Acute focal micro injury induces microglial driven activation and proliferation in CNS astrocytes
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Schillner, Miriam
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Abstract
Multiple Sclerosis (MS) is a severe autoimmune disease, characterized by demyelination and neurodegeneration of the central nervous system (CNS). The cause of the disease is still debated with theories claiming that it is either initiated by autoreactive T cells invading the CNS (outside-in hypothesis) or by events within the CNS followed by infiltration (inside-out hypothesis). While the exact mechanisms of the disease development are unclear, disease progression has been demonstrated to be driven by the crosstalk of the adaptive and innate immune system. The present thesis is focused on the crosstalk between astrocytes and microglia in CNS damage and a modulation of their interaction by encephalitogenic T cells. A focal laser injury model was established producing a consistent and reproducible non-inflammatory injury in intact organotypic hippocampal slice cultures (OHSCs). A distinct temporal response pattern was observed for both microglia and astrocytes, with microglia being the first responders towards the damage and subsequently driving the ensuing astrocyte reaction. The astrocytic response consisted either in parts of the formation of a glial scar entirely through spatial rearrangements in the absence of microglia or aided by cellular proliferation driven by microglia signalling. Infiltration of pro-inflammatory Th17 cells increased astrogliosis and astrocytic proliferation, underlining the role of immune cell invasion in modulating glial activation. In a next step, the chemogenetic Designer Receptor Exclusively Activated by Designer Drugs (DREADD) model – as a non-invasive way to chronically activate cells in their intact cellular environment – was successfully established in microglia in OHSCs. Stable expression of the modified receptor hM3Dq under the CSF1R promotor could be shown in Iba1+ microglia. DREADD-mediated activation led to a significant increase in microglia displaying a rod-like morphology potentially signifying a primed microglia phenotype. Overall, the present thesis underlines a prominent role of microglia in driving astrocytic activation and proliferation under both inflammatory and non-inflammatory conditions. Furthermore, it demonstrates the occurrence of an elusive microglia phenotype after chemogenetic manipulation.