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Tackling mitochondrial diversity in brain function: from animal models to human brain organoids.
Mitochondria exhibit high degree of heterogeneity within various tissues, including differences in terms of morphology, quantity, or function. Mitochondria can even vary among distinct sub-compartments of the same cell. Emerging evidence suggest that the molecular diversity of mitochondria can influence the identity and functionality of a given cell type. Human pathologies affecting mitochondria typically cause tissue and cell-type-specific impairment. Mitochondrial diversity could thus play a contributing role not only in physiological cell fate specification but also during pathological disease development. In this review, we discuss the role of mitochondrial diversity in brain function during health and disease. Recent advances in induced pluripotent stem cells (iPSCs) research and the derivation of cerebral organoids could provide novel opportunities to unveil the role of mitochondrial heterogeneity for the function of the human brain. Mitochondrial diversity might be at the bases of the cell-type-specific vulnerability of mitochondrial disorders and may represent an underappreciated target of disease intervention.
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Single-Cell Transcriptome Landscape and Cell Fate Decoding in Human Brain Organoids after Transplantation.
Human stem cells and derivatives transplantation are widely used to treat nervous system diseases, while the fate determination of transplanted cells is not well elucidated. To explore cell fate changes of human brain organoids before and after transplantation, human brain organoids are transplanted into prefrontal cortex (PFC) and hippocampus (HIP), respectively. Single-cell sequencing is then performed. According to time-series sample comparison, transplanted cells mainly undergo neural development at 2 months post-transplantation (MPT) and then glial development at 4MPT, respectively. A different brain region sample comparison shows that organoids grafted to PFC have obtained cell fate close to those of host cells in PFC, other than HIP, which may be regulated by the abundant expression of dopamine (DA) and acetylcholine (Ach) in PFC. Meanwhile, morphological complexity of human astrocyte grafts is greater in PFC than in HIP. DA and Ach both activate the calcium activity and increase morphological complexity of astrocytes in vitro. This study demonstrates that human brain organoids receive host niche factor regulation after transplantation, resulting in the alignment of grafted cell fate with implanted brain regions, which may contribute to a better understanding of cell transplantation and regenerative medicine.
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Investigating the neurodevelopmental toxicity of graphene oxides using 3D human brain organoids and zebrafish models: emphasis on GABAergic neuron alterations at single-cell resolution.
Exposure to graphene oxides (GOs) may induce neurotoxic effects, but the mechanisms and extent of these effects remain unclear, partly due to limitations in traditional neurotoxicity models. In this study, we utilized 3D human brain organoids and zebrafish larvae to investigate GO-induced neurodevelopmental toxicity. In 3D human brain organoids, GOs significantly reduced cellular viability in a dose-dependent manner, with bulk RNA-sequencing indicating minimal impact on neuron-related pathways. However, single-cell RNA-sequencing revealed significant alterations in glutamatergic and GABAergic neurons, particularly in gene ontology terms related to synaptic function and neuronal development. Further analysis showed that GO exposure affected the localization, morphology, and electrophysiological activity of GABAergic neurons. Additionally, key genes involved in neurodegeneration (ko05022) and protein processing in endoplasmic reticulum (ko04141) were dysregulated in GABAergic neurons. In vivo studies using zebrafish larvae demonstrated altered locomotor behaviors, including hyperactivity during dark cycles and reduced swimming distances during light cycles, alongside a loss of GABAergic neurons in specific brain regions. These findings highlight the potential neurodevelopmental toxicity of GOs, especially through their impact on GABAergic neurons, and underscore the value of combining 3D human brain organoids and zebrafish models with single-cell resolution techniques to better understand the neurotoxic effects of nanomaterials.
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Aberrant choroid plexus formation drives the development of treatment-related brain toxicity.
Brain tumors are commonly treated with radiotherapy, but the efficacy of the treatment is limited by its toxicity to the normal tissue including post-irradiation contrast enhanced lesions often linked to necrosis. The poorly understood mechanisms behind such brain lesions were studied using cerebral organoids. Here we show that irradiation of such organoids leads to dose-dependent growth retardation and formation of liquid-filled cavities but is not correlated with necrosis. Instead, the radiation-induced changes comprise of an enhancement of cortical hem markers, altered neuroepithelial stem cell differentiation, and an increase of ZO1+/AQP1+/CLDN3+-choroid plexus (CP)-like structures accompanied by an upregulation of IGF2 mRNA, known to be expressed in CP and cerebrospinal fluid. The altered differentiation is attributed to changes in the WNT/BMP signaling pathways. We conclude that aberrant CP formation can be involved in radiation-induced brain lesions providing additional strategies for possible countermeasures.
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Activity-induced instabilities of brain organoids.
We present an analytical and numerical investigation of the activity-induced hydrodynamic instabilities in model brain organoids. While several mechanisms have been introduced to explain the experimental observation of surface instabilities in brain organoids, the role of activity has been largely overlooked. Our results show that the active stress generated by the cells can be a, previously overlooked, contributor to the emergence of surface deformations in brain organoids.
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Cerebral organoids to unravel the mechanisms underlying malformations of human cortical development.
Genetic studies identified multiple mutations associated with malformations of cortical development (MCD) in humans. When analyzing the underlying mechanisms in non-human experimental models it became increasingly evident, that these mutations accumulate in genes, which functions evolutionary progressed from rodents to humans resulting in an incomplete reflection of the molecular and cellular alterations in these models. Human brain organoids derived from human pluripotent stem cells resemble early aspects of human brain development to a remarkable extent making them an attractive model to investigate MCD. Here we review how human brain organoids enable the generation of fundamental new insight about the underlying pathomechanisms of MCD. We show how phenotypic features of these diseases are reflected in human brain organoids and discuss challenges and future considerations but also limitations for the use of human brain organoids to model human brain development and associated disorders.
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Divergence between neural and retinal lineage specification during human brain development by signal transduction.
INTRODUCTION: The physiological functions of p70 S6 kinase 1 (S6K1) have been extensively studied in S6K1-deficient mice. However, there is limited evidence demonstrating the influence of S6K1 deletion on human brain development. OBJECTIVES: In this study, we identify the role of S6K1 in human brain development utilizing genetically engineered human embryonic stem cell-derived brain organoids. METHODS: Dorsal forebrain organoids generated from S6K1-depleted human embryonic stem cells (hESCs) were analyzed through single-cell RNA sequencing at early (5 weeks) and late (14 weeks) stages. In addition, the brain organoids derived from co-cultured S6K1-deleted and wild-type hESCs were subjected to ATAC-sequencing. RESULTS: Genetic deletion of S6K1 significantly decreases the size of the dorsal forebrain organoids in the early stages. Single-cell RNA sequencing analysis shows an abnormal emergence of retinal cell lineages in S6K1-deleted brain organoids, which diverges from cortical neurons in the early stage, eventually leading to a decrease in the proportion of mature cortical neurons. The chromatin accessibility analysis of co-cultured brain organoids shows that retinal specification in S6K1 knockout organoids was due to non-cell-autonomous function, whereas incomplete maturation of neurons results from cell-autonomous function. CONCLUSION: Depletion of S6K1 signaling in the early stage of human brain development drives the formation of retinal cells distinct from cortical neurons. Our findings demonstrate that S6K1 signaling fine-tunes neuronal and retinal lineage specification during brain development.
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Engineered Alzheimer Organoids Validate the Link Between Intracellular and Soluble p-Tau Biomarkers and Highlight the Contribution of Astrocytic Tau.
Soluble phosphorylated tau has become a key biomarker for Alzheimer's disease pathology, yet further mechanistic studies are needed beyond observational investigations to clarify the relationship between soluble p-tau species and intracellular tau pathology. Here, we utilized chimeric human cerebral organoids (chCOs) to generate a series of organoids in which the endogenous MAPT gene was CRISPR/Cas9 edited in different cell types to create phosphorylation-deficient mutants at the AD-associated sites. We found that the APPswe mutation increased tau phosphorylation in both neurons and astrocytes. Notably, astrocyte-specific phosphorylation-deficient mutations of tau in organoids reduced soluble p-tau181 and p-tau217 levels, as detected by single-molecule array. These findings indicate that astrocytic tau plays a substantial role in contributing to the pool of extracellular phosphorylated tau and suggest it may be an overlooked source of AD biomarkers. Moreover, our engineered chCOs offer a versatile platform for exploring how cell-type-specific pathologies correlate with changes in biomarker profiles.
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Cerebral organoids in a dish: progress and prospects.
A three-dimensional culture of cortical tissues derived from pluripotent stem cells offers an opportunity to model human brain development and disorders. In a recent issue of Nature, Lancaster et al. describe a new method for generating cerebral organoids in a dish and use it to model microcephaly.
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Robust and Highly Reproducible Generation of Cortical Brain Organoids for Modelling Brain Neuronal Senescence In Vitro.
Brain organoids are three-dimensional models of the developing human brain and provide a compelling, cutting-edge platform for disease modeling and large-scale genomic and drug screening. Due to the self-organizing nature of cells in brain organoids and the growing range of available protocols for their generation, issues with heterogeneity and variability between organoids have been identified. In this protocol paper, we describe a robust and replicable protocol that largely overcomes these issues and generates cortical organoids from neuroectodermal progenitors within 1 month, and that can be maintained for more than 1 year. This highly reproducible protocol can be easily carried out in a standard tissue culture room and results in organoids with a rich diversity of cell types typically found in the developing human cortex. Despite their early developmental make-up, neurons and other human brain cell types will start to exhibit the typical signs of senescence in neuronal cells after prolonged in vitro culture, making them a valuable and useful platform for studying aging-related neuronal processes. This protocol also outlines a method for detecting such senescent cells in cortical brain organoids using senescence-associated beta-galactosidase staining.