Final Report Summary - NEUROSYSTEM (A Systems Level Approach to Proliferation and Differentiation Control in Neural Stem Cell Lineages) Our brain is the most complex organ nature has generated. Despite its complexity, however, it develops within a few months from a limited set of neural stem cells. The goal of this research project was to decipher the rules of brain development using a simple and easily accessible animal model, the fruitfly Drosophila. The fly brain develops from neural stem cells called neuroblasts. Neuroblasts divide into two different daughter cells, one of which remains a neuroblast while the other eventually stops dividing and specializes to become a nerve or glia cell. We were able to develop methodology for isolating those neuroblasts and their specialized daughter cells in large numbers. This has allowed us to identify all the genes that are active in the two cell types. We could find a set of key regulators that maintain the proliferative state in the neuroblast and the principles for how those regulators are turned off in the specializing cells. We were able to demonstrate that a large complex of proteins called the SWI/SNF complex is responsible to ensure that the specializing cells do not revert back into neuroblasts. We could identify the genes that are regulated by SWI/SNF in order to perform this task and develop a model for how this occurs.Although neuroblasts divide perpetually during development, they eventually disappear. This process of stem cell differentiation also occurs in our own brain where defects in cell cycle exit can lead to the formation of child tumors. We could identify the mechanism that is responsible for the timely stop in neuroblast division. Much to our surprise, we found that a change in how neuroblasts deal with nutrients is responsible for this. Normally, sugars like glucose are converted to other products that in turn can be used to build cellular components required for growth. At a certain time in development, however, a hormonal signal tells the cell to burn the sugars into carbon dioxide instead. This is called aerobic metabolism as it uses Oxygen and occurs in specialized organelles called mitochondria. We could show that this switch in metabolism is the key event that ends neuroblast divisions as it deprives cells of their building blocks for growth. As a consequence, they reduce their volume with each division until they are too small to divide and form specialized neurons. Ultimately, we want to apply our findings from fruit flies to the human brain. This is difficult as experiments in humans are impossible and experiments in highly developed animals are ethically problematic. To develop an alternative strategy, we have developed a 3-dimensional cell culture method where we build human brain tissue starting from pluripotent stem cells. Our method can recapitulate the first few weeks of human brain development in an incubator with remarkable precision. The human cortex, the largest and most complex area of our brain, is recapitulated particularly well. As we can start the culture with stem cells originating from any human individual, we can recapitulate individual brain formation and neurological disease. We have used stem cells from a patient suffering from a severe brain disorder named microcephaly where brain volume and number of neurons is dramatically reduced, We were able to recapitulate those defects in our culture model and could identify a potential mechanism that may be the root cause for this disease. Our model has enormous potential, not only for transferring our results from fruitflies to humans but also for generating culture models for major human brain disorders allowing drugs and chemicals to be tested in human tissue directly without the need for animal models.
