ASSEMBLOIDS: AN INSIGHT INTO NEURAL CIRCUITS
Sheen Tukra1*
1 Division of Veterinary Pharmacology and Toxicology, Faculty of Veterinary Science & Animal husbandry, S.K.U.A.S.T Jammu, 181102, India.
Corresponding author email: sheentukra97@gmail.com
Abstract
In order to understanding how organs organize and how physiology can be disturbed in disease requires first dissecting and then reassembling developmental processes ex vivo. Organoids and other human 3D stem cell derived systems have made this endeavor easier, but they frequently fail to record the cellular interactions across tissues or between lineages that result in emergent tissue characteristics during development. Assembloids are 3D biological systems that self organize when many organoids are integrated or when organoids with primary tissue explants or missing cell types are combined. The idea and types of assembloids are described here, along with their uses in the study of the nervous system and other tissues. We outline existing issues and the possibilities of this novel method to examine growth and illness, as well as the instruments used to probe and manipulate assembloids.
Key Words: Assembloids, Biological systems, Cellular interactions, Organoids.
Introduction
Assembloids are mainly three dimensional (3D) cell cultures that are constitute by combining various types of organoids or combining certain cell types with self-organizing characteristics. Therefore, a self-organizing three-dimensional in vitro culture system that replicates a particular function of an organ or a portion of an organ and contains a variety of specialized cell types is called an organoid. A paradigm for the nomenclature of neuronal organoids and assembloids has been proposed recently. The multi-region assembloids are created by assembling many organoids. Human induced pluripotent stem cells (hiPSCs), human embryonic stem cells (hESCs), or even primary tissue sources such isolated adult stem cells, excised tumors with growth potential, or fetal tissue can be used to create these organoids. As an alternative to traditional organoids system, “multi-lineage assembloids” can be created by integrating specialized cell types that have been isolated from primary tissue or that were produced from stem cells through controlled differentiation in 2D cultures. “Inter-individual assembloids” are made from the organoids of different individuals and are used to investigate cell-autonomous effects; “inter-species assembloids” are made by merging organoids from different species, such as chimps and humans. Cellular self-organization and novel emergent characteristics, such as cell fate specification, morphological alterations, accelerated maturation, or circuit development, are typically required to accompany the assembly of these components.
Multiple-region nervous system assembloids
Till now many of two part assembloid systems have been introduced to simulate various inter regional interactions.
Dorsal and ventral forebrain organoids can be used to simulate the migration and integration of interneuron’s generated in the ventral forebrain into dorsal cortical circuits. This migration is based on nucleokinesis and is followed by morphological and synaptic integration with glutamatergic neurons. This assembloid system demonstrated how the morphology and migratory dynamics of humans and other species varied. Circuit creation and nervous system maturation depend on axonal connections between various brain areas. Input from the thalamus into the cortex is essential for cortical maturation, and the thalamus acts as a hub for information relaying between the cortex and several other brain regions. Thalamic and cortical organoids can be put together to create bi-lateral projections in order to simulate thalamo cortical interactions. In addition to controlling motivated actions, corticostriatal circuits are implicated in a number of illnesses and disorders, such as schizophrenia and autism spectrum disorder. Only cortical neurons project into the striatum when striatal and cortical organoids are combined as a reminiscent of the circuitry directionality. This causes medium spiny neurons to become more intrinsically active, exposing disease related abnormalities in patient derived assembloids. The combining of cortical, spinal, and skeletal muscle organoids, cortico-spinal projections can be used to investigate different affections of these systems. In these assembloids, contraction is accomplished through electrical or optogenetic activation of cortical glutamatergic neurons. Last but not least, retino thalamic and thalamo cortical projections can be created by assembling retinal organoids with thalamic and cortical organoids to simulate elements of the ascending visual pathway.
Difference type of Assembloids
The absence of functional vasculature to provide nutrition and other trophic elements is a drawback of organoids and assembloids. Brain organoids have been produced from ETV2-induced endothelial differentiation or integrated with hESC-derived vascular organoids to accomplish vascularization. They have also been mixed with endothelial organoids and mesenchymal cells. Neural crest-derived pericytes have a strong relationship with astrocytes and endothelial cells. As a result, the differentiated pericytes free basement membrane components that resulted in more mature astrocytes when hiPSC-derived neural crest cells were united with cortical organoids. The immune cells of the central nervous system, known as microglia, are essential for infection defense, neurodegeneration, and the phagocytosis of apoptotic cells. Microglia-like cells made from hiPSCs can be added to midbrain and cortical organoids to simulate neuro-immune interactions. Myelinating support cells in the brain called oligodendrocytes frequently have to migrate in order to get to their destination, such as from the ventral to the dorsal forebrain. It is possible to create oligodendrocytes in neural organoids, and it is theoretically possible to model their migration by creating two-part assemblies. Finally, intestine organoids created from hiPSCs can be combined with neural crest cells to create assembloids of the peripheral nervous system (such as the enteric). Hepato-biliary-pancreatic domains can be produced by juxtaposing anterior and posterior gut organoids in order to explore the specification of and interactions between endoderm-derived organs. Furthermore, long-term bladder organoids can be combined with muscle cells and stromal fibroblasts to create multilayered bladder assembloids, which may promote the maturation of bladder cells. Due to ethical and technical constraints, studying the very early phases of human development such as embryo implantation and gastrulation—is difficult. Indeed, understanding the early phases of embryogenesis and organogenesis can be aided by modeling interactions between the different cell lineages of the blastocyst and the uterus. Post-implantation phases of embryonic development can be studied using assembloids made by fusing extra-embryonic cells and hESC-derived epiblasts. Furthermore, senescent cells play a crucial role in averting implantation failures, according to endometrial assembloids produced by reassembling stromal and epithelial cell fractions from endometrial biopsies. By producing organizer-like cellular structures that can be incorporated into organoids to produce “polarized organoids,” another kind of assembloid seeks to replicate spatiotemporal patterning. When hPSCs are incorporated into early-stage forebrain organoids, for example, they can be made to express sonic hedgehog (SHH), which causes local patterning and spatial organization within the organoid. This, in turn, leads to the formation of dorsal and ventral forebrain, hypothalamic, and diencephalic domains.
Applications of assembloids in disease modeling
Disease characteristics have been modeled using assemblages made from patient cells or genetically modified cell lines. For example, a deficiency in cortical interneuron migration and function has been discovered in forebrain assembloids taken from patients with Timothy syndrome (TS), a hereditary condition linked to autism and epilepsy. The changes in actomyosin signaling and GABAergic receptor sensitivity are linked to TS neuron’s shorter saltation length and higher saltation frequency. Patients with Phelan-McDermid syndrome, who frequently lose the striatal-enriched SHANK3 gene, produce cortico-striatal assembloids that have decreased network synchronization and elevated calcium activity. Crucially, non-assembled striatal organoids do not exhibit this deficit, underscoring the significance of the interaction with cortical glutamatergic neurons in exposing this functional abnormality, which would be difficult to analyze in an animal model. Assembloids that are combined used vascular cells and immunological components have shown some of the cellular mechanisms of SARS-CoV-2 and Zika virus infection in central nervous system. Pericytes have been identified as replication hubs for the virus’s propagation to astrocytes, as evidenced by the SARS-CoV-2 infection of assembloids created by combining cortical organoids and pericytes. Zika virus-infected neuro-immune assembloids (cortical organoids and microglia) have demonstrated synaptic pruning during infection. Enteric nervous system neurons can be built with intestinal organoids to explore neuro-intestinal interactions in Hirschsprung’s illness. This reveals that a mutation in PHOX2B results in problems in neural crest cell development and functional integration with smooth muscle cells. The Combining glioblastoma tumor organoids (GBOs) or assembloids of GBOs and neural organoids with engineered T cells [chimeric antigen receptor T cells (CAR-Ts)] used to examine patient-specific responses to different cell therapies or autoimmune reactions is one example of how the assembloids made from tumor organoids and stem cell derived organoids hold promise for examining invasion into healthy tissues and screening for targeted therapies. Microenvironment modification frequently plays a role in the genesis and development of malignancies. Bladder assembloids have been used to study invasiveness into the surrounding stromal and muscle tissue as well as responses to T cells. They have also been used to model various types (basal and luminal) of urothelial carcinomas by combining tumor organoids with endothelial cells and matched fibroblasts.
Resources for working with assembloids
A variety of tools have been created and used to describe and work with assembloid’s constituent parts. Gene regulatory dynamics across cell types can be studied or cellular diversity in assembled components can be characterized using single cell RNA sequencing and chromatin accessibility (ATAC-seq, CUT&RUN) tests. These methods are especially useful for identifying more subtle transcriptional alterations that take place when various organoids assemble after migration or axonal projection. Future research into tissue architecture in organoids and assembloids, as well as the computational reconstruction of cellular crosstalk of interacting cells in assembloids, will be made possible by high-resolution spatial transcriptomics. Axonal projections have been specially used cell type with specific viral tagging efficiency, including retrograde rabies tracing, and imaging of projections of assembloids. After opsins and genetically encoded calcium indicators are delivered, light-induced activity on a single assembloid segment can be recorded to investigate functional connectivity. As an alternative, patch-clamping in slice culture can be used to record the electrical characteristics of cells. Long-term, non-invasive, and large-scale electrophysiological investigations of assembloids are probably possible due to recent advancements in extracellular recordings and flexible electrodes.
Future trends and challenges
Assembloids have already shown promise in revealing the pathophysiology of disease and mimicking intricate cell-cell interactions both within and between tissues. There are still a lot of restrictions to be fixed and features to add in the future. The complexity of current assembloid models is still restricted when compared to the intricate inter regional interactions in the brain and the inter organ crosstalk in the body. However, three, four, or even five-part assembloids will be constructed as a result of recent developments in strengthening culture conditions and specifying additional nervous system regions in 3D cultures. Recent advancements in automating organoid phenotyping and large scale manufacture. Additionally, it will make screening for intercellular faults and circuit characteristics easier. Because primary tissue is inaccessible, benchmarking assembloids to in vivo tissues and circuits is still difficult.A useful method for investigating the maturation potential of assembloids is transplantation into rodents, followed by vascularization of the grafts. Together with new genomic and functional technologies, assembloids and organoids will continue to provide insights into human biology and disease mechanisms.
REFERENCES
- Barmpa, K., Saraiva, C., Lopez-Pigozzi, D., Gomez-Giro, G., Gabassi, E., Spitz, S., & Schwamborn, J. C. (2024). Modeling early phenotypes of Parkinson’s disease by age-induced midbrain-striatum assembloids. Communications biology, 7(1): 1561.
- Kanton, S., & Paşca, S. P. (2022). Human assembloids. Development, 149(20), dev201120.
- Levy, R. J., & Paşca, S. P. (2023). What have organoids and assembloids taught us about the pathophysiology of neuropsychiatric disorders?. Biological Psychiatry, 93(7): 632-641.
- Makrygianni, E. A., & Chrousos, G. P. (2021). From brain organoids to networking assembloids: implications for neuroendocrinology and stress medicine. Frontiers inphysiology, 12(2): 621970.
- Marton, R. M., & Pașca, S. P. (2020). Organoid and assembloid technologies for investigating cellular crosstalk in human brain development and disease. Trends in cell biology, 30(2): 133-143.
- Miura, Y., Li, M. Y., Revah, O., Yoon, S. J., Narazaki, G., & Pașca, S. P. (2022). Engineering brain assembloids to interrogate human neural circuits. Nature protocols, 17(1): 15-35.
- Pan, T., Jaklic, D. C., Vaid, S., Lin, G., VanHeyningen, D., & Dang, L. T. (2024). A Multielectrode Array Platform for Modeling Epilepsy Using Human Pluripotent Stem Cell-Derived Brain Assembloids. Journal of visualized experiments: JoVE, 254(211): 10-3791.
- Paşca, S. P. (2019). Assembling human brain organoids. Science, 363(6423): 126-127.
- Pașca, S. P., Arlotta, P., Bateup, H. S., Camp, J. G., Cappello, S., Gage, F. H., & Young-Pearse, T. (2025). A framework for neural organoids, assembloids and transplantation studies. Nature, 639(8054): 315-320.
- Vogt, N. (2021). Assembloids. Nature Methods, 18(1): 27-27.
