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Abstract

Schizophrenia (SCZ) is a heritable psychiatric disorder which affects approximately 1% of the world population. The disease is characterized by a range of symptoms such as hallucinations, disorganized speech, lack of motivation and depression, as well as cognitive impairments. Genome-wide association studies (GWAS) have identified over 100 genomic regions that associate with SCZ risk and the major histocompatibility complex (MHC) region on chromosome 6, more specifically the complement component 4 (C4) gene, was shown to be the most significantly associated genetic locus within this region. Thereby, alleles leading to a higher C4A expression were correlated to an increased risk for SCZ. The complement cascade has been shown to be involved in microglia-mediated synapse elimination during development. Given the reduced numbers of synapses in brains of individuals with SCZ, it has been suggested that excessive complement activity and synaptic pruning contributes to the development of the disease during late adolescence and early adulthood. In this thesis, a human in vitro model to study synaptic pruning in the context of SCZ pathogenesis was established using induced pluripotent stem cell (iPSC)-derived cortical neurons and microglia. A differentiation protocol to generate iPSC-derived microglia was established which allowed the generation of human microglia-like cells in high purity. These cells expressed key microglia markers, showed a high phagocytic capacity and responded to pro-inflammatory stimuli. Using an in vitro synaptic pruning assay, it was shown that synaptosomes derived from C4A overexpressing neurons were more efficiently phagocytosed compared to control synaptosomes. Overexpression of C4A in mature iPSC-derived cortical neurons in co-cultures with microglia, led to an activation of the NFκB pathway in microglia and an increased synaptic loss in iPSC-derived cortical neurons. Furthermore, it was shown that oridonin, a specific NLRP3 inflammasome inhibitor, has anti-inflammatory effects on iPSC-derived microglia, can inhibit phagocytosis of synaptosomes and can increase synaptic density in C4A overexpressing iPSC-derived cortical neurons after co-culture with microglia. Taken together, this study investigates genotype-phenotype relationships with respect to synaptic pruning and C4A expression in the context of SCZ pathogenesis using a human in vitro model. The applied experimental setting identified the microglial NLRP3 inflammasome as a potential novel therapeutic option for SCZ as the NLRP3 inhibitor oridonin was able to inhibit C4A-induced microglial activation and rescue synaptic densities in iPSC-derived cortical neurons.