Single-cell transcriptomics captures features of human midbrain development and dopamine neuron diversity in brain organoids

CelltypeR: A flow cytometry pipeline to characterize single cells from brain organoids

Motivated by the cellular heterogeneity in complex tissues, particularly in brain and induced pluripotent stem cell (iPSC)-derived brain models, we developed a complete workflow to reproducibly characterize cell types in complex tissues. Our approach combines a flow cytometry (FC) antibody panel with our computational pipeline CelltypeR, enabling dataset aligning, unsupervised clustering optimization, cell type annotating, and statistical comparisons. Applied to human iPSC derived midbrain organoids, it successfully identified the major brain cell types. We performed fluorescence-activated cell sorting of CelltypeR-defined astrocytes, radial glia, and neurons, exploring transcriptional states by single-cell RNA sequencing. Among the sorted neurons, we identified subgroups of dopamine neurons: one reminiscent of substantia nigra cells most vulnerable in Parkinson's disease. Finally, we used our workflow to track cell types across a time course of organoid differentiation. Overall, our adaptable analysis framework provides a generalizable method for reproducibly identifying cell types across FC datasets in complex tissues.

Advances in neuronal reprogramming for neurodegenerative diseases: Strategies, controversies, and opportunities

Neuronal death is often observed in central nervous system injuries and neurodegenerative diseases. The mammalian central nervous system manifests limited neuronal regeneration capabilities, and traditional cell therapies are limited in their potential applications due to finite cell sources and immune rejection. Neuronal reprogramming has emerged as a novel technology, in which non-neuronal cells (e.g. glial cells) are transdifferentiated into mature neurons. This process results in relatively minimal immune rejection. The present review discuss the latest progress in this cutting-edge field, including starter cell selection, innovative technical strategies and methods of neuronal reprogramming for neurodegenerative diseases, as well as the potential problems and controversies. The further development of neuronal reprogramming technology may pave the way for novel therapeutic strategies in the treatment of neurodegenerative diseases.

Human brain organoids for understanding substance use disorders

Substance use disorders (SUDs) are complex mental health conditions involving a problematic pattern of substance use. Challenges remain in understanding their neural mechanisms, which are likely to lead to improved SUD treatments. Human brain organoids, brain-like 3D in vitro cultures derived from human stem cells, show unique potential in recapitulating the response of a developing human brain to substances. Here, we review the recent progress in understanding SUDs using human brain organoid models focusing on neurodevelopmental perspectives. We first summarize the background of SUDs in humans. Moreover, we introduce the development of various human brain organoid models and then discuss current progress and findings underlying the abuse of substances like nicotine, alcohol, and other addictive drugs using organoid models. Furthermore, we review efforts to develop organ chips and microphysiological systems to engineer better human brain organoids for advancing SUD studies. Lastly, we conclude by elaborating on the current challenges and future directions of SUD studies using human brain organoids.

Non-Invasive Quality Control of Organoid Cultures Using Mesofluidic CSTR Bioreactors and High-Content Imaging

Human brain organoids produce anatomically relevant cellular structures and recapitulate key aspects of in vivo brain function, which holds great potential to model neurological diseases and screen therapeutics. However, the long growth time of 3D systems complicates the culturing of brain organoids and results in heterogeneity across samples hampering their applications. We developed an integrated platform to enable robust and long-term culturing of 3D brain organoids. We designed a mesofluidic bioreactor device based on a reaction-diffusion scaling theory, which achieves robust media exchange for sufficient nutrient delivery in long-term culture. We integrated this device with longitudinal tracking and machine learning-based classification tools to enable non-invasive quality control of live organoids. This integrated platform allows for sample pre-selection for downstream molecular analysis. Transcriptome analyses of organoids revealed that our mesofluidic bioreactor promoted organoid development while reducing cell death. Our platform thus offers a generalizable tool to establish reproducible culture standards for 3D cellular systems for a variety of applications beyond brain organoids.

Bioengineered Silk Protein-Based 3D In Vitro Models for Tissue Engineering and Drug Development: From Silk Matrix Properties to Biomedical Applications

3D in vitro model has emerged as a valuable tool for studying tissue development, drug screening, and disease modeling. 3D systems can accurately replicate tissue microstructures and physiological features, mirroring the in vivo microenvironment departing from conventional 2D cell cultures. Various 3D in vitro models utilizing biomacromolecules like collagen and synthetic polymers have been developed to meet diverse research needs and address the complex challenges of contemporary research. Silk proteins, bearing structural and functional similarities to collagen, have been increasingly employed to construct advanced 3D in vitro systems, surpassing the limitations of 2D cultures. This review examines silk proteins' composition, structure, properties, and functions, elucidating their role in 3D in vitro models. Furthermore, recent advances in biomedical applications involving silk-based organoid models are discussed. In particular, the unique physiological attributes of silk matrix constituents in in vitro tissue constructs are highlighted, providing a meticulous evaluation of their importance. Additionally, it outlines the current research hurdles and complexities while contemplating future avenues, thereby paving the way for developing complex and biomimetic silk protein-based microtissues.

Multiscale engineering of brain organoids for disease modeling

Brain organoids hold great potential for modeling human brain development and pathogenesis. They recapitulate certain aspects of the transcriptional trajectory, cellular diversity, tissue architecture and functions of the developing brain. In this review, we explore the engineering strategies to control the molecular-, cellular- and tissue-level inputs to achieve high-fidelity brain organoids. We review the application of brain organoids in neural disorder modeling and emerging bioengineering methods to improve data collection and feature extraction at multiscale. The integration of multiscale engineering strategies and analytical methods has significant potential to advance insight into neurological disorders and accelerate drug development.

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.

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.

Massively parallel characterization of regulatory elements in the developing human cortex

Nucleotide changes in gene regulatory elements are important determinants of neuronal development and diseases. Using massively parallel reporter assays in primary human cells from mid-gestation cortex and cerebral organoids, we interrogated the cis-regulatory activity of 102,767 open chromatin regions, including thousands of sequences with cell type-specific accessibility and variants associated with brain gene regulation. In primary cells, we identified 46,802 active enhancer sequences and 164 variants that alter enhancer activity. Activity was comparable in organoids and primary cells, suggesting that organoids provide an adequate model for the developing cortex. Using deep learning we decoded the sequence basis and upstream regulators of enhancer activity. This work establishes a comprehensive catalog of functional gene regulatory elements and variants in human neuronal development.

Midbrain organoids for Parkinson's disease (PD) - A powerful tool to understand the disease pathogenesis

Brain Organiods (BOs) are a promising technique for researching disease progression in the human brain. These organoids, which are produced from human induced pluripotent stem cells (HiPSCs), can construct themselves into structured frameworks. In the context of Parkinson's disease (PD), recent advancements have been made in the development of Midbrain organoids (MBOs) models that consider key pathophysiological mechanisms such as alpha-synuclein (α-Syn), Lewy bodies, dopamine loss, and microglia activation. However, there are limitations to the current use of BOs in disease modelling and drug discovery, such as the lack of vascularization, long-term differentiation, and absence of glial cells. To address these limitations, researchers have proposed the use of spinning bioreactors to improve oxygen and nutrient perfusion. Modelling PD utilising modern experimental in vitro models is a valuable tool for studying disease mechanisms and elucidating previously unknown features of PD. In this paper, we exclusively review the unique methods available for cultivating MBOs using a pumping system that mimics the circulatory system. This mechanism may aid in delivering the required amount of oxygen and nutrients to all areas of the organoids, preventing cell death, and allowing for long-term culture and using co-culturing techniques for developing glial cell in BOs. Furthermore, we emphasise some of the significant discoveries about the BOs and the potential challenges of using BOs will be discussed.

Advanced 3D imaging and organoid bioprinting for biomedical research and therapeutic applications

Organoid cultures offer a valuable platform for studying organ-level biology, allowing for a closer mimicry of human physiology compared to traditional two-dimensional cell culture systems or non-primate animal models. While many organoid cultures use cell aggregates or decellularized extracellular matrices as scaffolds, they often lack precise biochemical and biophysical microenvironments. In contrast, three-dimensional (3D) bioprinting allows precise placement of organoids or spheroids, providing enhanced spatial control and facilitating the direct fusion for the formation of large-scale functional tissues in vitro. In addition, 3D bioprinting enables fine tuning of biochemical and biophysical cues to support organoid development and maturation. With advances in the organoid technology and its potential applications across diverse research fields such as cell biology, developmental biology, disease pathology, precision medicine, drug toxicology, and tissue engineering, organoid imaging has become a crucial aspect of physiological and pathological studies. This review highlights the recent advancements in imaging technologies that have significantly contributed to organoid research. Additionally, we discuss various bioprinting techniques, emphasizing their applications in organoid bioprinting. Integrating 3D imaging tools into a bioprinting platform allows real-time visualization while facilitating quality control, optimization, and comprehensive bioprinting assessment. Similarly, combining imaging technologies with organoid bioprinting can provide valuable insights into tissue formation, maturation, functions, and therapeutic responses. This approach not only improves the reproducibility of physiologically relevant tissues but also enhances understanding of complex biological processes. Thus, careful selection of bioprinting modalities, coupled with appropriate imaging techniques, holds the potential to create a versatile platform capable of addressing existing challenges and harnessing opportunities in these rapidly evolving fields.

FANCD2 deficiency sensitizes SHH medulloblastoma to radiotherapy via ferroptosis

Radiotherapy is one of the standard therapeutic regimens for medulloblastoma (MB). Tumor cells utilize DNA damage repair (DDR) mechanisms to survive and develop resistance during radiotherapy. It has been found that targeting DDR sensitizes tumor cells to radiotherapy in several types of cancer, but whether and how DDR pathways are involved in the MB radiotherapy response remain to be determined. Single-cell RNA sequencing was carried out on 38 MB tissues, followed by expression enrichment assays. Fanconi anemia group D2 gene (FANCD2) expression was evaluated in MB samples and public MB databases. The function of FANCD2 in MB cells was examined using cell counting assays (CCK-8), clone formation, lactate dehydrogenase activity, and in mouse orthotopic models. The FANCD2-related signaling pathway was investigated using assays of peroxidation, a malondialdehyde assay, a reduced glutathione assay, and using FerroOrange to assess intracellular iron ions (Fe2+ ). Here, we report that FANCD2 was highly expressed in the malignant sonic hedgehog (SHH) MB subtype (SHH-MB). FANCD2 played an oncogenic role and predicted worse prognosis in SHH-MB patients. Moreover, FANCD2 knockdown markedly suppressed viability, mobility, and growth of SHH-MB cells and sensitized SHH-MB cells to irradiation. Mechanistically, FANCD2 deficiency led to an accumulation of Fe2+ due to increased divalent metal transporter 1 expression and impaired glutathione peroxidase 4 activity, which further activated ferroptosis and reduced proliferation of SHH-MB cells. Using an orthotopic mouse model, we observed that radiotherapy combined with silencing FANCD2 significantly inhibited the growth of SHH-MB cell-derived tumors in vivo. Our study revealed FANCD2 as a potential therapeutic target in SHH-MB and silencing FANCD2 could sensitize SHH-MB cells to radiotherapy via inducing ferroptosis. © 2024 The Pathological Society of Great Britain and Ireland.

Gliomas: a reflection of temporal gliogenic principles

The hijacking of early developmental programs is a canonical feature of gliomas where neoplastic cells resemble neurodevelopmental lineages and possess mechanisms of stem cell resilience. Given these parallels, uncovering how and when in developmental time gliomagenesis intersects with normal trajectories can greatly inform our understanding of tumor biology. Here, we review how elapsing time impacts the developmental principles of astrocyte (AS) and oligodendrocyte (OL) lineages, and how these same temporal programs are replicated, distorted, or circumvented in pathological settings such as gliomas. Additionally, we discuss how normal gliogenic processes can inform our understanding of the temporal progression of gliomagenesis, including when in developmental time gliomas originate, thrive, and can be pushed towards upon therapeutic coercion.

Complex In Vitro Model Characterization for Context of Use in Toxicologic Pathology: Use Cases by Collaborative Teams of Biologists, Bioengineers, and Pathologists

Complex in vitro models (CIVMs) offer the potential to increase the clinical relevance of preclinical efficacy and toxicity assessments and reduce the reliance on animals in drug development. The European Society of Toxicologic Pathology (ESTP) and Society for Toxicologic Pathology (STP) are collaborating to highlight the role of pathologists in the development and use of CIVM. Pathologists are trained in comparative animal medicine which enhances their understanding of mechanisms of human and animal diseases, thus allowing them to bridge between animal models and humans. This skill set is important for CIVM development, validation, and data interpretation. Ideally, diverse teams of scientists, including engineers, biologists, pathologists, and others, should collaboratively develop and characterize novel CIVM, and collectively assess their precise use cases (context of use). Implementing a morphological CIVM evaluation should be essential in this process. This requires robust histological technique workflows, image analysis techniques, and needs correlation with translational biomarkers. In this review, we demonstrate how such tissue technologies and analytics support the development and use of CIVM for drug efficacy and safety evaluations. We encourage the scientific community to explore similar options for their projects and to engage with health authorities on the use of CIVM in benefit-risk assessment.

A cell therapy approach based on iPSC-derived midbrain organoids for the restoration of motor function in a Parkinson's disease mouse model

Parkinson's disease (PD) is a neurodegenerative disease characterized by the degeneration of dopaminergic (DA) neurons in the substantia nigra and loss of DA transmission in the striatum, thus making cell transplantation an effective treatment strategy. Here, we develop a cellular therapy based on induced pluripotent stem cell (iPSC)-derived midbrain organoids. By transplanting midbrain organoid cells into the striatum region of a 6-OHDA-lesioned PD mouse model, we found that the transplanted cells survived and highly efficiently differentiated into DA neurons. Further, using a dopamine sensor, we observed that the differentiated human DA neurons could efficiently release dopamine and were integrated into the neural network of the PD mice. Moreover, starting from four weeks after transplantation, the motor function of the transplanted mice could be significantly improved. Therefore, cell therapy based on iPSC-derived midbrain organoids can be a potential strategy for the clinical treatment of PD.

Humanized brain organoids-on-chip integrated with sensors for screening neuronal activity and neurotoxicity

The complex structure and function of the human central nervous system that develops from the neural tube made in vitro modeling quite challenging until the discovery of brain organoids. Human-induced pluripotent stem cells-derived brain organoids offer recapitulation of the features of early human neurodevelopment in vitro, including the generation, proliferation, and differentiation into mature neurons and micro-macroglial cells, as well as the complex interactions among these diverse cell types of the developing brain. Recent advancements in brain organoids, microfluidic systems, real-time sensing technologies, and their cutting-edge integrated use provide excellent models and tools for emulation of fundamental neurodevelopmental processes, the pathology of neurological disorders, personalized transplantation therapy, and high-throughput neurotoxicity testing by bridging the gap between two-dimensional models and the complex three-dimensional environment in vivo. In this review, we summarize how bioengineering approaches are applied to mitigate the limitations of brain organoids for biomedical and clinical research. We further provide an extensive overview and future perspectives of the humanized brain organoids-on-chip platforms with integrated sensors toward brain organoid intelligence and biocomputing studies. Such approaches might pave the way for increasing approvable clinical applications by solving their current limitations.

Neuron-Astrocyte Interactions: A Human Perspective

This chapter explores the intricate interactions between neurons and astrocytes within the nervous system with a particular emphasis on studies conducted in human tissue or with human cells. We specifically explore how neuron-astrocyte interactions relate to processes of cellular development, morphology, migration, synapse formation, and metabolism. These findings enrich our understanding of basic neurobiology and how disruptions in these processes are relevant to human diseases.The study of human neuron-astrocyte interactions is made possible because of transformative in vitro advancements that have facilitated the generation and sustained culture of human neural cells. In addition, the rise of techniques like sequencing at single-cell resolution has enabled the exploration of numerous human cell atlases and their comparisons to other animal model systems. Thus, the innovations outlined in this chapter illuminate the convergence and divergence of neuron-astrocyte interactions across species. As technologies progress, continually more sophisticated in vitro systems will increasingly reflect in vivo environments and deepen our command of neuron-glial interactions in human biology.

From cells to insights: the power of human pluripotent stem cell-derived cortical interneurons in psychiatric disorder modeling

Psychiatric disorders, such as schizophrenia (SCZ) and autism spectrum disorders (ASD), represent a global health challenge with their poorly understood and complex etiologies. Cortical interneurons (cINs) are the primary inhibitory neurons in the cortex and their subtypes, especially those that are generated from the medial ganglionic emission (MGE) region, have been shown to play an important role in the pathogenesis of these psychiatric disorders. Recent advances in induced pluripotent stem cell (iPSC) technologies provide exciting opportunities to model and study these disorders using human iPSC-derived cINs. In this review, we present a comprehensive overview of various methods employed to generate MGE-type cINs from human iPSCs, which are mainly categorized into induction by signaling molecules vs. direct genetic manipulation. We discuss their advantages, limitations, and potential applications in psychiatric disorder modeling to aid researchers in choosing the appropriate methods based on their research goals. We also provide examples of how these methods have been applied to study the pathogenesis of psychiatric disorders. In addition, we discuss ongoing challenges and future directions in the field. Overall, iPSC-derived cINs provide a powerful tool to model the developmental pathogenesis of psychiatric disorders, thus aiding in uncovering disease mechanisms and potential therapeutic targets. This review article will provide valuable resources for researchers seeking to navigate the complexities of cIN generation methods and their applications in the study of psychiatric disorders.

In vitro modeling of the human dopaminergic system using spatially arranged ventral midbrain-striatum-cortex assembloids

Ventral midbrain dopaminergic neurons project to the striatum as well as the cortex and are involved in movement control and reward-related cognition. In Parkinson's disease, nigrostriatal midbrain dopaminergic neurons degenerate and cause typical Parkinson's disease motor-related impairments, while the dysfunction of mesocorticolimbic midbrain dopaminergic neurons is implicated in addiction and neuropsychiatric disorders. Study of the development and selective neurodegeneration of the human dopaminergic system, however, has been limited due to the lack of an appropriate model and access to human material. Here, we have developed a human in vitro model that recapitulates key aspects of dopaminergic innervation of the striatum and cortex. These spatially arranged ventral midbrain-striatum-cortical organoids (MISCOs) can be used to study dopaminergic neuron maturation, innervation and function with implications for cell therapy and addiction research. We detail protocols for growing ventral midbrain, striatal and cortical organoids and describe how they fuse in a linear manner when placed in custom embedding molds. We report the formation of functional long-range dopaminergic connections to striatal and cortical tissues in MISCOs, and show that injected, ventral midbrain-patterned progenitors can mature and innervate the tissue. Using these assembloids, we examine dopaminergic circuit perturbations and show that chronic cocaine treatment causes long-lasting morphological, functional and transcriptional changes that persist upon drug withdrawal. Thus, our method opens new avenues to investigate human dopaminergic cell transplantation and circuitry reconstruction as well as the effect of drugs on the human dopaminergic system.

Modular 3D printed platform for fluidically connected human brain organoid culture

Brain organoid technology has transformed both basic and applied biomedical research and paved the way for novel insights into developmental processes and disease states of the human brain. While the use of brain organoids has been rapidly growing in the past decade, the accompanying bioengineering and biofabrication solutions have remained scarce. As a result, most brain organoid protocols still rely on commercially available tools and culturing platforms that had previously been established for different purposes, thus entailing suboptimal culturing conditions and excessive use of plasticware. To address these issues, we developed a 3D printing pipeline for the fabrication of tailor-made culturing platforms for fluidically connected but spatially separated brain organoid array culture. This all-in-one platform allows all culturing steps-from cellular aggregation, spheroid growth, hydrogel embedding, and organoid maturation-to be performed in a single well plate without the need for organoid manipulation or transfer. Importantly, the approach relies on accessible materials and widely available 3D printing equipment. Furthermore, the developed design principles are modular and highly customizable. As such, we believe that the presented technology can be easily adapted by other research groups and fuel further development of culturing tools and platforms for brain organoids and other 3D cellular systems.

Single-cell transcriptomics identifies perturbed molecular pathways in midbrain organoids using α-synuclein triplication Parkinson's disease patient-derived iPSCs

Three-dimensional (3D) brain organoids provide a platform to study brain development, cellular coordination, and disease using human tissue. Here, we generate midbrain dopaminergic (mDA) organoids from induced pluripotent stem cells (iPSC) from healthy and Parkinson's Disease (PD) donors and assess them as a human PD model using single-cell RNAseq. We characterize cell types in our organoid cultures and analyze our model's Dopamine (DA) neurons using cytotoxic and genetic stressors. Our study provides the first in-depth, single-cell analysis of SNCA triplication and shows evidence for molecular dysfunction in oxidative phosphorylation, translation, and ER protein-folding in DA neurons. We perform an in-silico identification of rotenone-sensitive DA neurons and characterization of corresponding transcriptomic profiles associated with synaptic signalling and cholesterol biosynthesis. Finally, we show a novel chimera organoid model from healthy and PD iPSCs allowing the study of DA neurons from different individuals within the same tissue.

Application of colloidal photonic crystals in study of organoids

As alternative disease models, other than 2D cell lines and patient-derived xenografts, organoids have preferable in vivo physiological relevance. However, both endogenous and exogenous limitations impede the development and clinical translation of these organoids. Fortunately, colloidal photonic crystals (PCs), which benefit from favorable biocompatibility, brilliant optical manipulation, and facile chemical decoration, have been applied to the engineering of organoids and have achieved the desirable recapitulation of the ECM niche, well-defined geometrical onsets for initial culture, in situ multiphysiological parameter monitoring, single-cell biomechanical sensing, and high-throughput drug screening with versatile functional readouts. Herein, we review the latest progress in engineering organoids fabricated from colloidal PCs and provide inputs for future research.

Organ-Chips Enhance the Maturation of Human iPSC-Derived Dopamine Neurons

While cells in the human body function in an environment where the blood supply constantly delivers nutrients and removes waste, cells in conventional tissue culture well platforms are grown with a static pool of media above them and often lack maturity, limiting their utility to study cell biology in health and disease. In contrast, organ-chip microfluidic systems allow the growth of cells under constant flow, more akin to the in vivo situation. Here, we differentiated human induced pluripotent stem cells into dopamine neurons and assessed cellular properties in conventional multi-well cultures and organ-chips. We show that organ-chip cultures, compared to multi-well cultures, provide an overall greater proportion and homogeneity of dopaminergic neurons as well as increased levels of maturation markers. These organ-chips are an ideal platform to study mature dopamine neurons to better understand their biology in health and ultimately in neurological disorders.

Midbrain organoids-development and applications in Parkinson's disease

Human brain development is spatially and temporally complex. Insufficient access to human brain tissue and inadequacy of animal models has limited the study of brain development and neurodegenerative diseases. Recent advancements of brain organoid technology have created novel opportunities to model human-specific neurodevelopment and brain diseases. In this review, we discuss the use of brain organoids to model the midbrain and Parkinson's disease. We critically evaluate the extent of recapitulation of PD pathology by organoids and discuss areas of future development that may lead to the model to become a next-generation, personalized therapeutic strategy for PD and beyond.

Application of Human Brain Organoids-Opportunities and Challenges in Modeling Human Brain Development and Neurodevelopmental Diseases

Brain organoids are three-dimensional (3D) structures derived from human pluripotent stem cells (hPSCs) that reflect early brain organization. These organoids contain different cell types, including neurons and glia, similar to those found in the human brain. Human brain organoids provide unique opportunities to model features of human brain development that are not well-reflected in animal models. Compared with traditional cell cultures and animal models, brain organoids offer a more accurate representation of human brain development and function, rendering them suitable models for neurodevelopmental diseases. In particular, brain organoids derived from patients' cells have enabled researchers to study diseases at different stages and gain a better understanding of disease mechanisms. Multi-brain regional assembloids allow for the investigation of interactions between distinct brain regions while achieving a higher level of consistency in molecular and functional characterization. Although organoids possess promising features, their usefulness is limited by several unresolved constraints, including cellular stress, hypoxia, necrosis, a lack of high-fidelity cell types, limited maturation, and circuit formation. In this review, we discuss studies to overcome the natural limitations of brain organoids, emphasizing the importance of combinations of all neural cell types, such as glia (astrocyte, oligodendrocytes, and microglia) and vascular cells. Additionally, considering the similarity of organoids to the developing brain, regionally patterned brain organoid-derived neural stem cells (NSCs) could serve as a scalable source for cell replacement therapy. We highlight the potential application of brain organoid-derived cells in disease cell therapy within this field.

Differentiation of astrocytes with characteristics of ventral midbrain from human embryonic stem cells

Molecular and functional diversity among region-specific astrocytes is of great interest in basic neuroscience and the study of neurological diseases. In this study, we present the generation and characterization of astrocytes from human embryonic stem cells with the characteristics of the ventral midbrain (VM). Fine modulation of WNT and SHH signaling during neural differentiation induced neural precursor cells (NPCs) with high expression of EN1 and NKX6.1, but less expression of FOXA2. Overexpression of nuclear factor IB in NPCs induced astrocytes, thereby maintaining the expression of region-specific genes acquired in the NPC stage. When cocultured with dopaminergic (DA) precursors or DA neurons, astrocytes with VM characteristics (VM-iASTs) promoted the differentiation and survival of DA neurons better than those that were not regionally specified. Transcriptomic analysis showed that VM-iASTs were more closely related to human primary midbrain astrocytes than to cortical astrocytes, and revealed the upregulation of WNT1 and WNT5A, which supports their VM identity and explains their superior activity in DA neurons. Taken together, we hope that VM-iASTs can serve to improve ongoing DA precursor transplantation for Parkinson's disease, and that their transcriptomic data provide a valuable resource for investigating regional diversity in human astrocyte populations.

A defined method for differentiating human iPSCs into midbrain dopaminergic progenitors that safely restore motor deficits in Parkinson's disease

Background

Parkinson's disease (PD) is a progressive neurodegenerative condition that primarily affects motor functions; it is caused by the loss of midbrain dopaminergic (mDA) neurons. The therapeutic effects of transplanting human-induced pluripotent stem cell (iPSC)-derived mDA neural progenitor cells in animal PD models are known and are being evaluated in an ongoing clinical trial. However, However, improvements in the safety and efficiency of differentiation-inducing methods are crucial for providing a larger scale of cell therapy studies. This study aimed to investigate the usefulness of dopaminergic progenitor cells derived from human iPSCs by our previously reported method, which promotes differentiation and neuronal maturation by treating iPSCs with three inhibitors at the start of induction.

Methods

Healthy subject-derived iPS cells were induced into mDA progenitor cells by the CTraS-mediated method we previously reported, and their proprieties and dopaminergic differentiation efficiency were examined in vitro. Then, the induced mDA progenitors were transplanted into 6-hydroxydopamine-lesioned PD model mice, and their efficacy in improving motor function, cell viability, and differentiation ability in vivo was evaluated for 16 weeks.

Results

Approximately ≥80% of cells induced by this method without sorting expressed mDA progenitor markers and differentiated primarily into A9 dopaminergic neurons in vitro. After transplantation in 6-hydroxydopamine-lesioned PD model mice, more than 90% of the engrafted cells differentiated into the lineage of mDA neurons, and approximately 15% developed into mature mDA neurons without tumour formation. The grafted PD model mice also demonstrated significantly improved motor functions.

Conclusion

This study suggests that the differentiation protocol for the preparation of mDA progenitors is a promising option for cell therapy in patients with PD.

Functional bioengineered tissue models of neurodegenerative diseases

Aging-associated neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases remain poorly understood and no disease-modifying treatments exist despite decades of investigation. Predominant in vitro (e.g., 2D cell culture, organoids) and in vivo (e.g., mouse) models of these diseases are insufficient mimics of human brain tissue structure and function and of human neurodegenerative pathobiology, and have thus contributed to this collective translational failure. This has been a longstanding challenge in the field, and new strategies are required to address both fundamental and translational needs. Bioengineered tissue culture models constitute a class of promising alternatives, as they can overcome the low cell density, poor nutrient exchange, and long term culturability limitations of existing in vitro models. Further, they can reconstruct the structural, mechanical, and biochemical cues of native brain tissue, providing a better mimic of human brain tissues for in vitro pathobiological investigation and drug development. We discuss bioengineering techniques for the generation of these neurodegenerative tissue models, including biomaterials-, organoid-, and microfluidics-based approaches, and design considerations for their construction. To aid the development of the next generation of functional neurodegenerative disease models, we discuss approaches to incorporate greater cellular diversity and simulate aging processes within bioengineered brain tissues.

Neural lineage differentiation of human pluripotent stem cells: Advances in disease modeling

Brain diseases affect 1 in 6 people worldwide. These diseases range from acute neurological conditions such as stroke to chronic neurodegenerative disorders such as Alzheimer’s disease. Recent advancements in tissue-engineered brain disease models have overcome many of the different shortcomings associated with the various animal models, tissue culture models, and epidemiologic patient data that are commonly used to study brain disease. One innovative method by which to model human neurological disease is via the directed differentiation of human pluripotent stem cells (hPSCs) to neural lineages including neurons, astrocytes, and oligodendrocytes. Three-dimensional models such as brain organoids have also been derived from hPSCs, offering more physiological relevance due to their incorporation of various cell types. As such, brain organoids can better model the pathophysiology of neural diseases observed in patients. In this review, we will emphasize recent developments in hPSC-based tissue culture models of neurological disorders and how they are being used to create neural disease models.

Glucocerebrosidase is imported into mitochondria and preserves complex I integrity and energy metabolism

Mutations in GBA1, the gene encoding the lysosomal enzyme β-glucocerebrosidase (GCase), which cause Gaucher's disease, are the most frequent genetic risk factor for Parkinson's disease (PD). Here, we employ global proteomic and single-cell genomic approaches in stable cell lines as well as induced pluripotent stem cell (iPSC)-derived neurons and midbrain organoids to dissect the mechanisms underlying GCase-related neurodegeneration. We demonstrate that GCase can be imported from the cytosol into the mitochondria via recognition of internal mitochondrial targeting sequence-like signals. In mitochondria, GCase promotes the maintenance of mitochondrial complex I (CI) integrity and function. Furthermore, GCase interacts with the mitochondrial quality control proteins HSP60 and LONP1. Disease-associated mutations impair CI stability and function and enhance the interaction with the mitochondrial quality control machinery. These findings reveal a mitochondrial role of GCase and suggest that defective CI activity and energy metabolism may drive the pathogenesis of GCase-linked neurodegeneration.

Unraveling the Complex Interplay between Alpha-Synuclein and Epigenetic Modification

Alpha-synuclein (αS) is a small, presynaptic neuronal protein encoded by the SNCA gene. Point mutations and gene multiplication of SNCA cause rare familial forms of Parkinson’s disease (PD). Misfolded αS is cytotoxic and is a component of Lewy bodies, which are a pathological hallmark of PD. Because SNCA multiplication is sufficient to cause full-blown PD, gene dosage likely has a strong impact on pathogenesis. In sporadic PD, increased SNCA expression resulting from a minor genetic background and various environmental factors may contribute to pathogenesis in a complementary manner. With respect to genetic background, several risk loci neighboring the SNCA gene have been identified, and epigenetic alterations, such as CpG methylation and regulatory histone marks, are considered important factors. These alterations synergistically upregulate αS expression and some post-translational modifications of αS facilitate its translocation to the nucleus. Nuclear αS interacts with DNA, histones, and their modifiers to alter epigenetic status; thereby, influencing the stability of neuronal function. Epigenetic changes do not affect the gene itself but can provide an appropriate transcriptional response for neuronal survival through DNA methylation or histone modifications. As a new approach, publicly available RNA sequencing datasets from human midbrain-like organoids may be used to compare transcriptional responses through epigenetic alterations. This informatic approach combined with the vast amount of transcriptomics data will lead to the discovery of novel pathways for the development of disease-modifying therapies for PD.

Recent Development of Brain Organoids for Biomedical Application

Over the years, scientists have studied the behavior and anatomy of many animals to understand the own species. However, despite the continuous efforts, it is often difficult to know for certain how the brain works due to the differences between the brains of animals and the human brain. While the use of animal models for research continues, the origin of human cognition and neurological disorders needs further elucidation. To that end, in vitro organoids that exhibit in vivo characteristics of the human brain have been recently developed. These brain-like organoids enable researchers to dive deeper into understanding the human brain, its neurological structures, and the causes of neurological pathologies. This paper reviews the recent developments in the regeneration of brain-like organoids using Matrigel and other alternatives. Further, gel-free methods that may enhance the regeneration process of organoids are discussed. Finally, the vascularized brain organoid growth and development in both in vitro and in vivo conditions are detailed.

Massively parallel characterization of psychiatric disorder-associated and cell-type-specific regulatory elements in the developing human cortex

Nucleotide changes in gene regulatory elements are important determinants of neuronal development and disease. Using massively parallel reporter assays in primary human cells from mid-gestation cortex and cerebral organoids, we interrogated the cis-regulatory activity of 102,767 sequences, including differentially accessible cell-type specific regions in the developing cortex and single-nucleotide variants associated with psychiatric disorders. In primary cells, we identified 46,802 active enhancer sequences and 164 disorder-associated variants that significantly alter enhancer activity. Activity was comparable in organoids and primary cells, suggesting that organoids provide an adequate model for the developing cortex. Using deep learning, we decoded the sequence basis and upstream regulators of enhancer activity. This work establishes a comprehensive catalog of functional gene regulatory elements and variants in human neuronal development.

Direct Cell Reprogramming and Phenotypic Conversion: An Analysis of Experimental Attempts to Transform Astrocytes into Neurons in Adult Animals

Central nervous system (CNS) repair after injury or disease remains an unresolved problem in neurobiology research and an unmet medical need. Directly reprogramming or converting astrocytes to neurons (AtN) in adult animals has been investigated as a potential strategy to facilitate brain and spinal cord recovery and advance fundamental biology. Conceptually, AtN strategies rely on forced expression or repression of lineage-specific transcription factors to make endogenous astrocytes become "induced neurons" (iNs), presumably without re-entering any pluripotent or multipotent states. The AtN-derived cells have been reported to manifest certain neuronal functions in vivo. However, this approach has raised many new questions and alternative explanations regarding the biological features of the end products (e.g., iNs versus neuron-like cells, neural functional changes, etc.), developmental biology underpinnings, and neurobiological essentials. For this paper per se, we proposed to draw an unconventional distinction between direct cell conversion and direct cell reprogramming, relative to somatic nuclear transfer, based on the experimental methods utilized to initiate the transformation process, aiming to promote a more in-depth mechanistic exploration. Moreover, we have summarized the current tactics employed for AtN induction, comparisons between the bench endeavors concerning outcome tangibility, and discussion of the issues of published AtN protocols. Lastly, the urgency to clearly define/devise the theoretical frameworks, cell biological bases, and bench specifics to experimentally validate primary data of AtN studies was highlighted.

Spatio-temporal dynamics enhance cellular diversity, neuronal function and further maturation of human cerebral organoids

The bioengineerined and whole matured human brain organoids stand as highly valuable three-dimensional in vitro brain-mimetic models to recapitulate in vivo brain development, neurodevelopmental and neurodegenerative diseases. Various instructive signals affecting multiple biological processes including morphogenesis, developmental stages, cell fate transitions, cell migration, stem cell function and immune responses have been employed for generation of physiologically functional cerebral organoids. However, the current approaches for maturation require improvement for highly harvestable and functional cerebral organoids with reduced batch-to-batch variabilities. Here, we demonstrate two different engineering approaches, the rotating cell culture system (RCCS) microgravity bioreactor and a newly designed microfluidic platform (µ-platform) to improve harvestability, reproducibility and the survival of high-quality cerebral organoids and compare with those of traditional spinner and shaker systems. RCCS and µ-platform organoids have reached ideal sizes, approximately 95% harvestability, prolonged culture time with Ki-67 + /CD31 + /β-catenin+ proliferative, adhesive and endothelial-like cells and exhibited enriched cellular diversity (abundant neural/glial/ endothelial cell population), structural brain morphogenesis, further functional neuronal identities (glutamate secreting glutamatergic, GABAergic and hippocampal neurons) and synaptogenesis (presynaptic-postsynaptic interaction) during whole human brain development. Both organoids expressed CD11b + /IBA1 + microglia and MBP + /OLIG2 + oligodendrocytes at high levels as of day 60. RCCS and µ-platform organoids showing high levels of physiological fidelity a high level of physiological fidelity can serve as functional preclinical models to test new therapeutic regimens for neurological diseases and benefit from multiplexing.

From 2D to 3D: Development of Monolayer Dopaminergic Neuronal and Midbrain Organoid Cultures for Parkinson's Disease Modeling and Regenerative Therapy

Parkinson's Disease (PD) is a prevalent neurodegenerative disorder that is characterized pathologically by the loss of A9-specific dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) of the midbrain. Despite intensive research, the etiology of PD is currently unresolved, and the disease remains incurable. This, in part, is due to the lack of an experimental disease model that could faithfully recapitulate the features of human PD. However, the recent advent of induced pluripotent stem cell (iPSC) technology has allowed PD models to be created from patient-derived cells. Indeed, DA neurons from PD patients are now routinely established in many laboratories as monolayers as well as 3D organoid cultures that serve as useful toolboxes for understanding the mechanism underlying PD and also for drug discovery. At the same time, the iPSC technology also provides unprecedented opportunity for autologous cell-based therapy for the PD patient to be performed using the patient's own cells as starting materials. In this review, we provide an update on the molecular processes underpinning the development and differentiation of human pluripotent stem cells (PSCs) into midbrain DA neurons in both 2D and 3D cultures, as well as the latest advancements in using these cells for drug discovery and regenerative medicine. For the novice entering the field, the cornucopia of differentiation protocols reported for the generation of midbrain DA neurons may seem daunting. Here, we have distilled the essence of the different approaches and summarized the main factors driving DA neuronal differentiation, with the view to provide a useful guide to newcomers who are interested in developing iPSC-based models of PD.

Enhanced peroxidase-like activity of 2(3), 9(10), 16(17), 23(24)-octamethoxyphthalocyanine modified CoFe LDH for a sensor array for reducing substances with catechol structure

Improving the catalytic activity of artificial nanozymes to realize the real-time detection of small molecules becomes an important task. Herein, a highly active nanozyme, 2(3), 9(10), 16(17), 23(24)-octamethoxyphthalocyanine (Pc(OH)₈) modified CoFe LDH microspheres (Pc(OH)₈-CoFe LDH) have been prepared by the two-step hydrothermal method. The 3,3',5,5'-tetramylbenzidine (TMB), a chromogenic substrate, was fast oxidized into blue oxTMB by H₂O₂ in the presence of Pc(OH)₈-CoFe LDH, indicating that Pc(OH)₈-CoFe LDH possesses high peroxidase-like activity rather than pure CoFe LDH. The enhancement peroxidase-like activity of the Pc(OH)₈-CoFe LDH is ascribed to the synergistic action between Pc(OH)₈ and CoFe LDH. Experimental results of radical scavenger and fluorescence probe verify that superoxide radical (•O₂^-) plays an important role during the catalytic reaction. Interestingly, the absorption intensity of reaction system has been enhanced largely, due to adding of the reducing substances containing catechol structure. Based on this, the three reducing substances (dopamine, procyanidin B2, catechins) containing catechol structure were distinguished from other reducing substances without catechol structure. Thus, a colorimetric array has been constructed using reaction time as the sensing element to realize the sensitive and selective recognition of catechol structures at a certain concentration.

Single-cell transcriptional and functional analysis of dopaminergic neurons in organoid-like cultures derived from human fetal midbrain

Significant efforts are ongoing to develop refined differentiation protocols to generate midbrain dopamine (DA) neurons from pluripotent stem cells for application in disease modeling, diagnostics, drug screening and cell-based therapies for Parkinson's disease. An increased understanding of the timing and molecular mechanisms that promote the generation of distinct subtypes of human midbrain DA during development will be essential for guiding future efforts to generate molecularly defined and subtype-specific DA neurons from pluripotent stem cells. Here, we use droplet-based single-cell RNA sequencing to transcriptionally profile the developing human ventral midbrain (VM) when the DA neurons are generated (6-11 weeks post-conception) and their subsequent differentiation into functional mature DA neurons in primary fetal 3D organoid-like cultures. This approach reveals that 3D cultures are superior to monolayer conditions for their ability to generate and maintain mature DA neurons; hence, they have the potential to be used for studying human VM development. These results provide a unique transcriptional profile of the developing human fetal VM and functionally mature human DA neurons that can be used to guide stem cell-based therapies and disease modeling approaches in Parkinson's disease.

Human cerebral organoids - a new tool for clinical neurology research

The current understanding of neurological diseases is derived mostly from direct analysis of patients and from animal models of disease. However, most patient studies do not capture the earliest stages of disease development and offer limited opportunities for experimental intervention, so rarely yield complete mechanistic insights. The use of animal models relies on evolutionary conservation of pathways involved in disease and is limited by an inability to recreate human-specific processes. In vitro models that are derived from human pluripotent stem cells cultured in 3D have emerged as a new model system that could bridge the gap between patient studies and animal models. In this Review, we summarize how such organoid models can complement classical approaches to accelerate neurological research. We describe our current understanding of neurodevelopment and how this process differs between humans and other animals, making human-derived models of disease essential. We discuss different methodologies for producing organoids and how organoids can be and have been used to model neurological disorders, including microcephaly, Zika virus infection, Alzheimer disease and other neurodegenerative disorders, and neurodevelopmental diseases, such as Timothy syndrome, Angelman syndrome and tuberous sclerosis. We also discuss the current limitations of organoid models and outline how organoids can be used to revolutionize research into the human brain and neurological diseases.

In this Review, Eichmüller and Knoblich discuss how human brain organoids can recapitulate the unique processes that occur in human brain development and how they can complement classical approaches to revolutionize research into neurological diseases.

Development of the human brain involves unique processes that are relevant to neurological disease but cannot be studied in animal models, so alternative model systems are required.

Organoids are 3D human cell culture models that originate from pluripotent stem cells and recapitulate the hallmarks of human neurodevelopment, enabling studies of human brain development in vitro.

Specific mutations can be introduced into organoids to study their effects on neurodevelopment; combined with high-throughput screening methods, this approach can determine the disease relevance of mutations in human tissue.

To study specific diseases, brain organoids can be generated from induced pluripotent stem cells from individual patients, thereby preserving the specific genetic background of the individual and generating an insightful model.

Through recapitulation of previously inaccessible periods of human brain development, brain organoids have enabled identification of novel mechanisms that underlie neurodevelopmental, neurodegenerative and infectious diseases.

Combining organoids, patient research and animal models enables us to take full advantage of each of these systems and will provide unprecedented insights into neurodevelopment and neurological diseases.

Silk scaffolding drives self-assembly of functional and mature human brain organoids

Human pluripotent stem cells (hPSCs) are intrinsically able to self-organize into cerebral organoids that mimic features of developing human brain tissue. These three-dimensional structures provide a unique opportunity to generate cytoarchitecture and cell-cell interactions reminiscent of human brain complexity in a dish. However, current in vitro brain organoid methodologies often result in intra-organoid variability, limiting their use in recapitulating later developmental stages as well as in disease modeling and drug discovery. In addition, cell stress and hypoxia resulting from long-term culture lead to incomplete maturation and cell death within the inner core. Here, we used a recombinant silk microfiber network as a scaffold to drive hPSCs to self-arrange into engineered cerebral organoids. Silk scaffolding promoted neuroectoderm formation and reduced heterogeneity of cellular organization within individual organoids. Bulk and single cell transcriptomics confirmed that silk cerebral organoids display more homogeneous and functionally mature neuronal properties than organoids grown in the absence of silk scaffold. Furthermore, oxygen sensing analysis showed that silk scaffolds create more favorable growth and differentiation conditions by facilitating the delivery of oxygen and nutrients. The silk scaffolding strategy appears to reduce intra-organoid variability and enhances self-organization into functionally mature human brain organoids.

Cerebral Organoids as an Experimental Platform for Human Neurogenomics

The cerebral cortex forms early in development according to a series of heritable neurodevelopmental instructions. Despite deep evolutionary conservation of the cerebral cortex and its foundational six-layered architecture, significant variations in cortical size and folding can be found across mammals, including a disproportionate expansion of the prefrontal cortex in humans. Yet our mechanistic understanding of neurodevelopmental processes is derived overwhelmingly from rodent models, which fail to capture many human-enriched features of cortical development. With the advent of pluripotent stem cells and technologies for differentiating three-dimensional cultures of neural tissue in vitro, cerebral organoids have emerged as an experimental platform that recapitulates several hallmarks of human brain development. In this review, we discuss the merits and limitations of cerebral organoids as experimental models of the developing human brain. We highlight innovations in technology development that seek to increase its fidelity to brain development in vivo and discuss recent efforts to use cerebral organoids to study regeneration and brain evolution as well as to develop neurological and neuropsychiatric disease models.

Single-cell transcriptomics reveals the cell fate transitions of human dopaminergic progenitors derived from hESCs

Background

Midbrain dopaminergic (DA) progenitors derived from human pluripotent stem cells are considered to be a promising treatment for Parkinson’s disease (PD). However, the differentiation process produces undesired cell types, which influence the in vivo evaluation of DA cells. In this paper, we analyze the cell fate choice during differentiation and provide valuable information on cell preparation.

Methods

Human embryonic stem cells were differentiated into DA progenitors. We applied single-cell RNA sequencing (scRNA-seq) of the differentiation cells at different time points and investigated the gene expression profiles. Based on the differentially expressed genes between DA and non-DA cells, we investigated the impact of LGI1 (DA enriched) overexpression on DA differentiation and the enrichment effect of CD99 (non-DA enriched) sorting.

Results

Transcriptome analyses revealed the DA differentiation trajectory as well as non-DA populations and three key lineage branch points. Using genetic gain- and loss-of-function approaches, we found that overexpression of LGI1, which is specific to EN1⁺ early DA progenitors, can promote the generation of TH⁺ neurons. We also found that choroid plexus epithelial cells and DA progenitors are major components of the final product (day 25), and CD99 was a specific surface marker of choroid plexus epithelial cells. Sorting of CD99⁻ cells eliminated major contaminant cells and improved the purity of DA progenitors.

Conclusions

Our study provides the single-cell transcriptional landscape of in vitro DA differentiation, which can guide future improvements in DA preparation and quality control for PD cell therapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03104-7.

Human Brain-Based Models Provide a Powerful Tool for the Advancement of Parkinson’s Disease Research and Therapeutic Development

Parkinson’s disease (PD) is the second most common neurodegenerative disease and affects approximately 2–3% of the population over the age of 65. PD is characterised by the loss of dopaminergic neurons from the substantia nigra, leading to debilitating motor symptoms including bradykinesia, tremor, rigidity, and postural instability. PD also results in a host of non-motor symptoms such as cognitive decline, sleep disturbances and depression. Although existing therapies can successfully manage some motor symptoms for several years, there is still no means to halt progression of this severely debilitating disorder. Animal models used to replicate aspects of PD have contributed greatly to our current understanding but do not fully replicate pathological mechanisms as they occur in patients. Because of this, there is now great interest in the use of human brain-based models to help further our understanding of disease processes. Human brain-based models include those derived from embryonic stem cells, patient-derived induced neurons, induced pluripotent stem cells and brain organoids, as well as post-mortem tissue. These models facilitate in vitro analysis of disease mechanisms and it is hoped they will help bridge the existing gap between bench and bedside. This review will discuss the various human brain-based models utilised in PD research today and highlight some of the key breakthroughs they have facilitated. Furthermore, the potential caveats associated with the use of human brain-based models will be detailed.

Human organotypic brain model as a tool to study chemical-induced dopaminergic neuronal toxicity

Oxidative stress is caused by an imbalance between the generation and detoxification of reactive oxygen and nitrogen species (ROS/RNS). This imbalance plays an important role in brain aging and age-related neurodegenerative diseases. In the context of Parkinson’s disease (PD), the sensitivity of dopaminergic neurons in the substantia nigra pars compacta to oxidative stress is considered a key factor of PD pathogenesis. Here we study the effect of different oxidative stress-inducing compounds (6-OHDA, MPTP or MPP⁺) on the population of dopaminergic neurons in an iPSC-derived human brain 3D model (aka BrainSpheres). Treatment with 6-OHDA, MPTP or MPP⁺ at 4 weeks of differentiation disrupted the dopaminergic neuronal phenotype in BrainSpheres at (50, 5000, 1000 μM respectively). 6-OHDA increased ROS production and decreased mitochondrial function most efficiently. It further induced the greatest changes in gene expression and metabolites related to oxidative stress and mitochondrial dysfunction. Co-culturing BrainSpheres with an endothelial barrier using a transwell system allowed the assessment of differential penetration capacities of the tested compounds and the damage they caused in the dopaminergic neurons within the BrainSpheres In conclusion, treatment with compounds known to induce PD-like phenotypes in vivo caused molecular deficits and loss of dopaminergic neurons in the BrainSphere model. This approach therefore recapitulates common animal models of neurodegenerative processes in PD at similarly high doses. The relevance as tool for drug discovery is discussed.