From Sox9 to astrocytes - Intrinsic and extrinsic influences that regulate the “neuron/glial switch” in adult neural stem cells of the hippocampus

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Due to its connectivity and plasticity, the dentate gyrus (DG) of the hippocampus is a unique structure in the brain. Persisting adult neurogenesis, the generation and integration of new neurons by neural stem cells (NSCs), warrants an individual´s ability to learn and adapt to a changing environment. While neurogenesis is predominantly responsible for the high level of plasticity in the DG, recent data indicate an active participation of niche astrocytes. Adult NSCs also generate astrocytes, although in lower numbers than neurons. Interestingly, preliminary work of our group revealed that the balance between neuro- and astrogenesis remains constant in the adult hippocampus even upon stimuli such as voluntary running. These findings imply the existence of a mechanism that tightly regulates the fate decision of adult NSCs and leads to the question, which factors are responsible in controlling the neuron-to-astrocyte ratio? The transcription factor Sox9 emerged as a master-regulator of the neuron/glial switch in NSCs during development. In order to reveal mechanisms by which the balance between adult neurogenesis and astrogenesis is mediated, I will use genetic mouse models to delete and overexpress Sox9 in adult hippocampal NSCs, respectively, and analyze the identity of newly generated cells by morphological and immunohistological criteria. In order to reveal mechanisms by which Sox9 exerts pro-astroglial or anti-neuronal effects in NSCs, I will first identify downstream targets of Sox9 in a comparative RNA-sequencing approach using Sox9 deleted and overexpressing animals. Subsequent in vitro experiments aim to confirm and rescue effects mediated by Sox9 manipulation by transducing NSCs with either downstream target-shRNA or -overexpressing constructs. Apart from NSC-intrinsic regulation, also newborn neurons affect stem cell behavior. To which extend does this apply to newborn astrocytes? Here, I aim to assess if an imbalance in the ratio between newborn neurons and astrocytes affects the fate choice of hippocampal NSCs. I expect an increase in astrogenesis upon overexpression of Sox9. Therefore, I will use this system to increase the number of newborn astrocytes in the adult DG at the expense of newborn neurons. Subsequently, will restore Sox9 expression to wildtype levels by application of Doxycyclin. Using this system, I will study whether overpopulation of the DG by astrocytes affects cell fate decisions of hippocampal NSCs. My research will significantly promote our understanding of intrinsic and extrinsic regulatory mechanism that control adult NSC behaviour and hence hippocampal plasticity. This presents an important prerequisite to better understand glio- and neuropathological phenotypes and their contribution to brain disease.