Mechanisms of neurodegeneration and neuronal cell loss in the hippocampus in murine scrapie
Transmissible spongiform encephalopathies (TSEs) or prion diseases are defined by infectivity and by the pathological damage they produce in the central nervous system (CNS), typically involving spongiform degeneration or vacuolation, deposition of abnormal PrP (PrPSc), glial activation and neuronal loss. Much of our understanding of the TSEs has derived from the study of murine scrapie models. The molecular basis of pathological changes is not clear, in particular the relationship between the deposition of PrPSc and neuronal dysfunction. A typical feature of TSE disease is neuronal loss, although the mechanisms leading to this loss are poorly understood. Apoptosis has been proposed as an important mechanism of TSE associated cell death, but which pathways are involved are still to be determined. The main aims of this thesis are to investigate the progression of the characteristic neuropathological changes observed in the TSE infected brain and to analyse the mechanisms involved in neuronal loss. In this study two contrasting scrapie mouse models were used : the ME7/CV model , and the 87V/VM model in which neuronal loss is targeted to different areas of the hippocampus, the CA1, and CA2 respectively. The role of the caspase-dependent pathway of apoptosis in the neuronal loss was investigated. The results of analysis of pro-apoptotic markers of disease in the two scrapie mouse models differed. The results observed in the ME7/CV scrapie mouse model suggest that apoptosis may not be the main mechanism of neuronal loss, whereas the 87V/VM model showed some indication that apoptosis may be involved. Detailed studies in the progression of neurodegenerative changes in the ME7/CV scrapie mouse model revealed that the initial pathological change observed in the hippocampus was the deposition of PrPSc followed by a glial response, spongiform change and subsequent neuronal degeneration. The role of the cytoskeleton and synaptic dysfunction in the neuronal damage observed in the CA1 of the ME7 infected hippocampus was analysed. Cytoskeletal disruption was observed in the post-synaptic dendritic spine, and the apical dendrites of CA1 neurons at 160days, a time point at which neurons are known to be lost. Changes in the expression of the pre-synaptic protein, synaptophysin and the post-synaptic protein PSD-95 were not observed until the terminal stage of disease when the neuronal loss is profound. In conclusion, this research suggests that the mechanisms of neuronal loss may follow different biochemical pathways, which might not necessarily involve an apoptotic mechanism. Cytoskeletal disruption in the post-synaptic dendritic spine plays a major role in the neuronal dysfunction observed in ME7 infected CA1 neurons, although the post synaptic density does not seem to be involved .Pre-synaptic changes and disruption to the innervation of CA1 neurons is not apparent until the end stages of disease. The trigger for this cytoskeletal disruption and the subsequent neuronal loss may be the early deposition of PrPSc in the extracellular space but the precise mechanisms involved are still to be elucidated. The identification of the key events involved in the mechanisms of neruodegeneration in TSE diseases may lead to the development of therapeutic strategies to inhibit the neurodegenerative process.