The project has involved several areas of research with important contributions to the scientific community and dissemination more broadly. Specifically, we have developed several new tools, as outlined in section 1 below, and used those tools to investigate evolutionary mechanisms of human brain expansion, as outlined in section 2 below.
1) Next generation brain organoids methods and their characterization
Over the course of this project, we have developed several new organoid methods, and carefully characterised the key aspects of their generation and use. First, we have developed a new method to mature brain organoids and achieve functional neural networks. This method was published in Nature Neuroscience in 2019, and disseminated at international meetings including the FENS summer school in 2018. The method is now being used within the lab, but also by at least a dozen labs in Europe and beyond to investigate human neuronal function.
Second, using these more mature new organoids, we developed a method to perform correlative light and electron microscopy using cryo tomography on organoids, specifically on the axons projecting from human neurons within. This allowed us to investigate the ultrastructure with very high resolutions, revealing unique endoplasmic reticulum morphologies and a paucity of ribosomes. The method resulted in a large number of datasets that can be used for further investigation of human axon biology, and was published in eLife in 2021, along with all the data which was made publicly available.
Third, we have developed a new organoid of the choroid plexus, the brain region responsible for generating cerebrospinal fluid. These organoids form a selective barrier, much like the actual barrier protecting the brain and spinal cord from blood-borne factors and pathogens, as well as secreting cerebrospinal fluid that can be used to investigate normal function as well as biomarkers. This new method was published in Science in 2020, presented at various meetings including the International Society of Stem Cell Research. It has been used in the lab to investigate how the virus causing COVID-19, SARS-CoV-2, enters and infects the brain, which we published in Cell Stem Cell in 2020 and was widely disseminated also in the popular press.
Finally, this project investigated how well different organoids model the human brain and which features of their morphology are key to their proper development. This work led to identification of several key methodological factors that lead to better quality organoids, and reveal an important role for tissue architecture in temporal progression of cell fate decisions. This study was published in Cell Stem Cell in 2023 and disseminated at several international meetings including an EMBO conference in 2022.
2) Using organoids to uncover human-specific features of brain development
Using these state-of-the-art organoid methods, we have uncovered new biological insight into human-specific neurodevelopment. First, by generating brain organoids from human cells and those of closely related apes, we uncovered a difference in neural stem cell behaviour, whereby human neural stem cells go through the early transition to neurogenesis more slowly that apes, leading to a larger founder progenitor pool that is able to generate more neurons. This explains how humans end up with a greater number of neurons than other apes, and thus answers an age-old question. This work was published in Cell in 2021, and was covered in various news outlets and presenting at several international meetings. Second, we investigated the potential role for sex hormones in brain organoids, revealing a role for androgens in the production of excitatory neurons, a role that seems to diverge in humans compared with mice. Thus, again, organoids have revealed evolutionary unique biology that may shed light on important differences in brain development in the presence of androgens like testosterone. This work was published in Nature in 2022 and presented at several international meetings.
