Understanding the complexities of the human brain and its associated disorders poses a significant challenge in neuroscience. Traditional research methods have limitations in replicating its intricacies, necessitating the development of in vitro models that can simulate its structure and function. Three-dimensional in vitro models, including organoids, cerebral organoids, bioprinted brain models, and functionalized brain organoids, offer promising platforms for studying human brain development, physiology, and disease. These models accurately replicate key aspects of human brain anatomy, gene expression, and cellular behavior, enabling drug discovery and toxicology studies while providing insights into human-specific phenomena not easily studied in animal models. The use of human-induced pluripotent stem cells has revolutionized the generation of 3D brain structures, with various techniques developed to generate specific brain regions. These advancements facilitate the study of brain structure development and function, overcoming previous limitations due to the scarcity of human brain samples. This technical review provides an overview of current 3D in vitro models of the human cortex, their development, characterization, and limitations, and explores the state of the art and future directions in the field, with a specific focus on their applications in studying neurodevelopmental and neurodegenerative disorders.

Maintaining the excitability of neurons and circuits is fundamental for healthy brain functions. The global compensatory increase in excitatory synaptic strength, in response to decreased activity, is one of the main homeostatic mechanisms responsible for such regulation. This type of plasticity has been extensively characterized in rodents in vivo and in vitro, but few data exist on human neurons maturation. We have generated an in vitro cortical model system, based on differentiated human-induced pluripotent stem cells, chronically treated with tetrodotoxin, to investigate homeostatic plasticity at different developmental stages. Our findings highlight the presence of homeostatic plasticity in human cortical networks and show that the changes in synaptic strength are due to both pre- and post-synaptic mechanisms. Pre-synaptic plasticity involves the potentiation of neurotransmitter release machinery, associated to an increase in synaptic vesicle proteins expression. At the post-synaptic level, we report an increase in the expression of post-synaptic density proteins, involved in glutamatergic receptor anchoring. These results extend our understanding of neuronal homeostasis and reveal the developmental regulation of its expression in human cortical networks. Since induced pluripotent stem cell-derived neurons can be obtained from patients with neurodevelopmental and neurodegenerative diseases, our platform offers a versatile model for assessing human neural plasticity under physiological and pathological conditions.

INTRODUCTION: The intronic MAPT mutation IVS10+16 is linked to familial frontotemporal dementia, causing hyperphosphorylation and accumulation of tau protein, resulting in synaptic and neuronal loss and neuroinflammation in patients. This mutation disrupts MAPT gene splicing, increasing exon 10 inclusion and leading to an imbalance of 3R and 4R Tau isoforms.METHODS: We generated patterned cortical organoids from isogenic control and mutant human induced pluripotent stem cell (iPSC) lines. Nanostring gene expression analysis, immunofluorescence, and calcium imaging recordings were used to study the impact of the MAPT IVS10+16 mutation on neuronal development and function.RESULTS: Tau mutant cortical organoids showed altered mitochondrial function and gene expression related to neuronal development, with synaptic markers and neuronal activity reduction. Bezafibrate treatment restored mitochondrial content and rescued synaptic functionality and tau physiology.DISCUSSION: These findings suggest that targeting mitochondrial function with bezafibrate could potentially reverse tau‐induced neurodevelopmental deficits, highlighting its therapeutic potential for tauopathies like frontotemporal dementia.Highlights: The IVS 10+16 MAPT mutation significantly disrupts cortical differentiation and synaptic maturation, evidenced by downregulated genes associated with synapses and neuronal development. Tau‐mutant cortical organoids exhibit mitochondrial dysfunction, with fewer and smaller mitochondria alongside tau hyperphosphorylation and aggregation, which further contribute to neuronal damage and disease progression. Treatment with bezafibrate effectively normalizes mitochondrial parameters, enhances neuronal integrity and synaptic maturation, and restores network functionality, showcasing its promise as a therapeutic strategy for tauopathies. The 3D in vitro disease model used in this study proves valuable for studying tauopathies and testing new drugs, effectively mimicking key aspects of tau‐related neurodegeneration.