Systemic inflammation is thought to be an important driver in chronic neurodegeneration. During systemic infection, the inflammatory status of the periphery is communicated to the brain and conserved sickness behaviours initiated. However, in the context of dementia the same inflammatory stimulus might trigger delirium. Delirium is a severe, transient neuropsychiatric condition characterised by altered levels of arousal, inattention, cognitive deficits and psychoses. Delirium and systemic inflammation exacerbate the trajectory of pre-existing dementia, and are associated with increased risk of future dementia. Accumulating experimental studies suggest microglia are “primed” by chronic neurodegeneration, such that a subsequent inflammatory insult – central or systemic – induces an increased inflammatory response which manifests as exaggerated sickness behaviours. To date there have been no studies of microglial priming in the context of pure tau pathology, without amyloid pathology, and none investigating acute sickness behaviour in such a model. The overarching aim of this thesis is to address this gap in the literature and further our understanding of the interactions between systemic inflammation, neuroinflammation and neurodegeneration in the context of tauopathy. The P301S mouse over-expresses human mutant tau protein under the Thy1.2 promoter. It develops hyperphosphorylated and insoluble tau accumulations and progressive neuronal loss. Consequently, P301S mice develop progressive hind limb paralysis. This study identified the horizontal bar task, a test of motor control and coordination, conducted at weekly intervals from 8-22 weeks of age, as a non-invasive measure of disease progression. In addition, a detailed temporal profile of pathological hallmarks at 8, 9, 10, 11, 12, 16 and 20 weeks of age was determined. Key results presented here demonstrate progressive, superficial neuronal loss in the cortex of P301S mice, with associated astrogliosis and surprisingly this occurs in the absence of apparent cortical microgliosis. In stark contrast, there is progressive microgliosis in the spinal cord of P301S mice. On this background, lipopolysaccharide (LPS), a chemical moiety found on the outer surface of gram-negative bacteria, was used to mimic a systemic bacterial infection. P301S mice and C57BL/6 control mice were injected, at 10 or 16 weeks of age, intraperitoneally with 500 μg/kg LPS or saline and were monitored in the following hours and weeks. Acutely, P301S mice showed signs of an exaggerated, longer lasting sickness response. Importantly, exaggerated acute symptoms extended beyond those typically associated with sickness behaviour; LPS induced an exaggerated acute impairment of horizontal bar performance in P301S mice and not C57BL/6 mice – a function which is known to be impaired in P301S mice later in disease. Impairments were age-dependent in terms of timing of injection. These data suggest an interaction between acute infection and existing CNS vulnerability leading to acute neurological dysfunction that is not a feature observed in sickness in a normal animal. LPS-injected P301S mice also showed, again age-dependent, increased rate of decline in motor performance compared with controls. There was no evidence of microglial priming in P301S mice. LPS caused an acute increase in AT8-positive phospho-tau however this did not persist until end stage. At 22 weeks of age there was significant disease-associated cortical neuronal loss in the vehicle-injected P301S mice, and additional superficial cortical neuronal loss in LPS-injected P301S mice and control mice. There was significant IBA1-positive microgliosis in the spinal cord of P301S mice at end stage which was further increased in LPS-injected P301S mice. Taken together these data indicate a clear and clinically relevant interaction between systemic inflammation and tau-associated neuropathology with acute and long-term functional consequences. In the absence of evidence of microglial priming, future work will explore potential mechanisms.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:743696 |
| Date | January 2018 |
| Creators | Torvell, Megan Isabel Lily |
| Contributors | Chandran, Siddharthan ; MacLullich, Alasdair |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/29605 |
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