CCL17^DTR mice as a new mouse model of epilepsy

Abstract

Epilepsy is a serious neurodegenerative disease affecting millions of people worldwide. Patients suffer from unpredictable seizure episodes that drastically impair the quality of life. Approximately one third of epilepsy patients do not respond to current anti-seizure medication. Especially patients suffering from a subgroup of epilepsy, temporal lobe epilepsy (TLE), make up the majority of drug-resistant epilepsy cases. The underlying mechanisms leading to seizure development and drug-resistant epilepsy are not fully uncovered to date. The need for new animal models to investigate the fundamental basics of seizure manifestation as well as the mode of action of contemporary anti-seizure medication is evident. <br/> The aim of this thesis was to characterize a newly developed potential animal model of TLE, the CCL17^DTR mouse model. In these mice, the simian receptor for Diphtheria toxin (DTR) is expressed under the control of the promotor of the <em>CCL17</em> gene. Administration of Diphtheria toxin (DT) depletes all cells expressing CCL17, which includes CCL17⁺ neurons in the murine brain. This leads to the emergence of recurring seizures, accompanied by gliosis. Spatiotemporal analysis of gliosis revealed the onset of microgliosis on day 5 post-DT, and astrogliosis was observed until at least day 60 post-DT. Two weeks following DT administration, pronounced neuroinflammation and neurodegeneration were present in the entire hippocampus and, to a lesser extent, in the neocortex. The fastest and strongest gliosis was identified in the hippocampus, especially in the CA2 region. Inhibition of TNFR1 signaling reduced seizure burden in CCL17^DTR mice but was insufficient to prevent gliosis or neurodegeneration. Spatial transcriptomics analysis revealed the neocortex as the area with most deregulated genes, followed by the hippocampus. Transcriptional changes primarily affected the deregulation of neuronal transmission, neurohormones, immediate-early genes and a type I interferon-driven immune response. <br/> In addition, the expression of the chemokine CCL17 in the developing murine brain was investigated. CCL17 emerged postnatally in numerous neurons in the hippocampus, neocortex and arcuate nucleus of the murine brain. In adulthood, the number of CCL17⁺ neurons was reduced and concentrated on the <em>stratum pyramidale</em> of the hippocampus, layer V neocortical neurons and the arcuate nucleus, suggesting potential functions of CCL17 on feeding behavior and neuronal migration in the postnatal brain. The phagocytic capacity of microglia <em>in vitro</em> was reduced upon CCL17 administration, indicating that the chemokine also potentially regulates the function of microglia. <br/> In summary, the depletion of CCL17⁺ neurons in CCL17^DTR mice can be used to model the development of TLE in mice. The new model shares several aspects of existing TLE mouse models regarding the seizure occurrence, spatiotemporal gliosis and neurodegeneration, as well as transcriptional alterations. Unique features comprise the neuronal loss in the CA2 region and the progression of neuroinflammatory and neurodegenerative processes despite successful seizure suppression. The novel CCL17^DTR mouse model could support the analysis of as yet unknown mechanisms of epileptogenesis and provide a new option for the test of anti-seizure medication.