Advancements in brain organoid models for neurodegenerative disease research
Article information
Abstract
Neurodegenerative diseases (NDs) such as Parkinson’s disease (PD) and Alzheimer’s disease (AD) are progressive disorders characterized by complex, human-specific pathology that poses challenges to drug discovery efforts. Traditional models, including two-dimensional cell cultures and animal models, often fall short in replicating the intricate cellular interactions observed in human neurodegeneration. This review explores the potential of brain organoid technology to address these limitations and offer a model more relevant to humans. Recent advancements in induced pluripotent stem cell (iPSC) technology have enabled the generation of patient-derived brain organoids that differentiate into various neural cell types within 3-dimensional structures. These iPSC-derived brain organoids establish a physiologically relevant microenvironment that mimics human brain architecture and cellular diversity. This review synthesizes studies on the application of brain organoids in modeling PD and AD pathology, including approaches to improve model fidelity. Brain organoids replicate disease-specific features, including dopaminergic neuron degeneration in PD and amyloid plaque formation in AD, offering valuable insights into disease mechanisms and potential therapeutic targets. However, challenges remain, including incomplete maturation, batch variability, and the absence of vascularization and complete cortical layering. Bioengineering approaches, including Clustered Regularly Interspaced Short Palindromic Repeats-based gene editing and organ-on-a-chip technologies, are being investigated to overcome these obstacles. Brain organoid technology presents a transformative platform for the study of NDs, facilitating detailed research into disease mechanisms and testing of therapeutics. Overcoming existing challenges is crucial for maximizing the translational value of brain organoids, advancing personalized medicine, and supporting the development of effective therapies for NDs.
Introduction
Neurodegenerative diseases (NDs) encompass a range of debilitating disorders characterized by the progressive degeneration of neurons, adversely impacting motor control, cognition, and ultimately, life expectancy [1–3]. Conditions such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease, and amyotrophic lateral sclerosis primarily affect older individuals and are becoming increasingly prevalent as life expectancy increases [4,5]. Although genetic mutations play a key role in these diseases, environmental factors and the cellular aging process also have a substantial impact [6,7]. Research has revealed several molecular pathways associated with the pathogenesis of NDs, including oxidative stress, mitochondrial dysfunction, and protein aggregation. However, these pathways only partially account for the complexity of these diseases [8,9].
Research into NDs is hindered by the limitations of traditional models. Two-dimensional (2D) cell cultures lack the spatial organization and cellular complexity found in brain tissue, and while animal models are valuable for whole-organism studies, they cannot replicate specific neuronal interactions and responses seen in humans [10,11]. This deficiency in modeling capabilities has constrained the translational potential of preclinical findings, highlighting the need for more sophisticated and physiologically relevant models (Fig. 1) [4,12].
Comparative overview of 2D iPSC cultures, animal models, and 3D iPSC-derived brain organoids in neurodegenerative disease research. 2D, 2-dimensional; 3D, 3-dimensional; iPSC, induced pluripotent stem cell.
The development of induced pluripotent stem cells (iPSCs) has provided a transformative approach to studying human neurological disorders [1,13]. Reprogrammed from somatic cells using Yamanaka factors (OCT4, SOX2, KLF4, and c-MYC), iPSCs can differentiate into various cell types across the 3 germ layers, enabling the generation of neural cells and tissues [1,14]. iPSCs have become instrumental in regenerative medicine, disease modeling, and drug testing [15,16].
Recent advances in iPSC-derived 3-dimensional (3D) brain organoids represent a transformative shift in the modeling of NDs [10,17]. Unlike 2D cultures, brain organoids offer a 3D microenvironment that supports cellular self-organization, fostering realistic neural architecture and cellular interactions [18,19]. By differentiating iPSCs into organoids specific to brain regions, researchers can investigate the molecular and cellular mechanisms of NDs in a system that more closely resembles human biology. This approach enables an unprecedented exploration of disease pathology and progression [20,21].
Generation and differentiation of brain organoids
Ethics statement: This study was a literature review of previously published studies and was therefore exempt from institutional review board approval.
Brain organoids are derived from iPSCs using specialized differentiation protocols that emulate key aspects of brain development [1,10]. These protocols generally involve culturing iPSCs in a 3D environment, which encourages the cells to self-organize into structures resembling early brain tissue [8,15]. Fig. 2 presents an overview of the process for generating iPSC-derived organoids for ND modeling. Recent advancements have refined these differentiation techniques, enabling the production of organoids that mimic specific brain regions, such as the cortex, midbrain, and hippocampus [19,20]. Researchers have optimized various parameters, including the timing of differentiation, the selection of growth factors, and the composition of the extracellular matrix, all of which meaningfully impact organoid development [11,22]. Improved protocols are under development to increase the reproducibility and scalability of organoids, positioning them as viable tools for large-scale research studies and clinical applications [16,18].
Overview of induced pluripotent stem cell-derived organoids for neurodegenerative disease modeling.
Schematic representation of the process used to generate brain organoids from human iPSCs and their subsequent application in ND modeling. The figure outlines key stages in organoid differentiation, including the 3D culture environment that promotes self-organization; the formation of region-specific structures such as cortical, midbrain, and hippocampal areas; and the addition of growth factors to increase cellular diversity. Region-specific brain organoids facilitate the study of disease-specific pathologies, including the degeneration of dopaminergic neurons in PD and the formation of amyloid plaques in AD. More advanced techniques, such as the creation of assembloids, enable the modeling of neuronal interactions between brain regions and the complex network activities relevant to neurodegenerative processes.
ND modeling with brain organoids
One of the most promising applications of brain organoids is the modeling of NDs, particularly PD and AD [7,11,23]. These organoids effectively replicate key pathological features, such as the loss of dopaminergic neurons characteristic of PD and the formation of amyloid-β plaques and neurofibrillary tangles observed in AD [5,24]. Patient-derived iPSCs facilitate the integration of disease-specific mutations, providing opportunities for detailed exploration of their role in disease progression [4,25]. Brain organoids also provide valuable platforms for examining the interplay between genetic predispositions and environmental factors in the onset of neurodegeneration [16,26]. This human-specific context enables more precise clarification of disease mechanisms that are challenging to replicate in animal models or traditional 2D cultures [17,18].
Recent studies have explored multi-lineage organoid models in which neural cells are co-cultured with glial cells, enhancing our understanding of the complex cellular interactions implicated in NDs. Researchers are increasingly integrating organoids with Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-based gene editing tools to introduce specific genetic mutations associated with PD, AD, and other NDs. This integration enables precise exploration of the molecular underpinnings of these diseases [12,27]. While CRISPR facilitates detailed investigation of disease mechanisms by replicating or correcting specific genetic variants, it does have certain limitations [28]. The technology allows for precise gene editing in organoids, but potential off-target effects and variability in editing efficiency hinder its utility in ND research [29]. These issues, and the associated potential for unintended genetic modifications, pose challenges for ensuring accuracy and reproducibility in ND modeling [30]. Moreover, further optimization of the CRISPR delivery system is essential to improve editing efficiency in organoid culture [31]. Methodologies using brain organoids in high-throughput drug screening are advancing the identification of compounds targeting neurodegenerative pathways, emphasizing their role in early-stage drug discovery [10,32]. The development of long-term culture techniques has further allowed researchers to model late-onset NDs by aging organoids, providing insights into how cellular and molecular changes over time contribute to neurodegeneration [15,22]. Such advancements underscore the translational potential of brain organoids, not only in disease modeling but also in drug discovery and personalized medicine.
Technological advancements impacting brain organoids
The advancement of brain organoids has greatly benefited from innovations in bioengineering [27,32,33]. Microfluidic systems offer controlled environments for nutrient and oxygen delivery, thereby improving the physiological relevance of these models and addressing limitations associated with nutrient diffusion and cellular growth [27,34]. Synthetic scaffolds and 3D bioprinting techniques provide structural support, which encourages more complex tissue organization and extends survival [8,11].
Vascularization remains one of the most critical advancements in brain organoids [35]. Bioengineered vascular scaffolds are designed to mimic the structure of blood vessels, enhancing oxygen and nutrient delivery to reduce necrosis and promote tissue maturation within organoids [36]. Advances in co-culture techniques, which involve culturing brain organoids with endothelial cells, have shown promise in forming rudimentary vascular networks within the organoid environment [37]. Emerging strategies include the integration of endothelial cells with bioengineered scaffolds and the use of dynamic microfluidic systems to replicate the vascular fluidic conditions found in vivo [36]. Additionally, innovations in microfluidic systems that enable dynamic perfusion of nutrients and oxygen are being applied to better simulate the fluidic conditions of the human brain, further advancing vascularization efforts [38]. These advancements address a critical limitation of brain organoids by enabling long-term culture and creating conditions that more closely resemble the in vivo environment [39]. Although current methods are still in early stages, they represent essential steps toward the creation of fully functional, vascularized brain organoids [40].
Applications in drug screening and therapeutic development
Brain organoids offer considerable potential for drug screening and the development of therapies, particularly within personalized medicine [14,15,41]. Fig. 3 illustrates the diverse applications and key features of human brain organoids, emphasizing their utility in studying brain evolution, regional identity, modeling of NDs, drug development, and more. Patient-specific brain organoids derived from iPSCs replicate the genetic and phenotypic characteristics of individuals, thus enabling highly relevant assessments of drug efficacy and safety in personalized medicine [9,14]. The use of brain organoids in high-throughput screening for therapeutic agents is on the rise, accelerating the discovery of new treatments for NDs [5,25]. Advances in high-throughput screening technologies have expedited the identification of compounds that target neurodegenerative pathways, further solidifying the role of organoids in the early stages of drug discovery [42]. Moreover, by modeling the progression of diseases like PD and AD in vitro, organoids facilitate the examination of therapeutic interventions at different stages of the disease [24,43].
Diverse applications and key features of human brain organoids.
This schematic highlights the wide-ranging applications of human brain organoids derived from iPSCs. Brain organoids are instrumental in various research domains, such as investigating brain evolution, regional identity, and the modeling of NDs, including ADs and PDs. Key attributes of brain organoids encompass their use in drug discovery, gene editing, transcriptomic analysis, and electrophysiological studies. Furthermore, brain organoids are gaining traction in cell therapy, optogenetics, and personalized medicine, offering a robust platform for advancing our understanding of human brain function, disease mechanisms, and potential therapies.
Challenges and limitations
Despite meaningful advances in the development of brain organoids, several challenges remain that limit their utility in modeling NDs [7,11]. One of the primary challenges is achieving full maturation; current organoids often mimic early developmental stages, making them less suitable for studying late-onset NDs such as AD and PD [18,19]. Furthermore, the absence of systemic interactions, such as those involving the immune and vascular systems, prevents organoids from fully reflecting the multifactorial nature of NDs [41]. The lack of fully developed synaptic networks and mature glial cells further limits their capacity to replicate the complex interactions of the adult brain [20,44].
Another critical limitation is the absence of vascularization [27,32]. Without functional blood vessels, nutrient and oxygen diffusion is insufficient, triggering necrosis within the interiors of organoids [10,44]. Despite recent bioengineering efforts that have introduced vascular structures, current models still fall short in replicating the intricate blood-brain barrier and vascular network of the human brain [10,44].
Reproducibility issues also continue to pose a challenge in organoid research. Variability in the generation of organoids across laboratories and between batches can lead to inconsistencies in their size, shape, and cellular composition [45]. This variability complicates the ability to conduct comparative studies and hinders the scalability of organoids for applications such as drug screening and personalized medicine [8,13]. Furthermore, the absence of complete cortical layering restricts studies of higher-order brain functions, while the prolonged culture times required for organoid maturation present practical challenges for high-throughput screening [22,24]. Finally, the lack of systemic integration with the immune and endocrine systems constrains the capacity of organoids to accurately model the multifactorial aspects of NDs [46,47].
Conclusion and future directions
Brain organoids derived from iPSCs have emerged as transformative tools for studying NDs, offering insights into human brain-specific pathologies that traditional models cannot replicate [7,10]. By recreating aspects of human brain structure, cellular diversity, and microenvironment, brain organoids provide an advanced platform for investigating neurodegenerative mechanisms, disease progression, and therapeutic interventions in a physiologically relevant context [6,32].
Despite these advancements, significant challenges remain that limit the translational potential of brain organoids in ND research [13,27]. A primary limitation is the incomplete maturation of organoids, which often resemble early developmental stages and lack fully developed synaptic networks and glial cells. To achieve more advanced neuronal and glial development, refinements in differentiation protocols and culture systems are necessary [26,48]. Additionally, the absence of vascularization impedes long-term survival and functional maturity, leading to necrosis within the organoid interiors. Advances in bioengineering, such as the development of microfluidic systems and organ-on-a-chip platforms, show promise for improving nutrient and oxygen delivery, which may help overcome this limitation [17,47]. Another challenge is the variability in organoid generation across batches, underscoring the need for standardized methodologies and automated production systems to improve reproducibility and scalability [26,48].
A critical future direction involves integrating advanced technologies such as CRISPR-based gene editing, which offers precision in modeling disease-specific genetic mutations [49]. Nevertheless, it is crucial to address potential challenges, including off-target effects, to ensure accuracy and utility [50]. Additionally, the incorporation of multi-organ systems or assembloids could improve the physiological relevance of brain organoids by facilitating the study of systemic interactions and inter-regional neuronal dynamics [44]. These methodologies are anticipated to propel the discovery of new treatments for complex NDs, including PD and AD, thereby advancing the fields of regenerative medicine and neuroscience.
Advancements in brain organoid technology are poised to revolutionize personalized medicine by enabling patient-specific disease modeling and therapeutic development, thus accelerating the discovery of targeted treatments [14,18]. Furthermore, integrating brain organoids with transcriptomics, proteomics, and high-throughput screening technologies will enable a deeper exploration of disease mechanisms and drug efficacy. By overcoming current challenges and leveraging emerging technologies, brain organoids are on track to become invaluable tools in the study of NDs and the field of regenerative medicine. These developments hold the promise of enhancing our understanding of brain pathologies and leading to effective treatments, ultimately improving patient outcomes and quality of life [8,24].
Notes
Conflict of interest
No potential conflict of interest relevant to this article was reported.
Funding
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) for Jong-Chan Park (RS-2023-00266110).
Additional contributions
Graphical figures were created with BioRender.com by Jong-Chan Park.
Data availability
Please contact the corresponding author for data availability.