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. [1] 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. [2] [3] [4] The term "assembloid" was coined by Sergiu P. Pașca's lab in 2017. [5]
Assembloids were described in 2017 in a study from a laboratory at Stanford to model forebrain development. [5] [6] Assembloids joining ventral and dorsal forebrain neural organoids demonstrated that cortical interneurons migrate and integrate into synaptically connected cortical microcircuits. [5] This was confirmed by multiple research groups applying similar approaches to model regionalized organoid interactions and study interneuron migration. [7] [8] Assembloids have subsequently been generated to model projections between brain regions, such as cortico-striatal, [9] cortico-spinal, [10] or retino-thalamic. [11] Methods such as Cre recombination combined with G-deleted rabies tracing can be used to identify cells projecting within assembloids; additionally, optogenetic stimulation can demonstrate the assembly of functional neural circuits in vitro. [12]
Assembloid formation starts with the generation of organoids. Initially, human induced pluripotent stem (hiPS) cells are aggregated to generate regionalized organoids through directed differentiation. [2] There are multiple ways in which organoids can be assembled. Regionalized organoids can be put in close proximity resulting in their fusion to generate multi-region assembloids. [13] Alternatively, organoids can be assembled by co-culture with other cell lineages, such as microglia or endothelial cells, or with tissue samples from animal dissection, leading to multi-lineage assembloids. [14] Lastly, organoids can be assembled with morphogenic or organizer-like cells, thus generating polarized assembloids. [15]
The assembloid type depends on the scientific question and the accessibility of cell types required. Major biological fields utilizing the assembloid technique include cancer, gastroenterology, cardiology, and neuroscience. For instance, there are liver assembloids, [16] kidney assembloids, [17] pericytes assembloids to study SARS-COVID2, [18] endometrium assembloids, [19] , stomach and colon assembloids, [20] and bladder assembloids. [21]
Assembloids are composed of at least two organoids and/or cells derived from stem cells or primary tissue. They can be assembled to form multi-region or multi-lineage assembloids, as described above. [22]
A. Multi-region assembloids of the nervous system There are techniques to guide organoid differentiation into specific regions of the nervous system. For example, fusion of thalamic and cortical neural organoids models thalamo-cortical projections of ascending sensory input while cortico-striatal assembloids generate the initial projections of motor planning circuits. [8] [9] Forebrain assembloids model interneuron migration into the cerebral cortex. [5] Cortico-motor assembloids can reconstitute aspects of the cortico-spinal-muscle circuit in vitro. [10] Finally, retinal organoids can be combined with thalamic and cortical organoids to model aspects of the ascending visual pathway. [11]
B. Multi-lineage assembloids of the nervous system Some cell types of interest are challenging to differentiate within organoids but can be isolated from tissue explants or derived in monolayer culture. These tissue samples or enriched cell populations can then be integrated with organoid(s) of interest to study their interaction. For example, one current limitation of organoids and assembloids is their lack of functional vasculature, which hinders the supply of nutrients and trophic factors. In a technical advancement, researchers have been able to achieve vascularization by combining neural organoids with endothelial organoids and mesenchymal cells or human embryonic stem cell-derived vascular organoids. [23] [24] [25] Next, microglia-like cells derived from hiPS cells can be introduced into midbrain neural organoids to model neuro-immune interactions. [26] Similarly, oligodendrocytes can be generated in neural organoids and then migrate from the ventral forebrain to the dorsal forebrain. [27] [28] [29] Lastly, combining hiPS cell-derived intestinal organoids with neural crest cells can derive assembloids of the enteric nervous system. [30]
Additionally, assembloids can be categorized as inter-individual or inter-species, depending on whether the organoids are combined from different stem cell lines (e.g., control with disease-associated lines) or different species, respectively. [31] These combinations help determine what aspects of development are cell-autonomous.
Assembloids help determine the complex pathophysiology of developmental disorders. For example, Timothy syndrome, which affects L-type calcium channels, was modeled in neural assembloid experiments. When dorsal and ventral forebrain organoids were integrated into an assembloid, interneurons migrated into the dorsal cortical neurons. Timothy syndrome-derived interneurons showed impaired migration. [5] The resulting assembloids developed hypersynchronous neuronal activity, hypothesized to be due to abnormal interneuron integration into circuits. [22] Next, Phelan-McDermid syndrome, also known as 22q13.3 deletion syndrome, is a neurodevelopmental disorder with a high risk of autism spectrum disorder that was modeled in assembloids containing cortical and striatal organoids. This research demonstrated increased striatal medium spiny neuron activity in Phelan-McDermid-derived assembloids after fusion of striatal and cortical organoids but not in isolated striatal organoids. [9] Rett syndrome-derived assembloids displayed hypersynchronous activity perhaps due to an increase in calretinin interneurons. [32] Alzheimer's disease risk allele APOE4, which increases the risk of dementia, has been modeled in assembloids. [14] APOE4-derived assembloids of neural organoids combined with microglia demonstrated increased amyloid-beta-42 secretion, a known Alzheimer biomarker. APOE4 microglia in assembloids had a more complex morphology than in two-dimensional culture and had limited amyloid-beta-42 clearance.
Despite the research benefits of assembloids, as for any model system, they have limitations. First, assembloids, like organoids, lack vascularisation, which impairs nutrient diffusion to the surface and eventually leads to necrosis in the core, thus limiting their growth. [33] One way to address this limitation is through transplantation. Grafting cortical organoids into the brains of laboratory rats leads to improved growth and neural development. [34] Another critique of both assembloids and organoids is the lack of sensory input, which is important for the maturation and shaping of circuits during embryonic development. [35] Assembloids and organoids do not currently have a blood brain barrier or immune cells, limiting the biological validity for drug screening or disease modeling. [36] There is a temporal limitation on the investigation of clinically relevant pathophysiology; organoids most closely model initial developmental stages corresponding to fetal and infant neurodevelopment and thus may not accurately model later-onset psychiatric disorders or degenerative conditions. Future directions to address this limitation include studies to understand and accelerate developmental clocks. [37] Next, organoids and assembloids have batch-to-batch variability. Guided differentiation methods reduce variability significantly, yet reproducibility still requires optimization. [35] Finally, the derivation and maintenance of organoids and assembloids require expertise and can be time-intensive and expensive.
The striatum or corpus striatum is a cluster of interconnected nuclei that make up the largest structure of the subcortical basal ganglia. The striatum is a critical component of the motor and reward systems; receives glutamatergic and dopaminergic inputs from different sources; and serves as the primary input to the rest of the basal ganglia.
The development of the nervous system, or neural development (neurodevelopment), refers to the processes that generate, shape, and reshape the nervous system of animals, from the earliest stages of embryonic development to adulthood. The field of neural development draws on both neuroscience and developmental biology to describe and provide insight into the cellular and molecular mechanisms by which complex nervous systems develop, from nematodes and fruit flies to mammals.
The nigrostriatal pathway is a bilateral dopaminergic pathway in the brain that connects the substantia nigra pars compacta (SNc) in the midbrain with the dorsal striatum in the forebrain. It is one of the four major dopamine pathways in the brain, and is critical in the production of movement as part of a system called the basal ganglia motor loop. Dopaminergic neurons of this pathway release dopamine from axon terminals that synapse onto GABAergic medium spiny neurons (MSNs), also known as spiny projection neurons (SPNs), located in the striatum.
Interneurons are neurons that connect to brain regions, i.e. not direct motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enabling communication between sensory or motor neurons and the central nervous system (CNS). They play vital roles in reflexes, neuronal oscillations, and neurogenesis in the adult mammalian brain.
A gamma wave or gamma rhythm is a pattern of neural oscillation in humans with a frequency between 30 and 100 Hz, the 40 Hz point being of particular interest. Gamma rhythms are correlated with large-scale brain network activity and cognitive phenomena such as working memory, attention, and perceptual grouping, and can be increased in amplitude via meditation or neurostimulation. Altered gamma activity has been observed in many mood and cognitive disorders such as Alzheimer's disease, epilepsy, and schizophrenia.
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.
Neural stem cells (NSCs) are self-renewing, multipotent cells that firstly generate the radial glial progenitor cells that generate the neurons and glia of the nervous system of all animals during embryonic development. Some neural progenitor stem cells persist in highly restricted regions in the adult vertebrate brain and continue to produce neurons throughout life. Differences in the size of the central nervous system are among the most important distinctions between the species and thus mutations in the genes that regulate the size of the neural stem cell compartment are among the most important drivers of vertebrate evolution.
Neural tissue engineering is a specific sub-field of tissue engineering. Neural tissue engineering is primarily a search for strategies to eliminate inflammation and fibrosis upon implantation of foreign substances. Often foreign substances in the form of grafts and scaffolds are implanted to promote nerve regeneration and to repair damage caused to nerves of both the central nervous system (CNS) and peripheral nervous system (PNS) by an injury.
The development of the nervous system in humans, or neural development, or neurodevelopment involves the studies of embryology, developmental biology, and neuroscience. These describe the cellular and molecular mechanisms by which the complex nervous system forms in humans, develops during prenatal development, and continues to develop postnatally.
The primary motor cortex is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex, and several subcortical brain regions, to plan and execute voluntary movements. Primary motor cortex is defined anatomically as the region of cortex that contains large neurons known as Betz cells, which, along with other cortical neurons, send long axons down the spinal cord to synapse onto the interneuron circuitry of the spinal cord and also directly onto the alpha motor neurons in the spinal cord which connect to the muscles.
In the human brain, the nucleus basalis, also known as the nucleus basalis of Meynert or nucleus basalis magnocellularis, is a group of neurons located mainly in the substantia innominata of the basal forebrain. Most neurons of the nucleus basalis are rich in the neurotransmitter acetylcholine, and they have widespread projections to the neocortex and other brain structures.
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.
Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). This occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs), radial glial cells (RGCs), basal progenitors (BPs), intermediate neuronal precursors (INPs), subventricular zone astrocytes, and subgranular zone radial astrocytes, among others.
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.
Madeline Lancaster is an American developmental biologist studying neurological development and diseases of the brain. Lancaster is a group leader at the Medical Research Council (MRC) Laboratory of Molecular Biology in Cambridge, UK.
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.
Brain cells make up the functional tissue of the brain. The rest of the brain tissue is structural or connective called the stroma which includes blood vessels. The two main types of cells in the brain are neurons, also known as nerve cells, and glial cells, also known as neuroglia.
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.
Kristen Kroll is an American developmental and stem cell biologist and Professor of Developmental Biology at Washington University School of Medicine. Her laboratory studies transcriptional and epigenetic regulation of brain development and its disruption to cause neurodevelopmental disorders.
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. Myelinoids have the capacity to recapitulate aspects of brain developmental processes, microenvironments, cell to cell interaction, structural organization and cellular composition. 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.
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