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Aerosol Jet-Printed High-Aspect Ratio Micro-Needle Electrode Arrays Applied for Human Cerebral Organoids and 3D Neurospheroid Networks.
The human brain is a complex and poorly accessible organ. Thus, new tools are required for studying the neural function in a controllable environment that preserves multicellular interaction and neuronal wiring. In particular, high-throughput methods that alleviate the need for animal experiments are essential for future studies. Recent developments of induced pluripotent stem cell technologies have enabled in vitro modeling of the human brain by creating three-dimensional brain tissue mimic structures. To leverage these new technologies, a systematic and versatile approach for evaluating neuronal activity at larger tissue depths within the regime of tens to hundreds of micrometers is required. Here, we present an aerosol-jet- and inkjet-printing-based method to fabricate microelectrode arrays, equipped with high-aspect ratio μ-needle electrodes that penetrate 3D neural network assemblies. The arrays have been successfully applied for electrophysiological recordings on interconnected neurospheroids formed on an engineered substrate and on cerebral organoids, both derived from human induced pluripotent stem cells.
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Robust production of uniform human cerebral organoids from pluripotent stem cells.
Human cerebral organoid (hCO) models offer the opportunity to understand fundamental processes underlying human-specific cortical development and pathophysiology in an experimentally tractable system. Although diverse methods to generate brain organoids have been developed, a major challenge has been the production of organoids with reproducible cell type heterogeneity and macroscopic morphology. Here, we have directly addressed this problem by establishing a robust production pipeline to generate morphologically consistent hCOs and achieve a success rate of >80%. These hCOs include both a radial glial stem cell compartment and electrophysiologically competent mature neurons. Moreover, we show using immunofluorescence microscopy and single-cell profiling that individual organoids display reproducible cell type compositions that are conserved upon extended culture. We expect that application of this method will provide new insights into brain development and disease processes.
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Fundamental Neurochemistry Review: Incorporating a greater diversity of cell types, including microglia, in brain organoid cultures improves clinical translation.
Brain organoids have the potential to improve clinical translation, with the added benefit of reducing any extraneous use of experimental animals. As brain organoids are three-dimensional in vitro constructs that emulate the human brain, they bridge in vitro and in vivo studies more appropriately than monocultures. Although many factors contribute to the failure of extrapolating monoculture-based information to animal-based experiments and clinical trials, for the purpose of this review, we will focus on glia (non-neuronal brain cells), whose functions and transcriptome are particularly abnormal in monocultures. As discussed herein, glia require signals from-and contact with-other cell types to exist in their homeostatic state, which likely contributes to some of the differences between data derived from monocultures and data derived from brain organoids and even two-dimensional co-cultures. Furthermore, we highlight transcriptomic differences between humans and mice in regard to aging and Alzheimer's disease, emphasizing need for a model using the human genome-again, a benefit of brain organoids-to complement data derived from animals. We also identify an urgency for guidelines to improve the reporting and transparency of research using organoids. The lack of reporting standards creates challenges for the comparison and discussion of data from different articles. Importantly, brain organoids mark the first human model enabling the study of brain cytoarchitecture and development.
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Neuroscience. What are mini-brains?
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Human brain organoids assemble functionally integrated bilateral optic vesicles.
During embryogenesis, optic vesicles develop from the diencephalon via a multistep process of organogenesis. Using induced pluripotent stem cell (iPSC)-derived human brain organoids, we attempted to simplify the complexities and demonstrate formation of forebrain-associated bilateral optic vesicles, cellular diversity, and functionality. Around day 30, brain organoids attempt to assemble optic vesicles, which develop progressively as visible structures within 60 days. These optic vesicle-containing brain organoids (OVB-organoids) constitute a developing optic vesicle's cellular components, including primitive corneal epithelial and lens-like cells, retinal pigment epithelia, retinal progenitor cells, axon-like projections, and electrically active neuronal networks. OVB-organoids also display synapsin-1, CTIP-positive myelinated cortical neurons, and microglia. Interestingly, various light intensities could trigger photosensitive activity of OVB-organoids, and light sensitivities could be reset after transient photobleaching. Thus, brain organoids have the intrinsic ability to self-organize forebrain-associated primitive sensory structures in a topographically restricted manner and can allow interorgan interaction studies within a single organoid.
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Tissue morphology influences the temporal program of human brain organoid development.
Progression through fate decisions determines cellular composition and tissue architecture, but how that same architecture may impact cell fate is less clear. We took advantage of organoids as a tractable model to interrogate this interaction of form and fate. Screening methodological variations revealed that common protocol adjustments impacted various aspects of morphology, from macrostructure to tissue architecture. We examined the impact of morphological perturbations on cell fate through integrated single nuclear RNA sequencing (snRNA-seq) and spatial transcriptomics. Regardless of the specific protocol, organoids with more complex morphology better mimicked in vivo human fetal brain development. Organoids with perturbed tissue architecture displayed aberrant temporal progression, with cells being intermingled in both space and time. Finally, encapsulation to impart a simplified morphology led to disrupted tissue cytoarchitecture and a similar abnormal maturational timing. These data demonstrate that cells of the developing brain require proper spatial coordinates to undergo correct temporal progression.
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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.
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Scalable production of human cortical organoids using a biocompatible polymer.
The generation of neural organoids from human pluripotent stem cells holds great promise in modelling disease and screening drugs, but current approaches are difficult to scale due to undesired organoid fusion. Here we develop a scalable cerebral cortical organoid platform by screening biocompatible polymers that prevent the fusion of organoids cultured in suspension. We identify a cost-effective polysaccharide that increases the viscosity of the culture medium, significantly enhancing the yield of cortical organoids while preserving key features such as regional patterning, neuronal morphology and functional activity. We further demonstrate that this platform enables straightforward screening of 298 FDA-approved drugs and teratogens for growth defects using over 2,400 cortical organoids, uncovering agents that disrupt organoid growth and development. We anticipate this approach to provide a robust and scalable system for modelling human cortical development, and facilitate efficient compound screening for neuropsychiatric disorders-associated phenotypes.
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Cerebral organoids model human brain development and microcephaly.
The complexity of the human brain has made it difficult to study many brain disorders in model organisms, highlighting the need for an in vitro model of human brain development. Here we have developed a human pluripotent stem cell-derived three-dimensional organoid culture system, termed cerebral organoids, that develop various discrete, although interdependent, brain regions. These include a cerebral cortex containing progenitor populations that organize and produce mature cortical neuron subtypes. Furthermore, cerebral organoids are shown to recapitulate features of human cortical development, namely characteristic progenitor zone organization with abundant outer radial glial stem cells. Finally, we use RNA interference and patient-specific induced pluripotent stem cells to model microcephaly, a disorder that has been difficult to recapitulate in mice. We demonstrate premature neuronal differentiation in patient organoids, a defect that could help to explain the disease phenotype. Together, these data show that three-dimensional organoids can recapitulate development and disease even in this most complex human tissue.
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Brain organoid models of SZT2-related disease reveal an overproduction of outer radial glial cells through mTORC1 activation.
Biallelic loss-of-function variants of Seizure Threshold 2 (SZT2) cause neurodevelopmental diseases with developmental delay, epilepsy, and macrocephaly. SZT2 forms the KICSTOR complex, which represses the mechanistic target of rapamycin complex 1 (mTORC1) amino acid-sensitive pathway. SZT2 dysfunction is thought to cause abnormal activation of the mTOR pathway, underlying the pathogenesis of SZT2-related diseases. We previously reported constitutive activation of mTORC1 in lymphoblastoid cell lines derived from patients with SZT2-related disease. However, the impact of SZT2 dysfunction on human brain development remains unclear. In this study, we examined the effects of SZT2 dysfunction on brain development using human brain organoids. We generated pluripotent stem cell-derived brain organoids and found a significantly greater number of outer radial glial cells (oRGCs) in the subventricular zone-like layer (SVZ) of SZT2 mutant (MT) brain organoids compared to control (WT) brain organoids. The number of upper-layer neurons, which generally originate from oRGCs, was also significantly greater in SZT2 MT brain organoids. Mechanistically, SZT2 MT brain organoids showed higher mTORC1 activity in the SVZ, where neural stem/progenitor cells amplify for cortical expansion in response to mTORC1 activity. Our data suggest that SZT2 dysfunction may cause macrocephaly through dysregulation of mTORC1 in early neural development.