The emergence of human pluripotent stem cells (PSCs) represents a excellent tool for regenerative medicine and disease modeling, because these cells can be differentiated into most cell lineages of the human body1,2. Since their discovery, PSC differentiation using diverse approaches has been reported to model different diseases, including neurodegenerative disorders3,4,5,6.
Recently, there have been reports of 3D cultures derived from PSCs resembling human cerebral structures; these are called brain organoids3,7,8. The generation of these structures from both healthy and patient-specific PSCs provides a valuable opportunity to model human development and neurodevelopmental disorders. However, the methods used to generate these well-organized cerebral structures are difficult to apply for their large-scale production. To produce structures that are large enough to recapitulate tissue morphogenesis without necrosis inside the organoids, protocols rely on the initial neural commitment in static conditions, followed by encapsulation in hydrogels and subsequent culture in dynamic systems3. However, such approaches may limit the potential scale-up of organoid production. Even though efforts have been made to direct PSC differentiation to specific regions of the central nervous system, including cortical, striatal, midbrain, and spinal cord neurons9,10,11,12, the generation of specific brain regions in dynamic conditions is still a challenge. In particular, the generation of mature cerebellar neurons in 3D structures has yet to be described. Muguruma et al. pioneered the generation of culture conditions that recapitulate early cerebellar development13 and recently reported a protocol that allows for human embryonic stem cells to generate a polarized structure reminiscent of the first trimester cerebellum7. However, the maturation of cerebellar neurons in the reported studies requires the dissociation of the organoids, sorting of cerebellar progenitors, and coculture with feeder cells in a monolayer culture system7,14,15,16. Therefore, the reproducible generation of the desired cerebellar organoids for disease modeling under defined conditions is still a challenge associated with culture and feeder source variability.
This protocol presents optimal culture conditions for 3D expansion and efficient differentiation of human PSCs into cerebellar neurons using single-use vertical wheel bioreactors (see Table of Materials for specifications), hereafter called bioreactors. Bioreactors are equipped with a large vertical impeller, which in combination with a U-shaped bottom, provide a more homogeneous shear distribution inside the vessel, allowing gentle, uniform mixing and particle suspension with reduced agitation speeds17. With this system, shape and size-controlled cell aggregates can be obtained, which is important for a more homogeneous and efficient differentiation. Moreover, a larger number of iPSC-derived organoids can be generated in a less laborious manner.
The main feature of the organoids, which are 3D multicellular structures usually formed from stem cells, is the self-organization of different cell types that forms specific shapes like those seen in human morphogenesis18,19,20. Therefore, organoid morphology is an important criterion to be evaluated during the differentiation process. Cryosectioning of organoids and further immunostaining of organoid slices with a specific set of antibodies allow for the spatial visualization of molecular markers to analyze cell proliferation, differentiation, cell population identity, and apoptosis. With this protocol, by immunostaining organoid cryosections, an initial efficient neural commitment is observed by the 7th day of differentiation. During differentiation, several cell populations with cerebellar identity are observed. After 35 days in this dynamic system, the cerebellar neuroepithelium organizes along an apicobasal axis, with an apical layer of proliferating progenitors and basally located postmitotic neurons. During the maturation process, from days 35–90 of differentiation, distinct types of cerebellar neurons can be seen, including Purkinje cells (Calbindin+), granule cells (PAX6+/MAP2+), Golgi cells (Neurogranin+), unipolar brush cells (TBR2+), and deep cerebellar nuclei projection neurons (TBR1+). Also, a nonsignificant amount of cell death is observed in the generated cerebellar organoids after 90 days in culture.
In this system, human iPSC-derived organoids mature into different cerebellar neurons and survive for up to 3 months without the need for dissociation and feeder coculture, providing a source of human cerebellar neurons for disease modeling.