Brain development and Neural Stem Cells
The human brain is the most complex and fascinating of all organs. We are interested in understanding how a relatively small number of stem and progenitor cells is able to generate the complex structure of the brain during development. Our group uses Drosophila, mouse and human genetics to understand how neural stem cells generate the right neurons at the right time, and how defects in neurogenesis lead to the formation of brain tumors or heritable brain disorders.
Neural Stem Cells in Drosophila
In the fruit fly Drosophila, about 400 neural stem cells known as neuroblasts create all neurons and glia cells in the adult brain (Fig. 1A). For this purpose they undergo repeated asymmetric cell divisions, giving rise to a large daughter cell that remains a dividing stem cell and a smaller daughter that differentiates after a limited number of transit amplifying divisions. During each neuroblast division, the cell fate determinants Numb, Prospero and Brat segregate into the smaller daughter cell where they prevent self-renewal and induce differentiation (Fig. 1A, B). When they are missing, both daughter cells become stem cells, leading to exponential proliferation and the formation of a lethal and transplantable brain tumor.
Eventually, neuroblasts exit the cell cycle and differentiate; there is no proliferation in the adult brain. We found that this is due to a cell-intrinsic mechanism that uncouples cell cycle progression from cell growth in a precisely defined period during development, resulting in cell shrinkage and differentiation. In a transgenic RNAi screen for factors responsible for neuroblast shrinkage, we identified several components of the mitochondrial respiratory chain. Our genetic and biochemical experiments support a model in which a change in energy metabolism induced by Ecdysone is responsible for reduced cell growth. We propose that the induction of oxidative phosphorylation deprives cells of the building blocks for lipid and amino acid biosynthesis, which accumulate as end products of glycolysis. Our data show that changes in energy metabolism may be a cause rather than a consequence of changes in the fate of the cell. Furthermore, they reveal a surprising connection between energy metabolism and stem cell self-renewal, which has not been observed earlier in vivo.
Brain development in humans and neurological disorders
The human brain is unique in terms of its size and complexity. While many of its characteristics have been successfully studied in model organisms, recent experiments have disclosed unique features that cannot easily be modeled in animals. We developed a 3D organoid culture system that permitted us to generate human brain tissue, starting from pluripotent stem cells (Lancaster et al., Nature 2013). Our culture model recapitulates the three-dimensional architecture of the developing human cortex in remarkable detail (Fig. 3). Cerebral organoids contain the human dorsal and ventral cortex, the choroid plexus, retina, and occasionally the hippocampus. They recapitulate human-specific cortical features such as the presence of an outer subventricular zone or an inner fiber layer. Furthermore, the stem cell properties and progenitor zone organization of human cerebral organoids is marked by characteristics very specific to humans. Most importantly, our organoid protocol can be combined with cellular reprogramming to model the genetically determined characteristics of brain development in any human individual.
We determined the power of this approach by modeling microcephaly, a genetic disorder resulting in a severe reduction of cortical volume and, consequently, intellectual ability. Our experiments revealed a premature switch from symmetric and expansive to asymmetric neurogenic progenitor cell divisions in microcephalic organoids. We showed that mutations in the centrosomal gene CDK5Rap2 lead to defects in mitotic spindle orientation, which were responsible for the differentiation defect in the specific patient we analyzed. We are currently extending this approach to other more common neurological disorders. In these settings organoid systems offer – for the first time – the possibility to recapitulate human disease without the need for animal experiments. Our work has set a precedence, showing that the development and physiology of even the most complex human organ can be recapitulated in 3D culture. It opens revolutionary possibilities for the analysis of neurological disorders and will hopefully translate into novel therapeutic strategies for these devastating diseases.