Harnessing cerebral organoids for Alzheimer's disease research

Engineering blood-brain barrier microphysiological systems to model Alzheimer's disease monocyte penetration and infiltration

Alzheimer's disease (AD) is a progressive and neurodegenerative disease, predominantly causing dementia. Despite increasing clinical evidence suggesting the involvement of peripheral immune cells such as monocytes in AD pathology, the dynamic penetration and infiltration of monocytes crossing blood-brain barrier (BBB) and inducing neuroinflammation is largely understudied in an AD brain. Herein, we engineer BBB-like microphysiological system (BBB-MPS) models for recapitulating the dynamic penetration and infiltration of monocytes in an AD patient's brain. Each BBB-MPS model can be engineered by integrating a functional BBB-like structure on a human cortical organoid using a 3D-printed device within a well of a plate. By coculturing these BBB-MPS models with monocytes from AD patients and age-matched healthy donors, we found that AD monocytes exhibit significantly greater BBB penetration and brain infiltration compared to age-matched control monocytes. Moreover, we also tested the interventions including Minocycline and Bindarit, and found they can effectively inhibit AD monocyte infiltration, subsequently reducing neuroinflammation and neuronal apoptosis. We believe these scalable and user-friendly BBB-MPS models may hold promising potential in modeling and advancing therapeutics for neurodegenerative and neuroinflammatory diseases.

Bioengineering innovations for neural organoids with enhanced fidelity and function

Neural organoids have been utilized to recapitulate different aspects of the developing nervous system. While hailed as promising experimental tools for studying human neural development and neuropathology, current neural organoids do not fully recapitulate the anatomy or microcircuitry-level functionality of the developing brain, spinal cord, or peripheral nervous system. In this review, we discuss emerging bioengineering approaches that control morphogen signals and biophysical microenvironments, which have improved the efficiency, fidelity, and utility of neural organoids. Furthermore, advancements in bioengineered tools have facilitated more sophisticated analyses of neural organoid functions and applications, including improved neural-bioelectronic interfaces and organoid-based information processing. Emerging bioethical issues associated with advanced neural organoids are also discussed. Future opportunities of neural organoid research lie in enhancing their fidelity, maturity, and complexity and expanding their applications in a scalable manner.

Cryopreserving 3D cell culture models of Alzheimer's disease in hydrogel microbeads

Long-term preservation of fully differentiated human neurons poses a longstanding challenge in neuroscience research. Numerous cellular disease models have been established using cultured human neuronal cells, including our three-dimensional (3D) human neural cell culture model of Alzheimer's disease (AD). However, the absence of a reliable method for preserving fully differentiated human neural cell cultures for a long time has hindered the sharing and standardization of these models. To address this critical limitation, we focused on cryopreservation, which is the gold standard for long-term preservation, and combined this with three key technological advancements. First, we employed parallelized microfluidic devices for the efficient generation of 3D cell cultures within uniform hydrogel microbeads (~ 220 μm), which facilitate the rapid exchange of media ingredients and cryoprotectants. Second, we implemented a cytophobic microwell system to safeguard neuron-encapsulated microbeads from fusion and aggregation. Third, we developed a novel inducible AD cell model optimized for cryopreservation and AD drug testing. We have successfully maintained encapsulated control and AD neural progenitor cells in microwells during differentiation for 12 days. Notably, fully differentiated human neural cells can be cryopreserved within Matrigel microbeads while retaining intact and mature neuronal processes, exhibiting no signs of damage to neurites following freeze/thaw cycles. Furthermore, we have demonstrated the successful cryopreservation, thawing, and induction of pathogenic Amyloid-β 42 (Aβ42) generation in fully differentiated AD neural progenitor cells. Our study offers a solution for one of the major challenges in neuroscience research, utilizing porous hydrogel microbead structures to facilitate rapid delivery of cryoprotectants and protect complex neuronal structures without undergoing damaging cell dissociation steps. The inducible "3D human microbead model of AD" enhances the speed, efficacy, and reproducibility of AD drug screening.

Two- and Three-Dimensional In Vitro Models of Parkinson's and Alzheimer's Diseases: State-of-the-Art and Applications

In vitro models play a pivotal role in advancing our understanding of neurodegenerative diseases (NDs) such as Parkinson's and Alzheimer's disease (PD and AD). Traditionally, 2D cell cultures have been instrumental in elucidating the cellular mechanisms underlying these diseases. Cultured cells derived from patients or animal models provide valuable insights into the pathological processes at the cellular level. However, they often lack the native tissue environment complexity, limiting their ability to fully recapitulate their features. In contrast, 3D models offer a more physiologically relevant platform by mimicking the 3D brain tissue architecture. These models can incorporate multiple cell types, including neurons, astrocytes, and microglia, creating a microenvironment that closely resembles the brain's complexity. Bioengineering approaches allow researchers to better replicate cell-cell interactions, neuronal connectivity, and disease-related phenotypes. Both 2D and 3D models have their advantages and limitations. While 2D cultures provide simplicity and scalability for high-throughput screening and basic processes, 3D models offer enhanced physiological relevance and better replicate disease phenotypes. Integrating findings from both model systems can provide a better understanding of NDs, ultimately aiding in the development of novel therapeutic strategies. Here, we review existing 2D and 3D in vitro models for the study of PD and AD.

Brain organoid models for studying the function of iPSC-derived microglia in neurodegeneration and brain tumours

Microglia represent the main resident immune cells of the brain. The interplay between microglia and other cells in the central nervous system, such as neurons or other glial cells, influences the function and ability of microglia to respond to various stimuli. These cellular communications, when disrupted, can affect the structure and function of the brain, and the initiation and progression of neurodegenerative diseases including Alzheimer's disease and Parkinson's disease, as well as the progression of other brain diseases like glioblastoma. Due to the difficult access to patient brain tissue and the differences reported in the murine models, the available models to study the role of microglia in disease progression are limited. Pluripotent stem cell technology has facilitated the generation of highly complex models, allowing the study of control and patient-derived microglia in vitro. Moreover, the ability to generate brain organoids that can mimic the 3D tissue environment and intercellular interactions in the brain provide powerful tools to study cellular pathways under homeostatic conditions and various disease pathologies. In this review, we summarise the most recent developments in modelling degenerative diseases and glioblastoma, with a focus on brain organoids with integrated microglia. We provide an overview of the most relevant research on intercellular interactions of microglia to evaluate their potential to study brain pathologies.

Microglial Drivers of Alzheimer's Disease Pathology: An Evolution of Diverse Participating States

Microglia, the resident immune-competent cells of the brain, become dysfunctional in Alzheimer's disease (AD), and their aberrant immune responses contribute to the accumulation of pathological proteins and neuronal injury. Genetic studies implicate microglia in the development of AD, prompting interest in developing immunomodulatory therapies to prevent or ameliorate disease. However, microglia take on diverse functional states in disease, playing both protective and detrimental roles in AD, which largely overlap and may shift over the disease course, complicating the identification of effective therapeutic targets. Extensive evidence gathered using transgenic mouse models supports an active role of microglia in pathology progression, though results vary and can be contradictory between different types of models and the degree of pathology at the time of study. Here, we review microglial immune signaling and responses that contribute to the accumulation and spread of pathological proteins or directly affect neuronal health. We additionally explore the use of induced pluripotent stem cell (iPSC)-derived models to study living human microglia and how they have contributed to our knowledge of AD and may begin to fill in the gaps left by mouse models. Ultimately, mouse and iPSC-derived models have their own limitations, and a comprehensive understanding of microglial dysfunction in AD will only be established by an integrated view across models and an appreciation for their complementary viewpoints and limitations.

Limitations of human brain organoids to study neurodegenerative diseases: a manual to survive

In 2013, M. Lancaster described the first protocol to obtain human brain organoids. These organoids, usually generated from human-induced pluripotent stem cells, can mimic the three-dimensional structure of the human brain. While they recapitulate the salient developmental stages of the human brain, their use to investigate the onset and mechanisms of neurodegenerative diseases still faces crucial limitations. In this review, we aim to highlight these limitations, which hinder brain organoids from becoming reliable models to study neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS). Specifically, we will describe structural and biological impediments, including the lack of an aging footprint, angiogenesis, myelination, and the inclusion of functional and immunocompetent microglia—all important factors in the onset of neurodegeneration in AD, PD, and ALS. Additionally, we will discuss technical limitations for monitoring the microanatomy and electrophysiology of these organoids. In parallel, we will propose solutions to overcome the current limitations, thereby making human brain organoids a more reliable tool to model neurodegeneration.

Landscape of human organoids: Ideal model in clinics and research

In the last decade, organoid research has entered a golden era, signifying a pivotal shift in the biomedical landscape. The year 2023 marked a milestone with the publication of thousands of papers in this arena, reflecting exponential growth. However, amid this burgeoning expansion, a comprehensive and accurate overview of the field has been conspicuously absent. Our review is intended to bridge this gap, providing a panoramic view of the rapidly evolving organoid landscape. We meticulously analyze the organoid field from eight distinctive vantage points, harnessing our rich experience in academic research, industrial application, and clinical practice. We present a deep exploration of the advances in organoid technology, underpinned by our long-standing involvement in this arena. Our narrative traverses the historical genesis of organoids and their transformative impact across various biomedical sectors, including oncology, toxicology, and drug development. We delve into the synergy between organoids and avant-garde technologies such as synthetic biology and single-cell omics and discuss their pivotal role in tailoring personalized medicine, enhancing high-throughput drug screening, and constructing physiologically pertinent disease models. Our comprehensive analysis and reflective discourse provide a deep dive into the existing landscape and emerging trends in organoid technology. We spotlight technological innovations, methodological evolution, and the broadening spectrum of applications, emphasizing the revolutionary influence of organoids in personalized medicine, oncology, drug discovery, and other fields. Looking ahead, we cautiously anticipate future developments in the field of organoid research, especially its potential implications for personalized patient care, new avenues of drug discovery, and clinical research. We trust that our comprehensive review will be an asset for researchers, clinicians, and patients with keen interest in personalized medical strategies. We offer a broad view of the present and prospective capabilities of organoid technology, encompassing a wide range of current and future applications. In summary, in this review we attempt a comprehensive exploration of the organoid field. We offer reflections, summaries, and projections that might be useful for current researchers and clinicians, and we hope to contribute to shaping the evolving trajectory of this dynamic and rapidly advancing field.

Human Glial Cells as Innovative Targets for the Therapy of Central Nervous System Pathologies

In vitro and preclinical in vivo research in the last 35 years has clearly highlighted the crucial physiopathological role of glial cells, namely astrocytes/microglia/oligodendrocytes and satellite glial cells/Schwann cells in the central and peripheral nervous system, respectively. Several possible pharmacological targets to various neurodegenerative disorders and painful conditions have therefore been successfully identified, including receptors and enzymes, and mediators of neuroinflammation. However, the translation of these promising data to a clinical setting is often hampered by both technical and biological difficulties, making it necessary to perform experiments on human cells and models of the various diseases. In this review we will, therefore, summarize the most relevant data on the contribution of glial cells to human pathologies and on their possible pharmacological modulation based on data obtained in post-mortem tissues and in iPSC-derived human brain cells and organoids. The possibility of an in vivo visualization of glia reaction to neuroinflammation in patients will be also discussed.

Brain organoid protocols and limitations

Stem cell-derived organoid technology is a powerful tool that revolutionizes the field of biomedical research and extends the scope of our understanding of human biology and diseases. Brain organoids especially open an opportunity for human brain research and modeling many human neurological diseases, which have lagged due to the inaccessibility of human brain samples and lack of similarity with other animal models. Brain organoids can be generated through various protocols and mimic whole brain or region-specific. To provide an overview of brain organoid technology, we summarize currently available protocols and list several factors to consider before choosing protocols. We also outline the limitations of current protocols and challenges that need to be solved in future investigation of brain development and pathobiology.

TUNEL-n-DIFL Method for Detection and Estimation of Apoptosis Specifically in Neurons and Glial Cells in Mixed Culture and Animal Models of Central Nervous System Diseases and Injuries

Detection of merely apoptosis does not reveal the type of central nervous system (CNS) cells that are dying in the CNS diseases and injuries. In situ detection and estimation of amount of apoptosis specifically in neurons or glial cells (astrocytes, oligodendrocytes, and microglia) can unveil valuable information for designing therapeutics for protection of the CNS cells and functional recovery. A method was first developed and reported from our laboratory for in situ detection and estimation of amount of apoptosis precisely in neurons and glial cells using in vitro and in vivo models of CNS diseases and injuries. This is a combination of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and double immunofluorescent labeling (DIFL) or simply TUNEL-n-DIFL method for in situ detection and estimation of amount of apoptosis in a specific CNS cell type. An anti-digoxigenin (DIG) IgG antibody conjugated with 7-amino-4-methylcoumarin-3-acetic acid (AMCA) for blue fluorescence, fluorescein isothiocyanate (FITC) for green fluorescence, or Texas Red (TR) for red fluorescence can be used for in situ detection of apoptotic cell DNA, which is earlier labeled with TUNEL using alkali-stable DIG-11-dUTP. A primary anti-NeuN (neurons), anti-GFAP (astrocytes), anti-MBP (oligodendrocytes), or anti-OX-42 (microglia) IgG antibody and a secondary IgG antibody conjugated with one of the above fluorophores (other than that of ani-DIG antibody) are used for in situ detection of apoptosis in a specific CNS cell type in the mixed culture and animal models of the CNS diseases and injuries.

Emerging concepts towards a translational framework in Alzheimer's disease

Over the past decades, significant efforts have been made to understand the precise mechanisms underlying the pathogenesis of Alzheimer's disease (AD), the most common cause of dementia. However, clinical trials targeting AD pathological hallmarks have consistently failed. Refinement of AD conceptualization, modeling, and assessment is key to developing successful therapies. Here, we review critical findings and discuss emerging ideas to integrate molecular mechanisms and clinical approaches in AD. We further propose a refined workflow for animal studies incorporating multimodal biomarkers used in clinical studies - delineating critical paths for drug discovery and translation. Addressing unresolved questions with the proposed conceptual and experimental framework may accelerate the development of effective disease-modifying strategies for AD.

Pushing the boundaries of brain organoids to study Alzheimer's disease

Progression of Alzheimer's disease (AD) entails deterioration or aberrant function of multiple brain cell types, eventually leading to neurodegeneration and cognitive decline. Defining how complex cell-cell interactions become dysregulated in AD requires novel human cell-based in vitro platforms that could recapitulate the intricate cytoarchitecture and cell diversity of the human brain. Brain organoids (BOs) are 3D self-organizing tissues that partially resemble the human brain architecture and can recapitulate AD-relevant pathology. In this review, we highlight the versatile applications of different types of BOs to model AD pathogenesis, including amyloid-β and tau aggregation, neuroinflammation, myelin breakdown, vascular dysfunction, and other phenotypes, as well as to accelerate therapeutic development for AD.

Current analysis of hypoxic-ischemic encephalopathy research issues and future treatment modalities

Hypoxic-ischemic encephalopathy (HIE) is the leading cause of long-term neurological disability in neonates and adults. Through bibliometric analysis, we analyzed the current research on HIE in various countries, institutions, and authors. At the same time, we extensively summarized the animal HIE models and modeling methods. There are various opinions on the neuroprotective treatment of HIE, and the main therapy in clinical is therapeutic hypothermia, although its efficacy remains to be investigated. Therefore, in this study, we discussed the progress of neural circuits, injured brain tissue, and neural circuits-related technologies, providing new ideas for the treatment and prognosis management of HIE with the combination of neuroendocrine and neuroprotection.

Consciousness in a Rotor? Science and Ethics of Potentially Conscious Human Cerebral Organoids

Human cerebral organoids are three-dimensional biological cultures grown in the laboratory to mimic as closely as possible the cellular composition, structure, and function of the corresponding organ, the brain. For now, cerebral organoids lack blood vessels and other characteristics of the human brain, but are also capable of having coordinated electrical activity. They have been usefully employed for the study of several diseases and the development of the nervous system in unprecedented ways. Research on human cerebral organoids is proceeding at a very fast pace and their complexity is bound to improve. This raises the question of whether cerebral organoids will also be able to develop the unique feature of the human brain, consciousness. If this is the case, some ethical issues would arise. In this article, we discuss the necessary neural correlates and constraints for the emergence of consciousness according to some of the most debated neuroscientific theories. Based on this, we consider what the moral status of a potentially conscious brain organoid might be, in light of ethical and ontological arguments. We conclude by proposing a precautionary principle and some leads for further investigation. In particular, we consider the outcomes of some very recent experiments as entities of a potential new kind.

3D models of neurodegeneration: implementation in drug discovery

A lack of in vitro models that robustly represent the complex cellular pathologies underlying neurodegeneration has resulted in a translational gap between in vitro and in vivo results, creating a bottleneck in the development of new therapeutics. In the past decade, new and complex 3D models of the brain have been published at an exponential rate. However, many novel 3D models of neurodegeneration overlook the validation and throughput requirements for implementation in drug discovery. This therefore represents a knowledge gap that could hinder the translation of these models to drug discovery efforts. We review the recent progress in the development of 3D models of neurodegeneration, examining model design benefits and validation techniques, and discuss opportunities and standards for 3D models of neurodegeneration to be implemented in drug discovery and development.

Roles of Siglecs in neurodegenerative diseases

Microglia are resident myeloid cells in the central nervous system (CNS) with a unique developmental origin, playing essential roles in developing and maintaining the CNS environment. Recent studies have revealed the involvement of microglia in neurodegenerative diseases, such as Alzheimer's disease, through the modulation of neuroinflammation. Several members of the Siglec family of sialic acid recognition proteins are expressed on microglia. Since the discovery of the genetic association between a polymorphism in the CD33 gene and late-onset Alzheimer's disease, significant efforts have been made to elucidate the molecular mechanism underlying the association between the polymorphism and Alzheimer's disease. Furthermore, recent studies have revealed additional potential associations between Siglecs and Alzheimer's disease, implying that the reduced signal from inhibitory Siglec may have an overall protective effect in lowering the disease risk. Evidences suggesting the involvement of Siglecs in other neurodegenerative diseases are also emerging. These findings could help us predict the roles of Siglecs in other neurodegenerative diseases. However, little is known about the functionally relevant Siglec ligands in the brain, which represents a new frontier. Understanding how microglial Siglecs and their ligands in CNS contribute to the regulation of CNS homeostasis and pathogenesis of neurodegenerative diseases may provide us with a new avenue for disease prevention and intervention.

Current progress of cerebral organoids for modeling Alzheimer's disease origins and mechanisms

Alzheimer's disease (AD) is a progressive, neurodegenerative disease that has emerged as a leading risk factor for dementia associated with increasing age. Two-dimensional (2D) cell culture and animal models, which have been used to analyze AD pathology and search for effective treatments for decades, have significantly contributed to our understanding of the mechanism of AD. Despite their successes, 2D and animal models can only capture a fraction of AD mechanisms due to their inability to recapitulate human brain-specific tissue structure, function, and cellular diversity. Recently, the emergence of three-dimensional (3D) cerebral organoids using tissue engineering and induced pluripotent stem cell technology has paved the way to develop models that resemble features of human brain tissue more accurately in comparison to prior models. In this review, we focus on summarizing key research strategies for engineering in vitro 3D human brain-specific models, major discoveries from using AD cerebral organoids, and its future perspectives.

Building in vitro models of the brain to understand the role of APOE in Alzheimer's disease

Alzheimer's disease (AD) is a devastating, complex, and incurable disease that represents an increasingly problematic global health issue. The etiology of sporadic AD that accounts for a vast majority of cases remains poorly understood, with no effective therapeutic interventions. Genetic studies have identified AD risk genes including the most prominent, APOE, of which the ɛ4 allele increases risk in a dose-dependent manner. A breakthrough discovery enabled the creation of human induced pluripotent stem cells (hiPSCs) that can be differentiated into various brain cell types, facilitating AD research in genetically human models. Herein, we provide a brief background on AD in the context of APOE susceptibility and feature work employing hiPSC-derived brain cell and tissue models to interrogate the contribution of APOE in driving AD pathology. Such models have delivered crucial insights into cellular mechanisms and cell type-specific roles underlying the perturbed biological functions that trigger pathogenic cascades and propagate neurodegeneration. Collectively, hiPSC-based models are envisioned to be an impactful platform for uncovering fundamental AD understanding, with high translational value toward AD drug discovery and testing.

Research models of neurodevelopmental disorders: The right model in the right place

Neurodevelopmental disorders (NDDs) are a heterogeneous group of impairments that affect the development of the central nervous system leading to abnormal brain function. NDDs affect a great percentage of the population worldwide, imposing a high societal and economic burden and thus, interest in this field has widely grown in recent years. Nevertheless, the complexity of human brain development and function as well as the limitations regarding human tissue usage make their modeling challenging. Animal models play a central role in the investigation of the implicated molecular and cellular mechanisms, however many of them display key differences regarding human phenotype and in many cases, they partially or completely fail to recapitulate them. Although in vitro two-dimensional (2D) human-specific models have been highly used to address some of these limitations, they lack crucial features such as complexity and heterogeneity. In this review, we will discuss the advantages, limitations and future applications of in vivo and in vitro models that are used today to model NDDs. Additionally, we will describe the recent development of 3-dimensional brain (3D) organoids which offer a promising approach as human-specific in vitro models to decipher these complex disorders.

Opportunities and challenges in delivering biologics for Alzheimer's disease by low-intensity ultrasound

Low-intensity ultrasound combined with intravenously injected microbubbles (US^+MB) is a novel treatment modality for brain disorders, including Alzheimer's disease (AD), safely and transiently allowing therapeutic agents to overcome the blood-brain barrier (BBB) that constitutes a major barrier for therapeutic agents. Here, we first provide an update on immunotherapies in AD and how US^+MB has been applied to AD mouse models and in clinical trials, considering the ultrasound and microbubble parameter space. In the second half of the review, we compare different in vitro BBB models and discuss strategies for combining US^+MB with BBB modulators (targeting molecules such as claudin-5), and highlight the insight provided by super-resolution microscopy. Finally, we conclude with a short discussion on how in vitro findings can inform the design of animal studies, and how the insight gained may aid treatment optimization in the clinical ultrasound space.

A Perspective: Challenges in Dementia Research

Although dementia is a common and devastating disease that has been studied intensely for more than 100 years, no effective disease modifying treatment has been found. At this impasse, new approaches are important. The purpose of this paper is to provide, in the context of current research, one clinician's perspective regarding important challenges in the field in the form of specific challenges. These challenges not only illustrate the scope of the problems inherent in finding treatments for dementia, but can also be specific targets to foster discussion, criticism and new research. One common theme is the need to transform research activities from small projects in individual laboratories/clinics to larger multinational projects, in which each clinician and researcher works as an integral part. This transformation will require collaboration between researchers, large corporations, regulatory/governmental authorities and the general population, as well as significant financial investments. However, the costs of transforming the approach are small in comparison with the cost of dementia.

The multifaceted role of LRRK2 in Parkinson's disease: From human iPSC to organoids

Parkinson's disease (PD) is the second most common neurodegenerative disease affecting elderly people. Pathogenic mutations in Leucine-Rich Repeat Kinase 2 (LRRK2) are the most common cause of autosomal dominant PD. LRRK2 activity is enhanced in both familial and idiopathic PD, thereby studies on LRRK2-related PD research are essential for understanding PD pathology. Finding an appropriate model to mimic PD pathology is crucial for revealing the molecular mechanisms underlying disease progression, and aiding drug discovery. In the last few years, the use of human-induced pluripotent stem cells (hiPSCs) grew exponentially, especially in studying neurodegenerative diseases like PD, where working with brain neurons and glial cells was mainly possible using postmortem samples. In this review, we will discuss the use of hiPSCs as a model for PD pathology and research on the LRRK2 function in both neuronal and immune cells, together with reviewing the recent advances in 3D organoid models and microfluidics.