Spatiotemporal pattern of Wnt-canonical pathway in dorsal telencephalic organoids.

In the last decade, organoid technology has filled the gap generated by the poor accessibility and unique features of the human brain. Organoids present several advantages in comparison with two-dimensional culturing, since they recapitulate cytoarchitecture in space and functionality of the tissue. Therefore, they mimic the unique features of human brain better than animal models. Although significant discoveries having already been achieved using this system, as a new technology it is still a major challenge. Hence, the aim of this work is to develop and understand the mechanism of this technique for an extensive and revolutionary further application in biomedical research. We focus our study on the Wnt pathway, an essential signalling pathway in the developing brain and its organization and patterning. Since organoids lack external anatomical patterning molecules, it is important to understand how they self-organize and if they generate signalling centres. In particular, we studied the cortical hem in the forebrain, a well-known centre that releases Wnt ligands and is crucial for patterning and formation of the cortical and hippocampal structures. Understanding the physiological scenario for Wnt signalling in organoids will allow us to study pathologies in what this pathway is altered i.e. FOXG1 syndrome.

Our results show, for the first time, a spatiotemporal pattern in canonical Wnt activity in forebrain organoids. This activity pattern is consistent with the described changes of cell identity in the organoid during its formation. At late stages, when the organoids have achieved cortical telencephalic fate, Wnt/ß-catenin dependent pathway activity was restricted to highly-localized regions that we suggest could be analogous to the embryonic cortical hem by expressing some of the morphogens expressed in vivo in that location: Wnt2b, Wnt3a, Wnt7b and Wnt8b. In conclusion, our findings allowed us to move one step closer in the understanding of Wnt signalling in cerebral organoids.