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Understanding Human Cortical Expansion Using Pluripotent Stem Cell-Derived Brain Organoids: Insights Into Progenitor Evolution and Diversity.
The evolution of the human brain underlies our higher-order cognitive functions. In particular, the cerebral cortex, the outermost layer of the brain, has rapidly evolved to contain a disproportionately large number of neurons relative to the rest of the brain. Much of this expansion is attributed to the enlargement and diversification of the pool of neural precursors, which proliferate and differentiate into the neurons and glia of the brain. How the human cerebral cortex has evolved remains an active area of investigation. With the advent of pluripotent stem cell and brain organoid technologies, comparative genomic studies between humans, mice, and nonhuman primates have identified human-specific genes or pathways during neurodevelopment. The utility of these models relies on the ability of brain organoids to preserve the cytoarchitecture and species-specific developmental trajectories of diverse neural and glial cell types that are observed in vivo. This review will discuss how brain organoids recapitulate aspects of interspecies differences during cortical development, specifically neural precursor expansion, neurogenesis, and gliogenesis, and how these models can be improved to enable a deeper understanding of human brain evolution. Unraveling the cellular and molecular mechanisms underlying cross-species differences in brain expansion could also provide key insights into neurodevelopmental diseases, particularly those where brain size is affected.
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Chronic Opioid Treatment Arrests Neurodevelopment and Alters Synaptic Activity in Human Midbrain Organoids.
Understanding the impact of long-term opioid exposure on the embryonic brain is critical due to the surging number of pregnant mothers with opioid dependency. However, this has been limited by human brain inaccessibility and cross-species differences in animal models. Here, a human midbrain model is established that uses hiPSC-derived midbrain organoids to assess cell-type-specific responses to acute and chronic fentanyl treatment and fentanyl withdrawal. Single-cell mRNA sequencing of 25,510 cells from organoids in different treatment groups reveals that chronic fentanyl treatment arrests neuronal subtype specification during early midbrain development and alters synaptic activity and neuron projection. In contrast, acute fentanyl treatment increases dopamine release but does not significantly alter gene expression related to cell lineage development. These results provide the first examination of the effects of opioid exposure on human midbrain development at the single-cell level.
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AUTS2 disruption causes neuronal differentiation defects in human cerebral organoids through hyperactivation of the WNT/β-catenin pathway.
Individuals with the Autism Susceptibility Candidate 2 (AUTS2) gene disruptions exhibit symptoms such as intellectual disability, microcephaly, growth retardation, and distinct skeletal and facial differences. The role of AUTS2 in neurodevelopment has been investigated using animal and embryonic stem cell models. However, the precise molecular mechanisms of how AUTS2 influences neurodevelopment, particularly in humans, are not thoroughly understood. Our study employed a 3D human cerebral organoid culture system, in combination with genetic, genomic, cellular, and molecular approaches, to investigate how AUTS2 impacts neurodevelopment through cellular signaling pathways. We used CRISPR/Cas9 technology to create AUTS2-deficient human embryonic stem cells and then generated cerebral organoids with these cells. Our transcriptomic analyses revealed that the absence of AUTS2 in cerebral organoids reduces the populations of cells committed to the neuronal lineage, resulting in an overabundance of cells with a transcription profile resembling that of choroid plexus (ChP) cells. Intriguingly, we found that AUTS2 negatively regulates the WNT/β-catenin signaling pathway, evidenced by its overactivation in AUTS2-deficient cerebral organoids and in luciferase reporter cells lacking AUTS2. Importantly, treating the AUTS2-deficient cerebral organoids with a WNT inhibitor reversed the overexpression of ChP genes and increased the downregulated neuronal gene expression. This study offers new insights into the role of AUTS2 in neurodevelopment and suggests potential targeted therapies for neurodevelopmental disorders.
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Organoid intelligence (OI) - The ultimate functionality of a brain microphysiological system.
Understanding brain function remains challenging as work with human and animal models is complicated by compensatory mechanisms, while in vitro models have been too simple until now. With the advent of human stem cells and the bioengineering of brain microphysiological systems (MPS), understanding how both cognition and long-term memory arise is now coming into reach. We suggest combining cutting-edge AI with MPS research to spearhead organoid intelligence (OI) as synthetic biological intelligence. The vision is to realize cognitive functions in brain MPS and scale them to achieve relevant short- and long-term memory capabilities and basic information processing as the ultimate functional experimental models for neurodevelopment and neurological function and as cell-based assays for drug and chemical testing. By advancing the frontiers of biological computing, we aim to (a) create models of intelligence-in-a-dish to study the basis of human cognitive functions, (b) provide models to advance the search for toxicants contributing to neurological diseases and identify remedies for neurological maladies, and (c) achieve relevant biological computational capacities to complement traditional computing. Increased understanding of brain functionality, in some respects still superior to today's supercomputers, may allow to imitate this in neuromorphic computer architectures or might even open up biological computing to complement silicon computers. At the same time, this raises ethical questions such as where sentience and consciousness start and what the relationship between a stem cell donor and the respective OI system is. Such ethical discussions will be critical for the socially acceptable advance of brain organoid models of cognition.
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Brain Organoids as Emerging Platforms for Modeling Neurodegenerative Diseases: Progress, Challenges, and Future Directions.
Neurodegenerative diseases are a group of disorders (such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis) characterized by loss of function and death of neurons in different parts of the nervous system. These pathologies constitute a global burden, especially for aging populations. This circumstance leads to an increasing demand for understanding the fundamental mechanisms and development of therapeutic strategies. Conventional models, including two-dimensional cell culture and animal models, postmortem brain tissue provide an overview about neurodegenerative disorders but do not completely recapitulate cellular and molecular mechanisms of the human brain. Although three-dimensional (3D) brain organoids exhibit similar properties with physiological and pathological conditions of human brain, including interaction of neuronal, glial cells and self-organizing structure, protein aggregation, neuroinflammation, and neuronal degeneration. The integration of reprogrammed human induced pluripotent stem cells (iPSCs) with 3D brain organoid systems provides a clinical platform as a bridge between bench to bedside. Brain organoids have been used to elucidate novel insights into the molecular and genetic mechanisms underlying neurodegenerative diseases. Furthermore, brain organoids serve as a tool for in vitro disease modeling, drug screening and emergence of new treatments. Despite these clinical benefits, there are various limitations such as incomplete tissue maturation, lack of vascularization and incomplete cellular diversity in this 3D culture system. This review describes in detail the advantages and disadvantages of brain organoids usage in modeling neurodegenerative diseases from a contemporary perspective.
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Human mini-brain models.
Engineered human mini-brains, made possible by knowledge from the convergence of precision microengineering and cell biology, permit systematic studies of complex neurological processes and of pathogenesis beyond what can be done with animal models. By culturing human brain cells with physiological microenvironmental cues, human mini-brain models reconstitute the arrangement of structural tissues and some of the complex biological functions of the human brain. In this Review, we highlight the most significant developments that have led to microphysiological human mini-brain models. We introduce the history of mini-brain development, review methods for creating mini-brain models in static conditions, and discuss relevant state-of-the-art dynamic cell-culture systems. We also review human mini-brain models that reconstruct aspects of major neurological disorders under static or dynamic conditions. Engineered human mini-brains will contribute to advancing the study of the physiology and aetiology of neurological disorders, and to the development of personalized medicines for them.
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Single-cell brain organoid screening identifies developmental defects in autism.
The development of the human brain involves unique processes (not observed in many other species) that can contribute to neurodevelopmental disorders1-4. Cerebral organoids enable the study of neurodevelopmental disorders in a human context. We have developed the CRISPR-human organoids-single-cell RNA sequencing (CHOOSE) system, which uses verified pairs of guide RNAs, inducible CRISPR-Cas9-based genetic disruption and single-cell transcriptomics for pooled loss-of-function screening in mosaic organoids. Here we show that perturbation of 36 high-risk autism spectrum disorder genes related to transcriptional regulation uncovers their effects on cell fate determination. We find that dorsal intermediate progenitors, ventral progenitors and upper-layer excitatory neurons are among the most vulnerable cell types. We construct a developmental gene regulatory network of cerebral organoids from single-cell transcriptomes and chromatin modalities and identify autism spectrum disorder-associated and perturbation-enriched regulatory modules. Perturbing members of the BRG1/BRM-associated factor (BAF) chromatin remodelling complex leads to enrichment of ventral telencephalon progenitors. Specifically, mutating the BAF subunit ARID1B affects the fate transition of progenitors to oligodendrocyte and interneuron precursor cells, a phenotype that we confirmed in patient-specific induced pluripotent stem cell-derived organoids. Our study paves the way for high-throughput phenotypic characterization of disease susceptibility genes in organoid models with cell state, molecular pathway and gene regulatory network readouts.
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Flash Assembloids: A Rapid Biofabrication of a Platform for Modeling Early Glioblastoma Invasion at the Glioblastoma-Brain Organoid Interfaces.
Glioblastoma (GBM) remains one of the most aggressive brain malignancies, characterized by rapid infiltration, therapeutic resistance, and dismal prognosis. Modeling GBM invasion in physiologically relevant systems has been hindered by the lack of reproducible platforms. Here, we present a bioengineered assembloid (ASM) system that integrates GBM cells encapsulated in self-degradable 5% oxidized alginate microgel (5OA) with dorsal forebrain organoids (DOs) to recapitulate early tumor-host interactions during glioblastoma invasion toward the brain. Live-cell imaging revealed GBM self-aggregation, leading to increased recruitment and invasion at the DO boundary, accompanied by strong cell-cell adhesion, nuclear compaction, and the infiltration fronts enriched in SOX2+/Vimentin+ tumor populations. Transcriptomic profiling demonstrated upregulation of adhesion, integrin clustering, and mechanosensing-associated genes, alongside downregulation of neuronal differentiation pathways, indicating a dual invasion and host suppression strategy. Comparative analyses of GBM-only constructs and DO-GBM ASMs revealed elevated expression of laminin subunits and enrichment of invasion-associated pathways, including PI3K-AKT-mTOR and TGF-β signaling, reflecting a shift toward an invasive state at the transcriptomic level. In vivo implantation of ASMs confirmed aggressive GBM infiltration and niche remodeling, highlighting the translational relevance. These findings suggest that it recapitulates the structural, molecular, and functional hallmarks of GBM invasion and tumor-driven remodeling of the host brain microenvironment.
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Self-organizing cortex generated from human iPSCs with combination of FGF2 and ambient oxygen.
Human brain development has generally been studied through the analysis of postmortem tissues because of limited access to fetal brain tissues. This approach, however, only provides information from the perspective of long-term development. To investigate the pathophysiology of neurodevelopmental disorders, it is necessary to understand the detailed mechanisms of human brain development. Recent advances in pluripotent stem cell (PSC) technologies enable us to establish in vitro brain models from human induced PSCs (hiPSCs), which can be used to examine the pathophysiological mechanisms of neurodevelopmental disorders. We previously demonstrated that self-organized cerebral tissues can be generated from human PSCs in a three-dimensional (3D) culture system. Here, we describe the cerebral tissues differentiated from hiPSCs in a further-optimized 3D culture. We found that treatment with FGF2 is helpful to form iPSC aggregates with efficiency. Neuroepithelial structures spontaneously formed with apico-basal polarity in the aggregates expressing forebrain marker FOXG1. The neuroepithelium self-forms a multilayered structure including progenitor zones (ventricular and subventricular zones) and neuronal zone (cortical plate). Furthermore, with the same level of oxygen (O2) as in ambient air (20% O2), we found that self-formation of cortical structures lasted for 70 days in culture. Thus, our optimized 3D culture for the generation of cortical structure from hiPSCs is a simple yet effective method.
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Non-destructive luminescence and PET imaging to monitor tissue microenvironment in microphysiological systems during brain metastasis using dissociated cerebral organoids.
During brain metastasis, tumor cells interact with the surrounding stroma, including neurons and astrocytes, to create a tumor-promoting microenvironment. However, the molecular and cellular factors driving tumor-neural stroma interactions remain unclear. Here, we developed a co-culture model of metastatic melanoma by combining metastatic melanoma cells with dissociated human iPSC-derived cerebral organoids, consisting of neurons and astrocytes, in a microfluidic device. We cultured these astrocytes and neurons in a 3D hydrogel that contained a domain with metastatic melanoma cells. This approach generated a spatially organized co-culture system with no physical boundary between the tumor and stromal compartments. Then, we leveraged several imaging modalities to study tumor-stroma interactions and changes in the microenvironment. Using non-destructive, luminescence-based methods, we spatially resolved changes in cell viability, metabolite concentration, and other biochemical parameters. We also used luminescence to analyze the effect of radionuclides on tumor cell viability and used PET imaging to monitor their diffusion across the system.