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Abstract

In contrast to mammals, zebrafish display an extensive capacity to generate new neurons and repair injuries of the central nervous system (CNS) at adult stages. During adult neurogenesis, new neurons are formed abundantly from a pool of neural stem cells (NSCs) in order to integrate into existing tissue. Pivotal for the regenerative ability of the zebrafish adult brain is the precise control of NSC behavior and long-term maintenance of the stem cell pool. The balance between quiescent NSCs and activated NSCs is fundamental to the successful functional regeneration of the brain. This balance is controlled by a gene regulatory network (GRN) which is tightly regulated in a spatial and temporal manner through the interplay of different factors including transcription regulators, signaling pathways, key genes and their associated regulatory elements and also small molecules arising from non-coding areas of the genome. Understanding which key players are involved in this regulatory network, how these factors are controlled, how they interact with each other and what their exact function is, will help to comprehend how brain regeneration in the adult zebrafish is facilitated and develop possible strategies for initiating similar regenerative processes in the human brain. Therefore, the overall aim of this thesis was to identify key genes and molecules of this complex GRN and investigate their regulation and function on a molecular level and their expression in the adult zebrafish brain. The gene midkine-a (mdka) was previously shown to be involved in the regeneration of the zebrafish retina, heart and fin. In this work, I investigated its expression pattern in the zebrafish adult telencephalon under constitutive and regenerative conditions and found that mdka expression is specifically restricted to cells in the ventricular zone of the zebrafish telencephalon, a stem cell niche of the zebrafish brain, and its expression pattern is mainly associated with quiescent NSCs. Furthermore, regulatory elements for mdka were identified and can be further investigated regarding their response to signaling pathways and their consequent influence on the cell and time specific expression of mdka. Moreover, micro RNAs (miRNAs), which are small molecules originating from non-coding areas of the genome, that showed an increased expression after injury of the zebrafish brain were investigated for their expression pattern and regulatory role in the GRN. Their increased expression and analysis of targeted genes suggested a role in the control of neuronal genes. However, since miRNAs can regulate a number of different targets, detailed investigation into their role in the GRN is still needed. Overall, this thesis provides different tools and applications for the investigation of the GRN that regulates NSC behavior in the adult zebrafish brain and identifies mdka as one of the key genes in the GRN controlling the proliferative behavior of NSCs and regulating stem cell quiescence.