Untersuchung der molekularen Mechanismen, die der Restriktion des neurogenen Potentials von Astrozyten zugrunde liegen
Zusammenfassung der Projektergebnisse
Astrocytes are a major population of glial cells, which are essential for normal brain function and possess a limited neural-stem-cell-like potential to generate new neurons. Maturation of astrocytes during late brain development is required to establish their essential homeostatic functions, but restricts their ability to generate new neurons. Therefore, understanding the regulation of astrocyte maturation is crucial to understand how defects of astrocyte function contribute to neurological disorders, and it might open up approaches to stimulate endogenous brain repair by reactivating the neural-stem-cell-potential of astrocytes. However, the mechanisms underlying astrocyte maturation are largely unknown. The main aim of the present project was to investigate mechanisms how astrocytes mature, and how this restricts their neurogenic potential. Initially this project studied the changes in gene expression and the epigenetic state (chromatin state) during maturation in astrocytes purified from mouse brain. These analyses showed major changes establishing transcriptional programmes required for mature astrocyte function and restricting neural stem cell programmes. Changes in gene expression were strongly linked to chromatin changes, showing that maturation is controlled by chromatin remodelling. These analyses also identified potential regulators of this process. Interestingly, while some regulators present in mature astrocytes seem to promote establishment of mature astrocyte functions, regulators present in immature astrocytes seem to be required to maintain an immature chromatin state allowing the activation of neurogenic programmes. To investigate the functions of these candidate regulators in more detail, an in vitro model of astrocyte differentiation was established. These astrocytes in culture showed incomplete maturation, which could be improved by adapting culture conditions, demonstrating that extrinsic signals regulate maturation. These signals promote maturation by activating expression of intrinsic maturation regulators and induce a more mature chromatin state. Some of these intrinsic maturation regulators were validated by genetica activation in astrocytes in vitro, which demonstrated their ability to induce mature astrocyte genes. In conclusion, these results suggest a model that astrocyte maturation is controlled by extrinsic signals inducing expression of cell-intrinsic regulators, which subsequently remodel the epigenetic state of astrocytes to allow induction of effector genes required for their mature functions. The results of this project also suggest that specific signals might be required to actively maintain the potential of immature astrocytes to generate new neurons. Future research will have to investigate whether reactivation of these signals can reactivate the ability of adult astrocytes to generate neurons, which might be a way to stimulate endogenous brain repair.
