Brain organoids as 3D in vitro models to investigate neurodevelopment and neurodegenerative diseases

Organoids are innovative self-organized 3D structures that are obtained from stem cells (embryonic or induced pluripotent stem cells) and grow in vitro to recreate models of specific organs, such as liver, gastrointestinal tract or brain. Within brain organoids, specific regions -such as the thalamus, the neocortex or the mesencephalon- can be grown in vitro, representing useful and complex tools to be studied from a cellular, molecular, physiological and structural point of view. Brain organoids can be obtained starting from pluripotent stem cells by two methodologies, the unguided and the guided methods, that are based, respectively, on the spontaneous differentiation of stem cells into mature neurons, to recreate a more complex structure, or on the addition of inductive signals to mimic the first stages of neurodevelopment. The results of these processes are 3D-organized in vitro structures, capable of generating consistent proportions of cell types, that show a well-defined architecture, that can be used instead of traditional 2D cultures to study cell-type diversity generation during brain development and the formation of brain cytoarchitecture. Moreover, iPSC-derived brain organoids can be used as models of neurodegenerative diseases to go beyond the use of animal models. Finally, organoids derived from different brain regions can be fused to study in vitro the formation of cellular connectivity between brain areas, named “assembloids”. In this thesis, I reported three papers that highlight the importance of cerebral organoids in the study of neurodevelopmental processes and in the investigation of neurodegenerative diseases. In particular, the first chosen paper demonstrates that organoids are effective models that can reliably generate a rich variety of cell types, appropriate for the human cerebral cortex, and that the similarity between the maturation of the organoid and the development of the cerebral cortex is maintained over time, with a high level of reproducibility between experimental conditions. The second work shows the possibility to model Alzheimer Disease (AD) with brain organoids, and highlights that these AD organoids are able to express the main hallmarks of the pathology, p-tau and Aβ amyloid aggregates, thus representing a proper model to study AD in vitro. The last article highlights the possibility to use fused organoids to study those mechanisms of brain development that are involved in the origin of specific cellular types and in the formation of functional circuits, together with the interaction of different brain regions (e.g. cortex and thalamus) during neurodevelopmental processes. Overall, the papers selected for this thesis demonstrate that brain organoids display structures that resemble defined brain regions and simulate specific changes of neurological disorders; thus, becoming an excellent model for investigating brain development and neurological diseases. In this perspective, organoids and fused organoids can be used to study how dysfunctions in a cerebral area can affect other brain regions; moreover, organoids can represent in vitro complex models to screen novel molecules for efficacy.