. Recent evidence indicates that brain somatic mutations contribute to several NDDs, in particular malformations of cortical development. While advances in sequencing technologies have enabled the detection of these somatic mutations, the mechanisms by which they alter brain development and function are not well understood due to limited model systems that recapitulate these events. Human brain organoids have emerged as powerful models to study the early developmental events of the human brain. Brain organoids capture the developmental progression of the human brain and contain human-enriched progenitor cell types. Advances in human stem cell and genome engineering provide an opportunity to model NDD-associated somatic mutations in brain organoids. These organoids can be tracked throughout development to understand the impact of somatic mutations on early human brain development and function. In this review, we discuss recent evidence that somatic mutations occur in the developing human brain, that they can lead to NDDs, and discuss how they could be modeled using human brain organoids.

Mechanistic target of rapamycin signaling in human nervous system development and disease

Mechanistic target of rapamycin (mTOR) is a highly conserved serine/threonine kinase that regulates fundamental cellular processes including growth control, autophagy and metabolism. mTOR has key functions in nervous system development and mis-regulation of mTOR signaling causes aberrant neurodevelopment and neurological diseases, collectively called mTORopathies. In this mini review we discuss recent studies that have deepened our understanding of the key roles of the mTOR pathway in human nervous system development and disease. Recent advances in single-cell transcriptomics have been exploited to reveal specific roles for mTOR signaling in human cortical development that may have contributed to the evolutionary divergence from our primate ancestors. Cerebral organoid technology has been utilized to show that mTOR signaling is active in and regulates outer radial glial cells (RGCs), a population of neural stem cells that distinguish the human developing cortex. mTOR signaling has a well-established role in hamartoma syndromes such as tuberous sclerosis complex (TSC) and other mTORopathies. New ultra-sensitive techniques for identification of somatic mTOR pathway mutations have shed light on the neurodevelopmental origin and phenotypic heterogeneity seen in mTORopathy patients. These emerging studies suggest that mTOR signaling may facilitate developmental processes specific to human cortical development but also, when mis-regulated, cause cortical malformations and neurological disease.

Gene Editing in Pluripotent Stem Cells and Their Derived Organoids

With the rapid rise in gene-editing technology, pluripotent stem cells (PSCs) and their derived organoids have increasingly broader and practical applications in regenerative medicine. Gene-editing technologies, from large-scale nucleic acid endonucleases to CRISPR, have ignited a global research and development boom with significant implications in regenerative medicine. The development of regenerative medicine technologies, regardless of whether it is PSCs or gene editing, is consistently met with controversy. Are the tools for rewriting the code of life a boon to humanity or a Pandora's box? These technologies raise concerns regarding ethical issues, unexpected mutations, viral infection, etc. These concerns remain even as new treatments emerge. However, the potential negatives cannot obscure the virtues of PSC gene editing, which have, and will continue to, benefit mankind at an unprecedented rate. Here, we briefly introduce current gene-editing technology and its application in PSCs and their derived organoids, while addressing ethical concerns and safety risks and discussing the latest progress in PSC gene editing. Gene editing in PSCs creates visualized in vitro models, providing opportunities for examining mechanisms of known and unknown mutations and offering new possibilities for the treatment of cancer, genetic diseases, and other serious or refractory disorders. From model construction to treatment exploration, the important role of PSCs combined with gene editing in basic and clinical medicine studies is illustrated. The applications, characteristics, and existing challenges are summarized in combination with our lab experiences in this field in an effort to help gene-editing technology better serve humans in a regulated manner. Current preclinical and clinical trials have demonstrated initial safety and efficacy of PSC gene editing; however, for better application in clinical settings, additional investigation is warranted.

WWOX-Related Neurodevelopmental Disorders: Models and Future Perspectives

The WW domain-containing oxidoreductase (WWOX) gene was originally discovered as a putative tumor suppressor spanning the common fragile site FRA16D, but as time has progressed the extent of its pleiotropic function has become apparent. At present, WWOX is a major source of interest in the context of neurological disorders, and more specifically developmental and epileptic encephalopathies (DEEs). This review article aims to introduce the many model systems used through the years to study its function and roles in neuropathies. Similarities and fundamental differences between rodent and human models are discussed. Finally, future perspectives and promising research avenues are suggested.

TSC1 loss increases risk for tauopathy by inducing tau acetylation and preventing tau clearance via chaperone-mediated autophagy

Age-associated neurodegenerative disorders demonstrating tau-laden intracellular inclusions are known as tauopathies. We previously linked a loss-of-function mutation in the TSC1 gene to tau accumulation and frontotemporal lobar degeneration. Now, we have identified genetic variants in TSC1 that decrease TSC1/hamartin levels and predispose to tauopathies such as Alzheimer’s disease and progressive supranuclear palsy. Cellular and murine models of TSC1 haploinsufficiency, as well as human brains carrying a TSC1 risk variant, accumulated tau protein that exhibited aberrant acetylation. This acetylation hindered tau degradation via chaperone-mediated autophagy, thereby leading to its accumulation. Aberrant tau acetylation in TSC1 haploinsufficiency resulted from the dysregulation of both p300 acetyltransferase and SIRT1 deacetylase. Pharmacological modulation of either enzyme restored tau levels. This study substantiates TSC1 as a novel tauopathy risk gene and includes TSC1 haploinsufficiency as a genetic model for tauopathies. In addition, these findings promote tau acetylation as a rational target for tauopathy therapeutics and diagnostic.

Genome Editing in iPSC-Based Neural Systems: From Disease Models to Future Therapeutic Strategies

Therapeutic advances for neurological disorders are challenging due to limited accessibility of the human central nervous system and incomplete understanding of disease mechanisms. Many neurological diseases lack precision treatments, leading to significant disease burden and poor outcome for affected patients. Induced pluripotent stem cell (iPSC) technology provides human neuronal cells that facilitate disease modeling and development of therapies. The use of genome editing, in particular CRISPR-Cas9 technology, has extended the potential of iPSCs, generating new models for a number of disorders, including Alzheimers and Parkinson Disease. Editing of iPSCs, in particular with CRISPR-Cas9, allows generation of isogenic pairs, which differ only in the disease-causing mutation and share the same genetic background, for assessment of phenotypic differences and downstream effects. Moreover, genome-wide CRISPR screens allow high-throughput interrogation for genetic modifiers in neuronal phenotypes, leading to discovery of novel pathways, and identification of new therapeutic targets. CRISPR-Cas9 has now evolved beyond altering gene expression. Indeed, fusion of a defective Cas9 (dCas9) nuclease with transcriptional repressors or activation domains allows down-regulation or activation of gene expression (CRISPR interference, CRISPRi; CRISPR activation, CRISPRa). These new tools will improve disease modeling and facilitate CRISPR and cell-based therapies, as seen for epilepsy and Duchenne muscular dystrophy. Genome engineering holds huge promise for the future understanding and treatment of neurological disorders, but there are numerous barriers to overcome. The synergy of iPSC-based model systems and gene editing will play a vital role in the route to precision medicine and the clinical translation of genome editing-based therapies.

Translating the Role of mTOR- and RAS-Associated Signalopathies in Autism Spectrum Disorder: Models, Mechanisms and Treatment

Mutations affecting mTOR or RAS signaling underlie defined syndromes (the so-called mTORopathies and RASopathies) with high risk for Autism Spectrum Disorder (ASD). These syndromes show a broad variety of somatic phenotypes including cancers, skin abnormalities, heart disease and facial dysmorphisms. Less well studied are the neuropsychiatric symptoms such as ASD. Here, we assess the relevance of these signalopathies in ASD reviewing genetic, human cell model, rodent studies and clinical trials. We conclude that signalopathies have an increased liability for ASD and that, in particular, ASD individuals with dysmorphic features and intellectual disability (ID) have a higher chance for disruptive mutations in RAS- and mTOR-related genes. Studies on rodent and human cell models confirm aberrant neuronal development as the underlying pathology. Human studies further suggest that multiple hits are necessary to induce the respective phenotypes. Recent clinical trials do only report improvements for comorbid conditions such as epilepsy or cancer but not for behavioral aspects. Animal models show that treatment during early development can rescue behavioral phenotypes. Taken together, we suggest investigating the differential roles of mTOR and RAS signaling in both human and rodent models, and to test drug treatment both during and after neuronal development in the available model systems.

Neocortical development and epilepsy: insights from focal cortical dysplasia and brain tumours

During the past decade, there have been considerable advances in understanding of the genetic and morphogenic processes underlying cortical malformations and developmental brain tumours. Focal malformations are caused by somatic (postzygotic) variants in genes related to cell growth (ie, in the mTOR pathway in focal cortical dysplasia type 2), which are acquired in neuronal progenitors during neurodevelopment. In comparison, developmental brain tumours result from somatic variants in genes related to cell proliferation (eg, in the MAP-kinase pathway in ganglioglioma), which affect proliferating glioneuronal precursors. The timing of the genetic event and the specific gene involved during neurodevelopment will drive the nature and size of the lesion, whether it is a developmental malformation or a brain tumour. There is also emerging evidence that epigenetic processes underlie a molecular memory in epileptogenesis. This knowledge will together facilitate understanding of why and how patients with these lesions have epilepsy, and could form a basis for a move towards precision medicine for this challenging cohort of patients.

Hybrid SMART spheroids to enhance stem cell therapy for CNS injuries

Although stem cell therapy holds enormous potential for treating debilitating injuries and diseases in the central nervous system, low survival and inefficient differentiation have restricted its clinical applications. Recently, 3D cell culture methods, such as stem cell–based spheroids and organoids, have demonstrated advantages by incorporating tissue-mimetic 3D cell-cell interactions. However, a lack of drug and nutrient diffusion, insufficient cell-matrix interactions, and tedious fabrication procedures have compromised their therapeutic effects in vivo. To address these issues, we developed a biodegradable nanomaterial-templated 3D cell assembly method that enables the formation of hybrid stem cell spheroids with deep drug delivery capabilities and homogeneous incorporation of 3D cell-matrix interactions. Hence, high survival rates, controlled differentiation, and functional recovery were demonstrated in a spinal cord injury animal model. Overall, our hybrid stem cell spheroids represent a substantial development of material-facilitated 3D cell culture systems and can pave the way for stem cell–based treatment of CNS injuries.

Tuberous Sclerosis Complex (TSC) Inactivation Increases Neuronal Network Activity by Enhancing Ca2+ Influx via L-Type Ca2+ Channels

Tuberous sclerosis complex (TSC) is a multisystem developmental disorder characterized by hamartomas in various organs, such as the brain, lungs, and kidneys. Epilepsy, along with autism and intellectual disability, is one of the neurologic impairments associated with TSC that has an intimate relationship with developmental outcomes and quality of life. Sustained activation of the mammalian target of rapamycin (mTOR) via TSC1 or TSC2 mutations is known to be involved in the onset of epilepsy in TSC. However, the mechanism by which mTOR causes seizures remains unknown. In this study, we showed that, human induced pluripotent stem cell-derived TSC2-deficient (TSC2 ^-/-) neurons exhibited elevated neuronal activity with highly synchronized Ca²⁺ spikes. Notably, TSC2 ^-/- neurons presented enhanced Ca²⁺ influx via L-type Ca²⁺ channels (LTCCs), which contributed to the abnormal neurite extension and sustained activation of cAMP response element binding protein (CREB), a critical mediator of synaptic plasticity. Expression of Cav1.3, a subtype of LTCCs, was increased in TSC2 ^-/- neurons, but long-term rapamycin treatment suppressed this increase and reversed the altered neuronal activity and neurite extensions. Thus, we identified Cav1.3 LTCC as a critical downstream component of TSC-mTOR signaling that would trigger enhanced neuronal network activity of TSC2 ^-/- neurons. We suggest that LTCCs could be potential novel targets for the treatment of epilepsy in TSC.SIGNIFICANCE STATEMENT There is a close relationship between elevated mammalian target of rapamycin (mTOR) activity and epilepsy in tuberous sclerosis complex (TSC). However, the underlying mechanism by which mTOR causes epilepsy remains unknown. In this study, using human TSC2 ^-/- neurons, we identified elevated Ca²⁺ influx via L-type Ca²⁺ channels as a critical downstream component of TSC-mTOR signaling and a potential cause of both elevated neuronal activity and neurite extension in TSC2 ^-/- neurons. Our findings demonstrate a previously unrecognized connection between sustained mTOR activation and elevated Ca²⁺ signaling via L-type Ca²⁺ channels in human TSC neurons, which could cause epilepsy in TSC.

Brain Organoids: Studying Human Brain Development and Diseases in a Dish

With the rapid development of stem cell technology, the advent of three-dimensional (3D) cultured brain organoids has opened a new avenue for studying human neurodevelopment and neurological disorders. Brain organoids are stem-cell-derived 3D suspension cultures that self-assemble into an organized structure with cell types and cytoarchitectures recapitulating the developing brain. In recent years, brain organoids have been utilized in various aspects, ranging from basic biology studies, to disease modeling, and high-throughput screening of pharmaceutical compounds. In this review, we overview the establishment and development of brain organoid technology, its recent progress, and translational applications, as well as existing limitations and future directions.

Defining the nature of human pluripotent stem cell-derived interneurons via single-cell analysis

The specification of inhibitory neurons has been described for the mouse and human brain, and many studies have shown that pluripotent stem cells (PSCs) can be used to create interneurons in vitro. It is unclear whether in vitro methods to produce human interneurons generate all the subtypes found in brain, and how similar in vitro and in vivo interneurons are. We applied single-nuclei and single-cell transcriptomics to model interneuron development from human cortex and interneurons derived from PSCs. We provide a direct comparison of various in vitro interneuron derivation methods to determine the homogeneity achieved. We find that PSC-derived interneurons capture stages of development prior to mid-gestation, and represent a minority of potential subtypes found in brain. Comparison with those found in fetal or adult brain highlighted decreased expression of synapse-related genes. These analyses highlight the potential to tailor the method of generation to drive formation of particular subtypes.

Real-Time Non-Invasive and Direct Determination of Lactate Dehydrogenase Activity in Cerebral Organoids-A New Method to Characterize the Metabolism of Brain Organoids?

Organoids are a powerful tool in the quest to understand human diseases. As the developing brain is extremely inaccessible in mammals, cerebral organoids (COs) provide a unique way to investigate neural development and related disorders. The aim of this study was to utilize hyperpolarized ¹³C NMR to investigate the metabolism of COs in real-time, in a non-destructive manner. The enzymatic activity of lactate dehydrogenase (LDH) was determined by quantifying the rate of [1-¹³C]lactate production from hyperpolarized [1-¹³C]pyruvate. Organoid development was assessed by immunofluorescence imaging. Organoid viability was confirmed using ³¹P NMR spectroscopy. A total of 15 organoids collated into 3 groups with a group total weight of 20-77 mg were used in this study. Two groups were at the age of 10 weeks and one was at the age of 33 weeks. The feasibility of this approach was demonstrated in both age groups, and the LDH activity rate was found to be 1.32 ± 0.75 nmol/s (n = 3 organoid batches). These results suggest that hyperpolarized NMR can be used to characterize the metabolism of brain organoids with a total tissue wet weight of as low as 20 mg (<3 mm³) and a diameter ranging from 3 to 6 mm.

Modeling genetic epileptic encephalopathies using brain organoids

Developmental and epileptic encephalopathies (DEE) are a group of disorders associated with intractable seizures, brain development, and functional abnormalities, and in some cases, premature death. Pathogenic human germline biallelic mutations in tumor suppressor WW domain-containing oxidoreductase (WWOX) are associated with a relatively mild autosomal recessive spinocerebellar ataxia-12 (SCAR12) and a more severe early infantile WWOX-related epileptic encephalopathy (WOREE). In this study, we generated an in vitro model for DEEs, using the devastating WOREE syndrome as a prototype, by establishing brain organoids from CRISPR-engineered human ES cells and from patient-derived iPSCs. Using these models, we discovered dramatic cellular and molecular CNS abnormalities, including neural population changes, cortical differentiation malfunctions, and Wnt pathway and DNA damage response impairment. Furthermore, we provide a proof of concept that ectopic WWOX expression could potentially rescue these phenotypes. Our findings underscore the utility of modeling childhood epileptic encephalopathies using brain organoids and their use as a unique platform to test possible therapeutic intervention strategies.

Brain Organoids: Filling the Need for a Human Model of Neurological Disorder

Neurological disorders are among the leading causes of death worldwide, accounting for almost all onsets of dementia in the elderly, and are known to negatively affect motor ability, mental and cognitive performance, as well as overall wellbeing and happiness. Currently, most neurological disorders go untreated due to a lack of viable treatment options. The reason for this lack of options is s poor understanding of the disorders, primarily due to research models that do not translate well into the human in vivo system. Current models for researching neurological disorders, neurodevelopment, and drug interactions in the central nervous system include in vitro monolayer cell cultures, and in vivo animal models. These models have shortcomings when it comes to translating research about disorder pathology, development, and treatment to humans. Brain organoids are three-dimensional (3D) cultures of stem cell-derived neural cells that mimic the development of the in vivo human brain with high degrees of accuracy. Researchers have started developing these miniature brains to model neurodevelopment, and neuropathology. Brain organoids have been used to model a wide range of neurological disorders, including the complex and poorly understood neurodevelopmental and neurodegenerative disorders. In this review, we discuss the brain organoid technology, placing special focus on the different brain organoid models that have been developed, discussing their strengths, weaknesses, and uses in neurological disease modeling.

Brain organoid: a 3D technology for investigating cellular composition and interactions in human neurological development and disease models in vitro

Abstract

The study of human brain physiology, including cellular interactions in normal and disease conditions, has been a challenge due to its complexity and unavailability. Induced pluripotent stem cell (iPSC) study is indispensable in the study of the pathophysiology of neurological disorders. Nevertheless, monolayer systems lack the cytoarchitecture necessary for cellular interactions and neurological disease modeling. Brain organoids generated from human pluripotent stem cells supply an ideal environment to model both cellular interactions and pathophysiology of the human brain. This review article discusses the composition and interactions among neural lineage and non-central nervous system cell types in brain organoids, current studies, and future perspectives in brain organoid research. Ultimately, the promise of brain organoids is to unveil previously inaccessible features of neurobiology that emerge from complex cellular interactions and to improve our mechanistic understanding of neural development and diseases.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02369-8.

The Use of Stem Cell-Derived Organoids in Disease Modeling: An Update

Organoids represent one of the most important advancements in the field of stem cells during the past decade. They are three-dimensional in vitro culturing models that originate from self-organizing stem cells and can mimic the in vivo structural and functional specificities of body organs. Organoids have been established from multiple adult tissues as well as pluripotent stem cells and have recently become a powerful tool for studying development and diseases in vitro, drug screening, and host-microbe interaction. The use of stem cells-that have self-renewal capacity to proliferate and differentiate into specialized cell types-for organoids culturing represents a major advancement in biomedical research. Indeed, this new technology has a great potential to be used in a multitude of fields, including cancer research, hereditary and infectious diseases. Nevertheless, organoid culturing is still rife with many challenges, not limited to being costly and time consuming, having variable rates of efficiency in generation and maintenance, genetic stability, and clinical applications. In this review, we aim to provide a synopsis of pluripotent stem cell-derived organoids and their use for disease modeling and other clinical applications.

Effects of Mutations in TSC Genes on Neurodevelopment and Synaptic Transmission

Mutations in TSC1 or TSC2 genes are linked to alterations in neuronal function which ultimately lead to the development of a complex neurological phenotype. Here we review current research on the effects that reduction in TSC1 or TSC2 can produce on the developing neural network. A crucial feature of the disease pathophysiology appears to be an early deviation from typical neurodevelopment, in the form of structural abnormalities. Epileptic seizures are one of the primary early manifestation of the disease in the CNS, followed by intellectual deficits and autism spectrum disorders (ASD). Research using mouse models suggests that morphological brain alterations might arise from the interaction of different cellular types, and hyperexcitability in the early postnatal period might be transient. Moreover, the increased excitation-to-inhibition ratio might represent a transient compensatory adjustment to stabilize the developing network rather than a primary factor for the development of ASD symptoms. The inhomogeneous results suggest region-specificity as well as an evolving picture of functional alterations along development. Furthermore, ASD symptoms and epilepsy might originate from different but potentially overlapping mechanisms, which can explain recent observations obtained in patients. Potential treatment is determined not only by the type of medicament, but also by the time point of treatment.

Screening Platforms for Genetic Epilepsies-Zebrafish, iPSC-Derived Neurons, and Organoids

Recent advances in molecular and cellular engineering, such as human cell reprogramming, genome editing, and patient-specific organoids, have provided unprecedented opportunities for investigating human disorders in both animals and human-based models at an improved pace and precision. This progress will inevitably lead to the development of innovative drug-screening platforms and new patient-specific therapeutics. In this review, we discuss recent advances that have been made using zebrafish and human-induced pluripotent stem cell (iPSC)-derived neurons and organoids for modeling genetic epilepsies. We also provide our prospective on how these models can potentially be combined to build new screening platforms for antiseizure and antiepileptogenic drug discovery that harness the robustness and tractability of zebrafish models as well as the patient-specific genetics and biology of iPSC-derived neurons and organoids.

From Brain Organoids to Networking Assembloids: Implications for Neuroendocrinology and Stress Medicine

Brain organoids are three-dimensional cultures that contain multiple types of cells and cytoarchitectures, and resemble fetal human brain structurally and functionally. These organoids are being used increasingly to model brain development and disorders, however, they only partially recapitulate such processes, because of several limitations, including inability to mimic the distinct cortical layers, lack of functional neuronal circuitry as well as non-neural cells and gyrification, and increased cellular stress. Efforts to create improved brain organoid culture systems have led to region-specific organoids, vascularized organoids, glia-containing organoids, assembloids, sliced organoids and polarized organoids. Assembloids are fused region-specific organoids, which attempt to recapitulate inter-regional and inter-cellular interactions as well as neural circuitry development by combining multiple brain regions and/or cell lineages. As a result, assembloids can be used to model subtle functional aberrations that reflect complex neurodevelopmental, neuropsychiatric and neurodegenerative disorders. Mammalian organisms possess a highly complex neuroendocrine system, the stress system, whose main task is the preservation of systemic homeostasis, when the latter is threatened by adverse forces, the stressors. The main central parts of the stress system are the paraventricular nucleus of the hypothalamus and the locus caeruleus/norepinephrine-autonomic nervous system nuclei in the brainstem; these centers innervate each other and interact reciprocally as well as with various other CNS structures. Chronic dysregulation of the stress system has been implicated in major pathologies, the so-called chronic non-communicable diseases, including neuropsychiatric, neurodegenerative, cardiometabolic and autoimmune disorders, which lead to significant population morbidity and mortality. We speculate that brain organoids and/or assembloids could be used to model the development, regulation and dysregulation of the stress system and to better understand stress-related disorders. Novel brain organoid technologies, combined with high-throughput single-cell omics and gene editing, could, thus, have major implications for precision medicine.

Human pluripotent stem cell-derived brain organoids as in vitro models for studying neural disorders and cancer

The sheer complexities of brain and resource limitation of human brain tissue greatly hamper our understanding of the brain disorders and cancers. Recently developed three-dimensional (3D) brain organoids (BOs) are self-organized and spontaneously differentiated from human pluripotent stem cells (hPSCs) in vitro, which exhibit similar features with cell type diversity, structural organization, and functional connectivity as the developing human brain. Based on these characteristics, hPSC-derived BOs (hPDBOs) provide new opportunities to recapitulate the complicated processes during brain development, neurodegenerative disorders, and brain cancers in vitro. In this review, we will provide an overview of existing BO models and summarize the applications of this technology in modeling the neural disorders and cancers. Furthermore, we will discuss the challenges associated with their use as in vitro models for disease modeling and the potential future direction.

Current Approaches and Future Directions for the Treatment of mTORopathies

The mechanistic target of rapamycin (mTOR) is a kinase at the center of an evolutionarily conserved signaling pathway that orchestrates cell growth and metabolism. mTOR responds to an array of intra- and extracellular stimuli and in turn controls multiple cellular anabolic and catabolic processes. Aberrant mTOR activity is associated with numerous diseases, with particularly profound impact on the nervous system. mTOR is found in two protein complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2), which are governed by different upstream regulators and have distinct cellular actions. Mutations in genes encoding for mTOR regulators result in a collection of neurodevelopmental disorders known as mTORopathies. While these disorders can affect multiple organs, neuropsychiatric conditions such as epilepsy, intellectual disability, and autism spectrum disorder have a major impact on quality of life. The neuropsychiatric aspects of mTORopathies have been particularly challenging to treat in a clinical setting. Current therapeutic approaches center on rapamycin and its analogs, drugs that are administered systemically to inhibit mTOR activity. While these drugs show some clinical efficacy, adverse side effects, incomplete suppression of mTOR targets, and lack of specificity for mTORC1 or mTORC2 may limit their utility. An increased understanding of the neurobiology of mTOR and the underlying molecular, cellular, and circuit mechanisms of mTOR-related disorders will facilitate the development of improved therapeutics. Animal models of mTORopathies have helped unravel the consequences of mTOR pathway mutations in specific brain cell types and developmental stages, revealing an array of disease-related phenotypes. In this review, we discuss current progress and potential future directions for the therapeutic treatment of mTORopathies with a focus on findings from genetic mouse models.

STRADA-mutant human cortical organoids model megalencephaly and exhibit delayed neuronal differentiation

Genetic diseases involving overactivation of the mechanistic target of rapamycin (mTOR) pathway, so-called "mTORopathies," often manifest with malformations of cortical development (MCDs), epilepsy, and cognitive impairment. How mTOR pathway hyperactivation results in abnormal human cortical development is poorly understood. To study the effect of mTOR hyperactivity on early stages of cortical development, we focused on Pretzel Syndrome (polyhydramnios, megalencephaly, symptomatic epilepsy; PMSE syndrome), a rare mTORopathy caused by homozygous germline mutations in the STRADA gene. We developed a human cortical organoid (hCO) model of PMSE and examined morphology and size for the first 2 weeks of organoid growth, and cell type composition at weeks 2, 8, and 12 of differentiation. In the second week, PMSE hCOs enlarged more rapidly than controls and displayed an abnormal Wnt pathway-dependent increase in neural rosette structures. PMSE hCOs also exhibited delayed neurogenesis, decreased subventricular zone progenitors, increased proliferation and cell death, and an abnormal architecture of primary cilia. At week 8, PMSE hCOs had fewer deep layer neurons. By week 12, neurogenesis recovered in PMSE organoids, but they displayed increased outer radial glia, a cell type thought to contribute to the expansion of the human cerebral cortex. Together, these findings suggest that megalencephaly in PMSE arises from the expansion of neural stem cells in early corticogenesis and potentially also from increased outer radial glial at later gestational stages. The delayed neuronal differentiation in PMSE organoids demonstrates the important role the mTOR pathway plays in the maintenance and expansion of the stem cell pool.

The Characterization of a Subependymal Giant Astrocytoma-Like Cell Line from Murine Astrocyte with mTORC1 Hyperactivation

Tuberous sclerosis complex (TSC) is a genetic disorder caused by inactivating mutations in TSC1 (hamartin) or TSC2 (tuberin), crucial negative regulators of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway. TSC affects multiple organs including the brain. The neurologic manifestation is characterized by cortical tubers, subependymal nodules (SEN), and subependymal giant cell astrocytoma (SEGA) in brain. SEGAs may result in hydrocephalus in TSC patients and mTORC1 inhibitors are the current recommended therapy for SEGA. Nevertheless, a major limitation in the research for SEGA is the lack of cell lines or animal models for mechanistic investigations and development of novel therapy. In this study, we generated TSC1-deficient neural cells from spontaneously immortalized mouse astrocytes in an attempt to mimic human SEGA. The TSC1-deficient cells exhibit mTORC1 hyperactivation and characteristics of transition from astrocytes to neural stem/progenitor cell phenotypes. Rapamycin efficiently decreased mTORC1 activity of these TSC1-deficient cells in vitro. In vivo, TSC1-deficient cells could form SEGA-like tumors and Rapamycin treatment decreased tumor growth. Collectively, our study generates a novel SEGA-like cell line that is invaluable for studying mTORC1-driven molecular and pathological alterations in neurologic tissue. These SEGA-like cells also provide opportunities for the development of novel therapeutic strategy for TSC patients with SEGA.

Convergent and Divergent Mechanisms of Epileptogenesis in mTORopathies

Hyperactivation of the mechanistic target of rapamycin complex 1 (mTORC1) due to mutations in genes along the PI3K-mTOR pathway and the GATOR1 complex causes a spectrum of neurodevelopmental disorders (termed mTORopathies) associated with malformation of cortical development and intractable epilepsy. Despite these gene variants’ converging impact on mTORC1 activity, emerging findings suggest that these variants contribute to epilepsy through both mTORC1-dependent and -independent mechanisms. Here, we review the literature on in utero electroporation-based animal models of mTORopathies, which recapitulate the brain mosaic pattern of mTORC1 hyperactivity, and compare the effects of distinct PI3K-mTOR pathway and GATOR1 complex gene variants on cortical development and epilepsy. We report the outcomes on cortical pyramidal neuronal placement, morphology, and electrophysiological phenotypes, and discuss some of the converging and diverging mechanisms responsible for these alterations and their contribution to epileptogenesis. We also discuss potential therapeutic strategies for epilepsy, beyond mTORC1 inhibition with rapamycin or everolimus, that could offer personalized medicine based on the gene variant.

Growing Glia: Cultivating Human Stem Cell Models of Gliogenesis in Health and Disease

Glia are present in all organisms with a central nervous system but considerably differ in their diversity, functions, and numbers. Coordinated efforts across many model systems have contributed to our understanding of glial-glial and neuron-glial interactions during nervous system development and disease, but human glia exhibit prominent species-specific attributes. Limited access to primary samples at critical developmental timepoints constrains our ability to assess glial contributions in human tissues. This challenge has been addressed throughout the past decade via advancements in human stem cell differentiation protocols that now offer the ability to model human astrocytes, oligodendrocytes, and microglia. Here, we review the use of novel 2D cell culture protocols, 3D organoid models, and bioengineered systems derived from human stem cells to study human glial development and the role of glia in neurodevelopmental disorders.

Pathological Features of Tumors of the Nervous System in Hereditary Cancer Predisposition Syndromes: A Review

Hereditary cancer predisposition syndromes (HCS) become more recognizable as the knowledge about them expands, and genetic testing becomes more affordable. In this review, we discussed the known HCS that predispose to central and peripheral nervous system tumors. Different genetic phenomena were highlighted, and the important cellular biological alterations were summarized. Genetic mosaicism and germline mutations are features of HCS, and recently, they were described in normal population and as modifiers for the genetic landscape of sporadic tumors. Description of the tumors arising in these conditions was augmented by representative cases explaining the main pathological findings. Clinical spectrum of the syndromes and diagnostic criteria were tabled to outline their role in defining these disorders. Interestingly, precision medicine has found its way to help these groups of patients by offering targeted preventive measures. Understanding the signaling pathway alteration of mammalian target of rapamycin (mTOR) in tuberous sclerosis helped introducing mTOR inhibitors as a prophylactic treatment in these patients. More research to define the germline genetic alterations and resulting cellular signaling perturbations is needed for effective risk-reducing interventions beyond prophylactic surgeries.

Patient-Derived Induced Pluripotent Stem Cells (iPSCs) and Cerebral Organoids for Drug Screening and Development in Autism Spectrum Disorder: Opportunities and Challenges

Autism spectrum disorder (ASD) represents a group of neurodevelopmental diseases characterized by persistent deficits in social communication, interaction, and repetitive patterns of behaviors, interests, and activities. The etiopathogenesis is multifactorial with complex interactions between genetic and environmental factors. The clinical heterogeneity and complex etiology of this pediatric disorder have limited the development of pharmacological therapies. The major limit to ASD research remains a lack of relevant human disease models which can faithfully recapitulate key features of the human pathology and represent its genetic heterogeneity. Recent advances in induced pluripotent stem cells (iPSCs), reprogrammed from somatic cells of patients into all types of patient-specific neural cells, have provided a promising cellular tool for disease modeling and development of novel drug treatments. The iPSCs technology allowed not only a better investigation of the disease etiopathogenesis but also opened up the potential for personalized therapies and offered new opportunities for drug discovery, pharmacological screening, and toxicity assessment. Moreover, iPSCs can be differentiated and organized into three-dimensional (3D) organoids, providing a model which mimics the complexity of the brain’s architecture and more accurately recapitulates tissue- and organ-level disease pathophysiology. The aims of this review were to describe the current state of the art of the use of human patient-derived iPSCs and brain organoids in modeling ASD and developing novel therapeutic strategies and to discuss the opportunities and major challenges in this rapidly moving field.

Building the brain from scratch: Engineering region-specific brain organoids from human stem cells to study neural development and disease

Human brain development is an intricate process that involves precisely timed coordination of cell proliferation, fate specification, neuronal differentiation, migration, and integration of diverse cell types. Understanding of these fundamental processes, however, has been largely constrained by limited access to fetal brain tissue and the inability to prospectively study neurodevelopment in humans at the molecular, cellular and system levels. Although non-human model organisms have provided important insights into mechanisms underlying brain development, these systems do not fully recapitulate many human-specific features that often relate to disease. To address these challenges, human brain organoids, self-assembled three-dimensional neural aggregates, have been engineered from human pluripotent stem cells to model the architecture and cellular diversity of the developing human brain. Recent advancements in neural induction and regional patterning using small molecules and growth factors have yielded protocols for generating brain organoids that recapitulate the structure and neuronal composition of distinct brain regions. Here, we first provide an overview of early mammalian brain development with an emphasis on molecular cues that guide region specification. We then focus on recent efforts in generating human brain organoids that model the development of specific brain regions and highlight endeavors to enhance the cellular complexity to better mimic the in vivo developing human brain. We also provide examples of how organoid models have enhanced our understanding of human neurological diseases and conclude by discussing limitations of brain organoids with our perspectives on future advancements to maximize their potential.

Electrophysiology Read-Out Tools for Brain-on-Chip Biotechnology

Brain-on-Chip (BoC) biotechnology is emerging as a promising tool for biomedical and pharmaceutical research applied to the neurosciences. At the convergence between lab-on-chip and cell biology, BoC couples in vitro three-dimensional brain-like systems to an engineered microfluidics platform designed to provide an in vivo-like extrinsic microenvironment with the aim of replicating tissue- or organ-level physiological functions. BoC therefore offers the advantage of an in vitro reproduction of brain structures that is more faithful to the native correlate than what is obtained with conventional cell culture techniques. As brain function ultimately results in the generation of electrical signals, electrophysiology techniques are paramount for studying brain activity in health and disease. However, as BoC is still in its infancy, the availability of combined BoC-electrophysiology platforms is still limited. Here, we summarize the available biological substrates for BoC, starting with a historical perspective. We then describe the available tools enabling BoC electrophysiology studies, detailing their fabrication process and technical features, along with their advantages and limitations. We discuss the current and future applications of BoC electrophysiology, also expanding to complementary approaches. We conclude with an evaluation of the potential translational applications and prospective technology developments.

Convergence of human pluripotent stem cell, organoid, and genome editing technologies

The last decade has seen many exciting technological breakthroughs that greatly expanded the toolboxes for biological and biomedical research, yet few have had more impact than induced pluripotent stem cells and modern-day genome editing. These technologies are providing unprecedented opportunities to improve physiological relevance of experimental models, further our understanding of developmental processes, and develop novel therapies. One of the research areas that benefit greatly from these technological advances is the three-dimensional human organoid culture systems that resemble human tissues morphologically and physiologically. Here we summarize the development of human pluripotent stem cells and their differentiation through organoid formation. We further discuss how genetic modifications, genome editing in particular, were applied to answer basic biological and biomedical questions using organoid cultures of both somatic and pluripotent stem cell origins. Finally, we discuss the potential challenges of applying human pluripotent stem cell and organoid technologies for safety and efficiency evaluation of emerging genome editing tools.

Neuron-Glia Interactions in Tuberous Sclerosis Complex Affect the Synaptic Balance in 2D and Organoid Cultures

Tuberous sclerosis complex (TSC) is a genetic disease affecting the brain. Neurological symptoms like epilepsy and neurodevelopmental issues cause a significant burden on patients. Both neurons and glial cells are affected by TSC mutations. Previous studies have shown changes in the excitation/inhibition balance (E/I balance) in TSC. Astrocytes are known to be important for neuronal development, and astrocytic dysfunction can cause changes in the E/I balance. We hypothesized that astrocytes affect the synaptic balance in TSC. TSC patient-derived stem cells were differentiated into astrocytes, which showed increased proliferation compared to control astrocytes. RNA sequencing revealed changes in gene expression, which were related to epidermal growth factor (EGF) signaling and enriched for genes that coded for secreted or transmembrane proteins. Control neurons were cultured in astrocyte-conditioned medium (ACM) of TSC and control astrocytes. After culture in TSC ACM, neurons showed an altered synaptic balance, with an increase in the percentage of VGAT⁺ synapses. These findings were confirmed in organoids, presenting a spontaneous 3D organization of neurons and glial cells. To conclude, this study shows that TSC astrocytes are affected and secrete factors that alter the synaptic balance. As an altered E/I balance may underlie many of the neurological TSC symptoms, astrocytes may provide new therapeutic targets.

Brain organoids: an ensemble of bioassays to investigate human neurodevelopment and disease

Understanding etiology of human neurological and psychiatric diseases is challenging. Genomic changes, protracted development, and histological features unique to human brain development limit the disease aspects that can be investigated using model organisms. Hence, in order to study phenotypes associated with human brain development, function, and disease, it is necessary to use alternative experimental systems that are accessible, ethically justified, and replicate human context. Human pluripotent stem cell (hPSC)-derived brain organoids offer such a system, which recapitulates features of early human neurodevelopment in vitro, including the generation, proliferation, and differentiation of neural progenitors into neurons and glial cells and the complex interactions among the diverse, emergent cell types of the developing brain in three-dimensions (3-D). In recent years, numerous brain organoid protocols and related techniques have been developed to recapitulate aspects of embryonic and fetal brain development in a reproducible and predictable manner. Altogether, these different organoid technologies provide distinct bioassays to unravel novel, disease-associated phenotypes and mechanisms. In this review, we summarize how the diverse brain organoid methods can be utilized to enhance our understanding of brain disorders. FACTS: Brain organoids offer an in vitro approach to study aspects of human brain development and disease. Diverse brain organoid techniques offer bioassays to investigate new phenotypes associated with human brain disorders that are difficult to study in monolayer cultures. Brain organoids have been particularly useful to study phenomena and diseases associated with neural progenitor morphology, survival, proliferation, and differentiation. OPEN QUESTION: Future brain organoid research needs to aim at later stages of neurodevelopment, linked with neuronal activity and connections, to unravel further disease-associated phenotypes. Continued improvement of existing organoid protocols is required to generate standardized methods that recapitulate in vivo-like spatial diversity and complexity.

Generation of human striatal organoids and cortico-striatal assembloids from human pluripotent stem cells

Cortico-striatal projections are critical components of forebrain circuitry that regulate motivated behaviors. To enable the study of the human cortico-striatal pathway and how its dysfunction leads to neuropsychiatric disease, we developed a method to convert human pluripotent stem cells into region-specific brain organoids that resemble the developing human striatum and include electrically active medium spiny neurons. We then assembled these organoids with cerebral cortical organoids in three-dimensional cultures to form cortico-striatal assembloids. Using viral tracing and functional assays in intact or sliced assembloids, we show that cortical neurons send axonal projections into striatal organoids and form synaptic connections. Medium spiny neurons mature electrophysiologically following assembly and display calcium activity after optogenetic stimulation of cortical neurons. Moreover, we derive cortico-striatal assembloids from patients with a neurodevelopmental disorder caused by a deletion on chromosome 22q13.3 and capture disease-associated defects in calcium activity, showing that this approach will allow investigation of the development and functional assembly of cortico-striatal connectivity using patient-derived cells.

Insight into developmental mechanisms of global and focal migration disorders of cortical development

Cortical development involves neurogenesis followed by migration, maturation, and myelination of immature neurons. Disruptions in these processes can cause malformations of cortical development (MCD). Radial glia (RG) are the stem cells of the brain, both generating neurons and providing the scaffold upon which immature neurons radially migrate. Germline mutations in genes required for cell migration, or cell-cell contact, often lead to global MCDs. Somatic mutations in RG in genes involved in homeostatic function, like mTOR signaling, often lead to focal MCDs. Two different mutations occurring in the same patient can combine in ways we are just beginning to understand. Our growing knowledge about MCD suggests mTOR inhibitors may have expanded utility in treatment-resistant epilepsy, while imaging techniques can better delineate the type and extent of these lesions.

Brain Organoids as Model Systems for Genetic Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs) are a group of disorders in which the development of the central nervous system (CNS) is disturbed, resulting in different neurological and neuropsychiatric features, such as impaired motor function, learning, language or non-verbal communication. Frequent comorbidities include epilepsy and movement disorders. Advances in DNA sequencing technologies revealed identifiable genetic causes in an increasingly large proportion of NDDs, highlighting the need of experimental approaches to investigate the defective genes and the molecular pathways implicated in abnormal brain development. However, targeted approaches to investigate specific molecular defects and their implications in human brain dysfunction are prevented by limited access to patient-derived brain tissues. In this context, advances of both stem cell technologies and genome editing strategies during the last decade led to the generation of three-dimensional (3D) in vitro-models of cerebral organoids, holding the potential to recapitulate precise stages of human brain development with the aim of personalized diagnostic and therapeutic approaches. Recent progresses allowed to generate 3D-structures of both neuronal and non-neuronal cell types and develop either whole-brain or region-specific cerebral organoids in order to investigate in vitro key brain developmental processes, such as neuronal cell morphogenesis, migration and connectivity. In this review, we summarized emerging methodological approaches in the field of brain organoid technologies and their application to dissect disease mechanisms underlying an array of pediatric brain developmental disorders, with a particular focus on autism spectrum disorders (ASDs) and epileptic encephalopathies.

Recent advances in gene therapy for neurodevelopmental disorders with epilepsy

Neurodevelopmental disorders can be caused by mutations in neuronal genes fundamental to brain development. These disorders have severe symptoms ranging from intellectually disability, social and cognitive impairments, and a subset are strongly linked with epilepsy. In this review, we focus on those neurodevelopmental disorders that are frequently characterized by the presence of epilepsy (NDD + E). We loosely group the genes linked to NDD + E with different neuronal functions: transcriptional regulation, intrinsic excitability and synaptic transmission. All these genes have in common a pivotal role in defining the brain architecture and function during early development, and when their function is altered, symptoms can present in the first stages of human life. The relationship with epilepsy is complex. In some NDD + E, epilepsy is a comorbidity and in others seizures appear to be the main cause of the pathology, suggesting that either structural changes (NDD) or neuronal communication (E) can lead to these disorders. Furthermore, grouping the genes that cause NDD + E, we review the uses and limitations of current models of the different disorders, and how different gene therapy strategies are being developed to treat them. We highlight where gene replacement may not be a treatment option, and where innovative therapeutic tools, such as CRISPR‐based gene editing, and new avenues of delivery are required. In general this group of genetically defined disorders, supported increasing knowledge of the mechanisms leading to neurological dysfunction serve as an excellent collection for illustrating the translational potential of gene therapy, including newly emerging tools.

Tuberous Sclerosis Complex as Disease Model for Investigating mTOR-Related Gliopathy During Epileptogenesis

Tuberous sclerosis complex (TSC) represents the prototypic monogenic disorder of the mammalian target of rapamycin (mTOR) pathway dysregulation. It provides the rational mechanistic basis of a direct link between gene mutation and brain pathology (structural and functional abnormalities) associated with a complex clinical phenotype including epilepsy, autism, and intellectual disability. So far, research conducted in TSC has been largely neuron-oriented. However, the neuropathological hallmarks of TSC and other malformations of cortical development also include major morphological and functional changes in glial cells involving astrocytes, oligodendrocytes, NG2 glia, and microglia. These cells and their interglial crosstalk may offer new insights into the common neurobiological mechanisms underlying epilepsy and the complex cognitive and behavioral comorbidities that are characteristic of the spectrum of mTOR-associated neurodevelopmental disorders. This review will focus on the role of glial dysfunction, the interaction between glia related to mTOR hyperactivity, and its contribution to epileptogenesis in TSC. Moreover, we will discuss how understanding glial abnormalities in TSC might give valuable insight into the pathophysiological mechanisms that could help to develop novel therapeutic approaches for TSC or other pathologies characterized by glial dysfunction and acquired mTOR hyperactivation.

Human Brain Organoid Models of Developmental Epilepsies

Epilepsy is a common neurological disorder characterized by recurrent and unprovoked seizures due to neuronal hyperactivity. A large proportion of epilepsy cases begin during childhood. Causes of epilepsy include stroke, infections, brain injury, genetic factors, or other factors that alter brain structure and development, but in up to 50% of cases the cause is unknown. Approximately 35% of patients have refractory seizures that do not respond to medication. Animal models and in vitro cultures have contributed to our understanding of epilepsy, but there is a clear need for better models to explore the human brain in normal and pathological conditions. Human pluripotent stem cell (PSC) technologies opened the door for new models for analyzing brain development and disease, especially conditions with a genetic component. Initially, PSCs were differentiated into 2-dimensional cultures of a homogenous population of neural cells, such as glutamatergic excitatory or γ-aminobutyric acidergic inhibitory neurons, as well as glial cells. Nevertheless, these cultures lacked the structure and complexity of a human brain. In the last decade, PSC technology has advanced to the next level through the development of 3-dimensional culture, called organoids. These organoids recapitulate features of the human brain that are missing in animal models, enabling a deeper study of the human brain. In this review, we will summarize the current status of organoid research and its application to epilepsy.

Decanoic acid inhibits mTORC1 activity independent of glucose and insulin signaling

The mTORC1 complex provides a critical role in cell function, regulating a variety of processes including growth and autophagy. mTORC1 signaling is hyperactivated in a range of common diseases including cancer, epilepsy, and neurodegenerative disorders. Hence, mTORC1 signaling provides an important target for regulation in many contexts. Here, we show that decanoic acid, a key component of a widely used medicinal diet, reduces mTORC1 activity. We identify this in a tractable model system, and validate it in ex vivo rat brain tissue and in human iPSC-derived astrocytes from patients with a clinically relevant disease. Thus, we provide insight into an easily accessible therapeutic approach for a range of diseases.

Low-glucose and -insulin conditions, associated with ketogenic diets, can reduce the activity of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway, potentially leading to a range of positive medical and health-related effects. Here, we determined whether mTORC1 signaling is also a target for decanoic acid, a key component of the medium-chain triglyceride (MCT) ketogenic diet. Using a tractable model system, Dictyostelium, we show that decanoic acid can decrease mTORC1 activity, under conditions of constant glucose and in the absence of insulin, measured by phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1). We determine that this effect of decanoic acid is dependent on a ubiquitin regulatory X domain-containing protein, mediating inhibition of a conserved Dictyostelium AAA ATPase, p97, a homolog of the human transitional endoplasmic reticulum ATPase (VCP/p97) protein. We then demonstrate that decanoic acid decreases mTORC1 activity in the absence of insulin and under high-glucose conditions in ex vivo rat hippocampus and in tuberous sclerosis complex (TSC) patient-derived astrocytes. Our data therefore indicate that dietary decanoic acid may provide a new therapeutic approach to down-regulate mTORC1 signaling.

DEPDC5 haploinsufficiency drives increased mTORC1 signaling and abnormal morphology in human iPSC-derived cortical neurons

Mutations in the DEPDC5 gene can cause epilepsy, including forms with and without brain malformations. The goal of this study was to investigate the contribution of DEPDC5 gene dosage to the underlying neuropathology of DEPDC5-related epilepsies. We generated induced pluripotent stem cells (iPSCs) from epilepsy patients harboring heterozygous loss of function mutations in DEPDC5. Patient iPSCs displayed increases in both phosphorylation of ribosomal protein S6 and proliferation rate, consistent with elevated mTORC1 activation. In line with these findings, we observed increased soma size in patient iPSC-derived cortical neurons that was rescued with rapamycin treatment. These data indicate that human cells heterozygous for DEPDC5 loss-of-function mutations are haploinsufficient for control of mTORC1 signaling. Our findings suggest that human pathology differs from mouse models of DEPDC5-related epilepsies, which do not show consistent phenotypic differences in heterozygous neurons, and support the need for human-based models to affirm and augment the findings from animal models of DEPDC5-related epilepsy.

Progression of Fetal Brain Lesions in Tuberous Sclerosis Complex

Tuberous sclerosis (TSC) is a multisystem autosomal dominant genetic disorder due to loss of function of TSC1/TSC2 resulting in increased mTOR (mammalian target of rapamycin) signaling. In the brain, TSC is characterized by the formation of specific lesions that include subependymal and white matter nodules and cortical tubers. Cells that constitute TSC lesions are mainly Giant cells and dysmorphic neurons and astrocytes, but normal cells also populate the tubers. Although considered as a developmental disorder, the histopathological features of brain lesions have been described in only a limited number of fetal cases, providing little information on how these lesions develop. In this report we characterized the development of TSC lesions in 14 fetal brains ranging from 19 gestational weeks (GW) to term and 2 postnatal cases. The study focused on the telencephalon at the level of the caudothalamic notch. Our data indicate that subcortical lesions, forming within and at the vicinity of germinative zones, are the first alterations (already detected in 19GW brains), characterized by the presence of numerous dysmorphic astrocytes and Giant, balloon-like, cells. Our data show that cortical tuber formation is a long process that initiates with the presence of dysmorphic astrocytes (by 19–21GW), progress with the apparition of Giant cells (by 24GW) and mature with the appearance of dysmorphic neurons by the end of gestation (by 36GW). Furthermore, the typical tuberal aspect of cortical lesions is only reached when bundles of neurofilament positive extensions delineate the bottom of the cortical lesion (by 36GW). In addition, our study reveals the presence of Giant cells and dysmorphic neurons immunopositive for interneuron markers such as calbindin and parvalbumin, suggesting that TSC lesions would be mosaic lesions generated from different classes of progenitors.

Studying Human Neurodevelopment and Diseases Using 3D Brain Organoids

In vitro differentiation of pluripotent stem cells provides a systematic platform to study development and disease. Recent advances in brain organoid technology have created new opportunities to investigate the formation and function of the human brain, under physiological and pathological conditions. Brain organoids can be generated to model the cellular and structural development of the human brain, and allow the investigation of the intricate interactions between resident neural and glial cell types. Combined with new advances in gene editing, imaging, and genomic analysis, brain organoid technology can be applied to address questions pertinent to human brain development, disease, and evolution. However, the current iterations of brain organoids also have limitations in faithfully recapitulating the in vivo processes. In this perspective, we evaluate the recent progress in brain organoid technology, and discuss the experimental considerations for its utilization.Dual Perspectives Companion Paper: Integrating CRISPR Engineering and hiPSC-Derived 2D Disease Modeling Systems, by Kristina Rehbach, Michael B. Fernando, and Kristen J. Brennand.

Ornithine decarboxylase, the rate-limiting enzyme of polyamine synthesis, modifies brain pathology in a mouse model of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is a rare autosomal dominant neurodevelopmental disorder characterized by variable expressivity. TSC results from inactivating variants within the TSC1 or TSC2 genes, leading to constitutive activation of mechanistic target of rapamycin complex 1 signaling. Using a mouse model of TSC (Tsc2-RG) in which the Tsc2 gene is deleted in radial glial precursors and their neuronal and glial descendants, we observed increased ornithine decarboxylase (ODC) enzymatic activity and concentration of its product, putrescine. To test if increased ODC activity and dysregulated polyamine metabolism contribute to the neurodevelopmental defects of Tsc2-RG mice, we used pharmacologic and genetic approaches to reduce ODC activity in Tsc2-RG mice, followed by histologic assessment of brain development. We observed that decreasing ODC activity and putrescine levels in Tsc2-RG mice worsened many of the neurodevelopmental phenotypes, including brain growth and neuronal migration defects, astrogliosis and oxidative stress. These data suggest a protective effect of increased ODC activity and elevated putrescine that modify the phenotype in this developmental Tsc2-RG model.

In vitro modeling for inherited neurological diseases using induced pluripotent stem cells: from 2D to organoid

Stem cells are characterized by self-renewal and by their ability to differentiate into cells of various organs. With massive progress in 2D and 3D cell culture techniques, in vitro generation of various types of such organoids from patient-derived stem cells is now possible. As in vitro differentiation protocols are usually made to resemble human developmental processes, organogenesis of patient-derived stem cells can provide key information regarding a range of developmental diseases. Human stem cell-based in vitro modeling as opposed to using animal models can particularly benefit the evaluation of neurological diseases because of significant differences in structure and developmental processes between the human and the animal brain. This review focuses on stem cell-based in vitro modeling of neurodevelopmental disorders, more specifically, the fundamentals and technical advancements in monolayer neuron and brain organoid cultures. Furthermore, we discuss the drawbacks of the conventional culture method and explore the advanced, cutting edge 3D organoid models for several neurodevelopmental diseases, including genetic diseases such as Down syndrome, Rett syndrome, and Miller-Dieker syndrome, as well as brain malformations like macrocephaly and microcephaly. Finally, we discuss the limitations of the current organoid techniques and some potential solutions that pave the way for accurate modeling of neurological disorders in a dish.

The Neurodevelopmental Pathogenesis of Tuberous Sclerosis Complex (TSC)

Tuberous sclerosis complex (TSC) is a model disorder for understanding brain development because the genes that cause TSC are known, many downstream molecular pathways have been identified, and the resulting perturbations of cellular events are established. TSC, therefore, provides an intellectual framework to understand the molecular and biochemical pathways that orchestrate normal brain development. The TSC1 and TSC2 genes encode Hamartin and Tuberin which form a GTPase activating protein (GAP) complex. Inactivating mutations in TSC genes (TSC1/TSC2) cause sustained Ras homologue enriched in brain (RHEB) activation of the mammalian isoform of the target of rapamycin complex 1 (mTORC1). TOR is a protein kinase that regulates cell size in many organisms throughout nature. mTORC1 inhibits catabolic processes including autophagy and activates anabolic processes including mRNA translation. mTORC1 regulation is achieved through two main upstream mechanisms. The first mechanism is regulation by growth factor signaling. The second mechanism is regulation by amino acids. Gene mutations that cause too much or too little mTORC1 activity lead to a spectrum of neuroanatomical changes ranging from altered brain size (micro and macrocephaly) to cortical malformations to Type I neoplasias. Because somatic mutations often underlie these changes, the timing, and location of mutation results in focal brain malformations. These mutations, therefore, provide gain-of-function and loss-of-function changes that are a powerful tool to assess the events that have gone awry during development and to determine their functional physiological consequences. Knowledge about the TSC-mTORC1 pathway has allowed scientists to predict which upstream and downstream mutations should cause commensurate neuroanatomical changes. Indeed, many of these predictions have now been clinically validated. A description of clinical imaging and histochemical findings is provided in relation to laboratory models of TSC that will allow the reader to appreciate how human pathology can provide an understanding of the fundamental mechanisms of development.