Myelinoid

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A myelinoid or myelin organoid is a three dimensional in vitro cultured model derived from human pluripotent stem cells (hPSCs) that represents various brain regions, the spinal cord or the peripheral nervous system in early fetal human development. [1] [2] [3] Myelinoids have the capacity to recapitulate aspects of brain developmental processes, microenvironments, cell to cell interaction, structural organization and cellular composition. [2] [3] The differentiating aspect dictating whether an organoid is deemed a cerebral organoid/brain organoid or myelinoid is the presence of myelination and compact myelin formation that is a defining feature of myelinoids. Due to the complex nature of the human brain, there is a need for model systems which can closely mimic complicated biological processes. Myelinoids provide a unique in vitro model through which myelin pathology, neurodegenerative diseases, developmental processes and therapeutic screening can be accomplished. [1] [2]

Contents

History

In vitro models have been a critical component of many biological studies. Monolayers, or 2D cultures, have been widely used in the past, however, they are limited by their lack of complexity and fail to recapitulate tissue architecture involved in biological processes occurring in vivo. [4] Model organisms, such as Mus musculus, Caenorhabditis elegans, Drosophila melanogaster, and Saccharomyces cerevisiae, recapitulate biological complexity better than 2D monolayer cultures. [5] [6] However, these model organisms do not perfectly capture human biology. Specifically, there are stark differences in brain development between mice and humans. Major developmental differences include variability in division patterns of neural stem cells and localization and types of glial cells that occur at specific stages in development. [7] [8]

Leveraging pluripotent stem cell technologies, brain organoids and cerebral organoids were developed to fill the gap in model systems to study human specific brain development and pathology in vitro. The first cerebral organoid was established in 2013. [9] Since then, various protocols have emerged for generating organoids for different brain regions such as cerebellar, [10] hippocampal, [11] midbrain, [12] forebrain, [13] and hypothalamic [14] organoids. Cerebral organoids provide a neurological model through which diseases, development and therapeutics can be studied. [15] However, a major constraint of cerebral organoids is that they lack robust myelin formation and are therefore not well suited to studies investigating white matter.

This limitation of cerebral organoids was addressed in 2018 when brain organoids containing a robust population of myelinating oligodendrocytes were generated. The process of generating these myelinated brain organoids lasted 210 days and involved the addition of various growth factors and media at specific time points. [2] Due to the prolonged duration of the 2018 protocol, there were efforts to speed up and streamline the differentiation and generation of these myelinated organoids. A similar protocol which differed slightly in growth factors added and timing of media changes was described in 2019. This protocol was able to generate organoids with compact myelin formation by day 160. [16]

Another protocol developed in 2019 demonstrated that myelinated organoid generation could be accelerated further. Using a novel protocol, myelin basic protein (MBP), a marker for oligodendrocyte differentiation and myelination in the CNS, was detectable as early as day 63 (9 weeks) and myelinated axons were observed by day 105 (15 weeks), effectively halving the duration of the protocol. [17] [18]

A protocol of similar duration was established in 2021, however, the resulting organoids differ slightly in their biological context. This protocol leveraged the fact that spinal cord myelination is observed prior to cortical myelination. [1] This protocol generated organoids with robust myelination with a ventral caudal cell fate. [1] These organoids, although not technically brain organoids, can also be used to study myelin disease pathology, validated in the study through generating organoids recapitulating the disease pathology observed in Nfasc 155-/- patients. In this protocol, they referred to their myelinated organoids as "myelinoids" thus creating the category of organoids referred to as myelinoids. [1]

In 2021, a group of researchers aimed to address the fact that the lengthy differentiation protocols renders myelinoids less practical for high throughput experimentation such as drug screening. [19] To do this, scientists developed a human induced pluripotent stem cell (hiPSC) line that relies on early expression of an oligodendroglial gene which enabled the accelerated generation of myelinated organoids in just 42 days. [19] To date, this is the fastest protocol for generating mature oligodendrocytes in a brain organoid. [19]

History of cerebral organoid to myelinoid generation protocols History of myelinoids.jpg
History of cerebral organoid to myelinoid generation protocols

Culturing methods

General workflow for generating myelin organoids. Myelinoid General Workflow.jpg
General workflow for generating myelin organoids.

To generate organoids, human pluripotent stem cells (hPSCs) are allowed to aggregate into embryoid bodies (EBs) in low attachment plates (in suspension), which are then cultivated in a rotating bioreactor with lineage specific factors to promote cell amplification, growth and differentiation. [2] [20] [21] EBs have the capacity to differentiate into all embryonic germ layers, mesoderm, endoderm and ectoderm. In vivo, the nervous system, including myelin, is generated from the ectoderm. [21] To recapitulate this in vitro and generate myelin organoids, the EBs are cultured in media with specific growth factors and supplements that lead to ectodermal differentiation specifically, followed by subsequent neural induction. [21] More specifically, neural induction factors are added to induce the formation of neural progenitor cells which give rise to neurons and glial cells, including oligodendrocytes, in vivo. [2]

A well established method used to efficiently differentiate hPSC into neural cells is by dual inhibition of SMAD signaling using dorsomorphin (also known as compound C) and SB431542. [1] [2] [22] [23] To promote further proliferation of neural precursor cells specific growth factors are added to the media such as epidermal growth factor (EGF) and fibroblast growth factor 2 (FGF-2). [2] [23] [24] Before neural and glial induction, the spheroids are generally embedded in an extracellular matrix, such as Matrigel, and transferred to a rotating bioreactor where different small molecules and growth factors are continuously supplemented to promote the differentiation of cells into specific structures and cell types. [2] [20] [23]

In vivo, neuronal induction precedes oligodendrocyte formation. [25] Therefore, in culture, neuronal induction factors are added first to induce neuro-cortical patterning of the spheroids, followed by factors that induce oligodendrocyte precursor cell (OPC) formation and differentiation into oligodendroglia. [2] [23] To promote formation of neurons from neural precursor cells, brain-derived neurotrophic factor (BDNF) and neurotrophic factor 3 (NT3) can be added to the media. [2] [23] Subsequently, factors such as platelet-derived growth factor AA (PDGF-AA) and insulin-like growth factor 1 (IGF-1) are added to the media to result in an expansion of the OPC populations present within the organoid by promoting OPC proliferation and survival. [1] [2]

Finally, factors that induce OPC differentiation into oligodendrocytes, and ultimately myelinating oligodendrocytes, are added. [1] [2] This includes thyroid hormone (T3), which has been shown to induce oligodendrocyte generation from OPCs in vivo. [1] [2] The organoids are maintained in suspension where they grow and mature until required for analysis. The fundamentals of this workflow are generally used to obtain myelin organoids; however, various protocols that rely on it have introduced multiple modifications for different purposes. Madhavan et al. was the first to establish a reproducible protocol that allowed for generating organoids with robust OPC and oligodendrocytes populations, and therefore myelination; they are referred to as myelin organoids, or myelinoids. [2]

Timeline of small molecules & growth factors for myelinoid differentiation and generation. Myelin Organoid Protocol Overview.jpg
Timeline of small molecules & growth factors for myelinoid differentiation and generation.

Properties and components

The generation of myelin organoids generally relies on neurocortical patterning factors that establish the structural and cellular framework necessary for the induction of oligodendrogenesis later on in the differentiation protocol. [2] Therefore, the properties and components of myelin organoids in the early stages of differentiation are very similar to that observed in cerebral organoids where populations of neural progenitor cells, precursors of neurons and glial cells, start to emerge and self-organize into distinct layers that recapitulate features of the cortex during early embryogenesis. [3]

At such early stages, myelin organoids start to form large continuous neuroepithelial that encompass a fluid filled cavity representative of a brain ventricle. [9] The progenitor cells surrounding the putative ventricle organize into distinct layers defined by specific neural markers that become more defined as the organoid matures. [9] The layers include a ventricular zone surrounding the cavity with cells expressing PAX6, SOX2 and Ki67, followed by the outer subventricular zone and intermediate zone with cells expressing Ki-67 and TBR2, and finally cortical plate layer with cells expressing CTIP2, MAP2 and TBR1. [3]

Following neurocortical patterning, the oligodendrocyte lineage growth factors drive the expansion of native populations of OPCs distributed causing a substantial increase in their numbers which express SOX6, SOX10 and OLIG2, markers of glial induction and OPC specification. [2] As the myelin organoid matures, the OPC cells differentiate into oligodendrocytes that express proteolipid protein 1 (PLP1), the predominant component of myelin, and MYRF25, an oligodendrocyte specific transcription factor. [2] The oligodendrocytes are distributed throughout the neuronal layers, where upon maturation, their processes express MBP and CNP (an early myelination marker), begin extending to wrap and myelinate the axons surrounding them. The myelin undergoes maturation, refinement and compaction eventually leading to the formation of functional neuronal networks with compactly wrapped myelin lamellae. [1] [2] Further myelin maturation leads to distinct axonal subdomains with a paranodal axo-glial junction (PNJ) and node of Ranvier. The observation of paranodal and nodal assembly is protocol dependent, some observe paranodal and nodal assembly, some do not. Overall, the oligodendrocytes in myelin organoids demonstrate the ability to form compact myelin that wraps and organizes around neuronal axons recapitulating the three dimensional architecture of myelinated axonal networks in humans.

Applications

Disease modelling

Myelinoids recapitulate various fundamental aspects of brain development and myelination, and therefore related disease and pathology. Given that, they can be used to model various diseases and understand disease mechanisms associated with myelin defects including neurodegenerative diseases, CNS injury, PMD, and NFASC. [1] [2] [17]

Pelizaeus-Merzbacher disease (PMD)

PMD is a rare monogenic disease caused by various mutations of the X-linked proteolipid protein 1 gene (PLP1). [26] PLP1 is a critical protein for myelin formation. PMD is classified as a leukodystrophy, meaning that it is a disease affecting the white matter of the brain. Madhavan et al. tested how well their myelinoid system could recapitulate the established cellular pathology of PMD. Organoids were derived from three patients with varying disease severity where the subject with a deletion, a duplication, and a point mutation had mild, moderate and severe phenotypes respectively. [2] Their results demonstrated that the myelinating oligocortical spheroids generated recapitulated the degrees of cellular pathology associated with the genetic variants, therefore can serve as models for understanding the relationships between PMD genotypes and phenotypes, which have not been fully characterized yet, therefore can serve as models for understanding the relationships between PMD genotypes and phenotypes, which have not been fully characterized yet. [2]

Neurofascin (NFASC) nonsense mutation

The NFASC gene encodes a cell adhesion molecule that is involved in neurite outgrowth and fasciculation. [27] Additionally, NFASC is involved in the organization of axonal initiation segment and the nodes of Ranvier during development. [27] Patients with nonsense mutations in NFASC have abnormalities in the paranodal axo-glial junction (PNJ). [1] James et al. demonstrated that patient derived myelinoids had widespread formation of myelinoids of both patient and control; however, as expected, the PNJ in patient derived myelinoids had disrupted paranode formation. [1]

Myelin structure and integrity analysis

Myelin structure and integrity is inherently hard to study in humans at a molecular level. MRI can shed light on myelin abnormalities in a human brain, however, many studies utilize animal models to study myelin related changes in response to genetic variants. Myelinoids provide a 3D human derived system to study myelin structure. [2] Measuring the number and length of myelin sheaths, paranodal/nodal organization and structure, myelin volume and compaction, cellular identity and composition, and cellular organization are all methods for quantifying myelin changes. [1]

Testing drugs and therapeutics

Studies have shown that in myelinoids, human myelination can be pharmacologically manipulated in a quantifiable manner at both cellular and global levels across the myelinoids. [1] Therefore, myelin organoids can be used as a preclinical model for evaluating myelin associated candidate therapeutics and drugs in a human physiologically relevant context. [2]

Promyelinating drugs

Myelin organoids can be used to study the therapeutic potential of possible myelination strategies for individuals with diseases associated with demyelination such as leukodystrophies and multiple sclerosis, an auto-immune demyelinating disease affecting the CNS. [2] [28] [29] Clemastine and ketoconazole are promyelinating drugs that function as potent stimulators of oligodendrocyte generation and myelination in rodent models. The previously known effects of both drugs have been recapitulated using myelin organoids as they enhanced and accelerated the extent and rate of oligodendrocyte generation, maturation and myelination in organoids. [2]

Er stress pathway small-molecule modulators

Certain classes of Pelizaeus-Merzbacher disease (PMD), proteolipid protein 1 (PLP1) show perinuclear retention in oligodendrocytes. [30] Perinuclear retention of misfolded proteins is a hallmark of endoplasmic reticulum (ER) stress, which might be implicated in the pathology observed in PMD. [30] In a myelinoid model of Pelizaeus-Merzbacher disease (PMD) developed in 2018, treatment with a modulator of ER stress pathways called GSK2656157, an inhibitor of protein-kinase-R-like ER kinase, partially rescued PLP1 perinuclear retention mobilizing it away from the ER and into the processes of oligodendrocytes. [2] In addition, treatment resulted in an increase in the number of cells that show MYRF expression, an oligodendrocyte specific transcription factor, which has been observed to be reduced in PMD oligodendrocytes compared to control. [2]

Gene-editing: CRISPR

In a myelinoid model of PMD caused by point mutations in proteolipid protein 1 (PLP1), a CRISPR correction to the wildtype sequence in the hPSCs used to generate it rescued some aspects of PMD pathology. [2] The treatment restored the perinuclear retention of PLP1 and mobilization into oligodendrocyte processes and increased the amount of oligodendrocytes that express MYRF, an oligodendrocyte specific marker, to levels observed in healthy controls. [2] The myelin organoids derived from hPSC after the CRISPR correction of PLP1 point mutations generated myelin after 20 weeks in culture. [2]

Genome analysis

'Omics' has a broad application to organoids and since the development of organoid technology, transcriptome, epigenome, proteome, and metabolome analysis have been used. [31] Additionally, targeted gene editing and host-microbiome interactions have been studied using organoids. [31]

Single-cell omics

It is not possible to study gene expression patterns of the brain in human subjects, so the ability to recapitulate some of the complexity of the human brain in vitro allows for aspects of human development and disease to be investigated. Single-cell omics is a powerful tool that has been used to identify different subpopulations of oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes in mouse models which were previously undefined. [32] The heterogeneity of oligodendrocytes was previously thought to be functionally homogeneous; however, distinct cell populations can be characterized through specific transcriptional signatures and gene ontology profiles.

Single-cell RNA sequencing (scRNA seq) analysis of myelinoids generated in 2018, confirmed that there were distinct populations of oligodendrocytes throughout multiple stages of development in oligocortical spheroids which closely matched the single-cell transcriptome data obtained from human fetal cortex. [2] Due to their close transcriptomic resemblance to human fetal brain data, the regulatory landscape of cells within cerebral organoids can inform on the underlying regulatory mechanisms governing human brain development. [33]

In 2020, researchers described an approach to obtain meaningful scRNA seq and assay for transposase-accessible chromatin using sequencing (ATAC-seq) data from brain organoids. [33] The protocol can likely translate to myelin organoids due to the similar biology between cerebral organoids and myelinoids.

Orgo-seq

Orgo-seq is a framework through which bulk RNA (bRNA) and scRNA sequencing data of organoids can be integrated. [34] This platform was developed to address challenges associated with phenotyping organoids and demonstrated its ability to identify critical cell types and cell type specific driver genes involved with neurodevelopmental disorders and disease manifestation. [34]

Using the Orgo-Seq framework, three datasets (bRNA-seq from donor derived organoids, scRNA-seq data from cerebral organoids and fetal brains in precious studies, and bRNA-seq from the BrainSpan Project of human post-mortem brains) were used to study copy number variants in autism spectrum disorder. They leveraged several datasets to identify the types of cells present and cell specific driver genes in patient derived organoids.

Brain organoids serve as a human-derived model through which genetic variation and its impact on cell specific processes and association with neurodevelopmental and neurodegenerative disorders can be studied. [34] Specifically, myelinoids provide a system to study the cell type specific effects in oligodendrocytes that are disrupted by genetic variants. Overall, Orgo-Seq provides a quantitative and validated framework for investigating driver genes and their role in neurological and neurological disorders. [34] In the future, Lim et al., aim to develop a precision medicine framework to identify gene networks and effects of genetic variants in an organoid system, which would include myelinoids, that recapitulates the patient's exact genetic background. [34]

Advantages

With the absence of human brain tissue, myelinoids offer unprecedented opportunities for studying oligogenesis and myelination. [17] While animal models are valuable for studying human diseases, they do not fully recapitulate human brain development and show many discrepancies affecting their translatability to human physiology. [20] Considering resemblance of myelin organoids to the human brain, they have been proposed as models bridging between animal models and human physiology. [3]

Other hPSC derived oligodendrocytes systems have been established, such as the two dimensional (2D) monolayer oligodendrocytes models. [2] However, when compared to 2D systems, myelin organoids more faithfully recapitulate the structure and functionality of the developing human brain containing a more physiologically relevant microenvironment including their 3D cytoarchitecture, neural circuits, cell interactions and an overall more physiologically relevant microenvironment. [2] [3]

While cerebral organoids form the brain cytoarchitecture and composition, they generally lack oligodendrocytes, the cells responsible for myelination in the central nervous system. [2] The myelinoid protocol pioneered in 2018, and subsequently modified by others, offer a reproducible method for generating organoids with robust OPC and oligodendrocytes populations that track the endogenous neurons forming functional neuronal networks ensheathed with myelin. [2] [19]

Finally, the ability to generate myelinoids from patient derived hPSCs (induced-PSCs) offer major advantages and opportunities to explore patient-specific pathogenesis over the developmental and maturation stages of oligodendrocytes. This allows for the development of personalized therapeutic approaches. [2] [18]

Limitations

As is the case with every model system, myelinoids have their limitations. Due to the methods involved with generating the organoids, there can be a large degree of experimental variability. [18] Additionally, due to the long duration over which myelination occurs, optimizing the dosage of molecules and treatments involved in myelin development can be difficult. [1] The advantage of drug screening in this model comes with its own limitations. It can be difficult to scale myelinoid experiment to an appropriate scale for high throughput screening due to the long duration of protocols and limited efficiency. [18]

Myelinoids capture a large number of cell types found in vivo, however, they fail to capture all cell types. Microglia are absent in some myelinoids as was observed in the 2021 protocol. [1] Myelinoids also do not capture any behavioral abnormalities.

Finally, a challenge with all organoid cultures is that they rely on diffusion for nutrients to reach cells. Therefore, many organoids will develop a necrotic center due to a lack of nutrients making their way to the innermost cells. [35] Recently, developing vascularized organoids has been of interest and may potentially alleviate this issue. [36] However, myelinoids as described in current protocols are not vascularized.

Related Research Articles

<span class="mw-page-title-main">Myelin</span> Fatty substance that surrounds nerve cell axons to insulate them and increase transmission speed

Myelin is a lipid-rich material that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses pass along the axon. The myelinated axon can be likened to an electrical wire with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, myelin ensheaths the axon segmentally: in general, each axon is encased in multiple long sheaths with short gaps between, called nodes of Ranvier. At the nodes of Ranvier, which are approximately one thousandth of a mm in length, the axon's membrane is bare of myelin.

<span class="mw-page-title-main">Pelizaeus–Merzbacher disease</span> X-linked leukodystrophy

Pelizaeus–Merzbacher disease is an X-linked neurological disorder that damages oligodendrocytes in the central nervous system. It is caused by mutations in proteolipid protein 1 (PLP1), a major myelin protein. It is characterized by a decrease in the amount of insulating myelin surrounding the nerves (hypomyelination) and belongs to a group of genetic diseases referred to as leukodystrophies.

<span class="mw-page-title-main">Schwann cell</span> Glial cell type

Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle. The two types of Schwann cells are myelinating and nonmyelinating. Myelinating Schwann cells wrap around axons of motor and sensory neurons to form the myelin sheath. The Schwann cell promoter is present in the downstream region of the human dystrophin gene that gives shortened transcript that are again synthesized in a tissue-specific manner.

<span class="mw-page-title-main">Oligodendrocyte</span> Neural cell type

Oligodendrocytes, also known as oligodendroglia, are a type of neuroglia whose main functions are to provide support and insulation to axons within the central nervous system (CNS) of jawed vertebrates. Their function is similar to that of Schwann cells, which perform the same task in the peripheral nervous system (PNS). Oligodendrocytes accomplish this by forming the myelin sheath around axons. Unlike Schwann cells, a single oligodendrocyte can extend its processes to cover around 50 axons, with each axon being wrapped in approximately 1 μm of myelin sheath. Furthermore, an oligodendrocyte can provide myelin segments for multiple adjacent axons.

<span class="mw-page-title-main">Demyelinating disease</span> Any neurological disease in which the myelin sheath of neurons is damaged

A demyelinating disease refers to any disease affecting the nervous system where the myelin sheath surrounding neurons is damaged. This damage disrupts the transmission of signals through the affected nerves, resulting in a decrease in their conduction ability. Consequently, this reduction in conduction can lead to deficiencies in sensation, movement, cognition, or other functions depending on the nerves affected.

Oligodendrocyte progenitor cells (OPCs), also known as oligodendrocyte precursor cells, NG2-glia, O2A cells, or polydendrocytes, are a subtype of glia in the central nervous system named for their essential role as precursors to oligodendrocytes and myelin. They are typically identified in the human by co-expression of PDGFRA and CSPG4.

<span class="mw-page-title-main">Organoid</span> Miniaturized and simplified version of an organ

An organoid is a miniaturised and simplified version of an organ produced in vitro in three dimensions that mimics the key functional, structural, and biological complexity of that organ. It is derived from one or a few cells from a tissue, embryonic stem cells, or induced pluripotent stem cells, which can self-organize in three-dimensional culture owing to their self-renewal and differentiation capacities. The technique for growing organoids has rapidly improved since the early 2010s, and The Scientist named it one of the biggest scientific advancements of 2013. Scientists and engineers use organoids to study development and disease in the laboratory, for drug discovery and development in industry, personalized diagnostics and medicine, gene and cell therapies, tissue engineering, and regenerative medicine.

Remyelination is the process of propagating oligodendrocyte precursor cells to form oligodendrocytes to create new myelin sheaths on demyelinated axons in the Central nervous system (CNS). This is a process naturally regulated in the body and tends to be very efficient in a healthy CNS. The process creates a thinner myelin sheath than normal, but it helps to protect the axon from further damage, from overall degeneration, and proves to increase conductance once again. The processes underlying remyelination are under investigation in the hope of finding treatments for demyelinating diseases, such as multiple sclerosis.

<span class="mw-page-title-main">Induced pluripotent stem cell</span> Pluripotent stem cell generated directly from a somatic cell

Induced pluripotent stem cells are a type of pluripotent stem cell that can be generated directly from a somatic cell. The iPSC technology was pioneered by Shinya Yamanaka and Kazutoshi Takahashi in Kyoto, Japan, who together showed in 2006 that the introduction of four specific genes, collectively known as Yamanaka factors, encoding transcription factors could convert somatic cells into pluripotent stem cells. Shinya Yamanaka was awarded the 2012 Nobel Prize along with Sir John Gurdon "for the discovery that mature cells can be reprogrammed to become pluripotent."

Myelinogenesis is the formation and development of myelin sheaths in the nervous system, typically initiated in late prenatal neurodevelopment and continuing throughout postnatal development. Myelinogenesis continues throughout the lifespan to support learning and memory via neural circuit plasticity as well as remyelination following injury. Successful myelination of axons increases action potential speed by enabling saltatory conduction, which is essential for timely signal conduction between spatially separate brain regions, as well as provides metabolic support to neurons.

<span class="mw-page-title-main">LINGO1</span> Protein-coding gene in the species Homo sapiens

Leucine-rich repeat and Immunoglobulin-like domain-containing protein 1 also known as LINGO-1 is a protein which is encoded by the LINGO1 gene in humans. It belongs to the family of leucine-rich repeat proteins which are known for playing key roles in the biology of the central nervous system. LINGO-1 is a functional component of the Nogo receptor also known as the reticulon 4 receptor.

<span class="mw-page-title-main">Myelin regulatory factor</span> Mammalian protein found in Homo sapiens

Myelin regulatory factor, also known as myelin gene regulatory factor (MRF), is a protein that in humans is encoded by the MYRF gene.

<span class="mw-page-title-main">Cerebral organoid</span> Artificial miniature brain like organ

A neural, or brain organoid, describes an artificially grown, in vitro, tissue resembling parts of the human brain. Neural organoids are created by culturing pluripotent stem cells into a three-dimensional culture that can be maintained for years. The brain is an extremely complex system of heterogeneous tissues and consists of a diverse array of neurons and glial cells. This complexity has made studying the brain and understanding how it works a difficult task in neuroscience, especially when it comes to neurodevelopmental and neurodegenerative diseases. The purpose of creating an in vitro neurological model is to study these diseases in a more defined setting. This 3D model is free of many potential in vivo limitations. The varying physiology between human and other mammalian models limits the scope of animal studies in neurological disorders. Neural organoids contain several types of nerve cells and have anatomical features that recapitulate regions of the nervous system. Some neural organoids are most similar to neurons of the cortex. In some cases, the retina, spinal cord, thalamus and hippocampus. Other neural organoids are unguided and contain a diversity of neural and non-neural cells. Stem cells have the potential to grow into many different types of tissues, and their fate is dependent on many factors. Below is an image showing some of the chemical factors that can lead stem cells to differentiate into various neural tissues; a more in-depth table of generating specific organoid identity has been published. Similar techniques are used on stem cells used to grow cerebral organoids.

<span class="mw-page-title-main">Sergiu P. Pașca</span> Romanian-American scientist and physician at Stanford University

Sergiu P. Pașca is a Romanian-American scientist and physician at Stanford University in California. He is renowned for his groundbreaking work creating and developing stem cell-based models of the human brain to gain insights into neuropsychiatric disease. His lab was the first to develop and name assembloids: multi-unit self-organizing structures created in 3D cultures that allow for the study of human neural circuit and systems functions in vitro. Pașca’s lab generated and published human cortico-striatal and cortico-motor assembloids in 2020. Combining regionalized neural organoids pioneered in the lab and studies with human forebrain assembloids and transplantation, in 2024, Pașca developed a therapeutic for a severe genetic disorder called Timothy Syndrome, which was published on the cover of Nature.

<span class="mw-page-title-main">Jürgen Knoblich</span> German molecular biologist

Jürgen Knoblich is a German molecular biologist. Since 2018, he is the interim Scientific Director of the Institute of Molecular Biotechnology (IMBA) of the Austrian Academy of Sciences in Vienna.  

<span class="mw-page-title-main">Paul J. Tesar</span> American developmental biologist

Paul J. Tesar is an American developmental biologist. He is the Dr. Donald and Ruth Weber Goodman Professor of Innovative Therapeutics at Case Western Reserve University School of Medicine. His research is focused on regenerative medicine.

Paola Arlotta is the Golub Family Professor of Stem Cell and Regenerative Biology at Harvard University and chair of the Harvard Stem Cell and Regenerative Biology (HSCRB). Her research focuses on the development of neuron types in the cerebral cortex. She is best known for her work using 3D cerebral organoids derived from human induced pluripotent stem cells (iPSCs) to study cortical development in neurodegenerative and neuropsychiatric disorders.

<span class="mw-page-title-main">Experimental models of Alzheimer's disease</span>

Experimental models of Alzheimer's disease are organism or cellular models used in research to investigate biological questions about Alzheimer's disease as well as develop and test novel therapeutic treatments. Alzheimer's disease is a progressive neurodegenerative disorder associated with aging, which occurs both sporadically or due to familial passed mutations in genes associated with Alzheimer's pathology. Common symptoms associated with Alzheimer's disease include: memory loss, confusion, and mood changes.

Valentina Fossati is an Italian stem cell biologist. She is a Senior Research Investigator at the New York Stem Cell Foundation. Her research is focused on developing human stem cell-based models to study the role of glia in neurodegeneration and neuroinflammation.

<span class="mw-page-title-main">Assembloid</span> Biological model of 2 or more cell types

An assembloid is an in vitro model that combines two or more organoids, spheroids, or cultured cell types to recapitulate structural and functional properties of an organ. They are typically derived from induced pluripotent stem cells. Assembloids have been used to study cell migration, neural circuit assembly, neuro-immune interactions, metastasis, and other complex tissue processes. The term "assembloid" was coined by Sergiu P. Pașca's lab in 2017.

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