- Title
- Environmental determinants of neurovascular remodelling and strategies to enhance recovery after stroke
- Creator
- Zidan, Zhao
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Background: Stroke is currently the biggest cause of long-term disability in the world. Despite the numerous efforts that have been made, we currently only have one effective treatment for stroke, a drug known as recombinant tissue plasminogen activator (tPA). Problematically, to be effective, tPA must be administered with 4.5 h of the initial occlusion event. This has led to a challenging situation in which only about 5% of patients with stroke receive the drug. As such a vast majority of patients suffering a stroke will go on to experience significant and often life-long impairments of cognition and behaviour. As such the experimental work provided in thesis is focused on examining how the brain responds after stroke, with the aim of developing strategies to enhance optimal repair. Now it is well recognised that successful stroke recovery depends on the restoration of the neurovascular unit (NVU) function. The neurovascular unit (NVU) is considered to be a functionally and structurally interdependent multicellular complex, comprising endothelial cells, the basal lamina, pericytes, astrocytes, microglia and neurons. From the available evidence, it appears that remodelling of the NVU continues for many weeks and potentially months following the initial injury. It is widely considered that if we can achieve a better understanding of these endogenous neurorestorative processes, one or more of the processes may be targeted to further enhance recovery. As such, the components of the NVU will be a major focus of the studies in this thesis. Secondary neurodegeneration (SND) involving the inexorable death of tissues that are connected to but not originally damaged by the primary stroke, is a particularly interesting phenomenon for those interested in enhancing optimal recovery. The interest here is because SND occurs over a timescale of months to years following stroke, the very same time when efforts to recover are at the most intense. There is also emerging evidence that SND is associated with functional declines post stroke. Against this background it was also motivating to develop a better understanding of how SND develops and how it could relate to exposure to different environmental factors. There are two environmental factors that have been considered in this thesis, psychological stress and oxygen modulation, in the form of high altitude training; it was particularly interesting in how they influence the NVU and SND during stroke recovery. The rationale here was that it is well recognised now that many stroke patients will experience high levels of stress due to significant difficulties with movement, communication, and cognition. However, the effect of stress on stroke recovery has received surprisingly little attention to date. Second, either stroke or stress has been reported to induce significantly high level of Amyloid-β (Aβ), which is neurotoxic and highly associated with Alzheimer’s disease and cognition decline. This might be the underlying mechanism for exacerbated cognitive decline in stroke or stress patients. However, whether stress would induce extra negative effect in Aβ accumulation or cognitive decline is largely unknown. Third, studies have shown that Aβ clearance is dependent on the glymphatic system, a paravascular pathway formed by the end-feet of astrocytes binding to the outer liminal surface of vessels in the brain. High altitude training is well established the ability to induce erythropoiesis, angiogenesis, neuroprotective effect and glial promotion. All these effects not only potentially contribute to NVU remodelling that successful stroke recovery relies on, but also might contribute to the improved glymphatic system to reduce Aβ following stroke. However, few studies have ever examined these changes in stroke research. In this project, it was worthwhile to examine how exposure to chronic stress could influence post-stroke functional recovery, NVU remodelling, and SND development and how exposure to delayed normobaric hypoxia could change glymphatic system and whether that may be capable of improving functional recovery post-stroke. Aims: The overall aim of my project was to investigate how difference environmental factors, as chronic stress and high altitude training, could affect cellular and molecular remodelling at both the primary injury site and within the thalamus, a major site of SND. The project is combined of four individual studies and the individual aims of these four studies were: 1) to investigate how chronic stress exposure changes functional outcomes after stroke and the remodelling of components in NVU, including cerebral vasculature, neurons, astrocyte and microglia and their key relevant molecules, for instance vascular endothelial growth factor and postsynaptic density protein 95 (Chapter 2), 2) to investigate how chronic stress exposure influences SND process, including neuronal loss, Aβ accumulation and altered oligomerisation at sites of thalamic secondary neurodegeneration after stroke (Chapter 3), 3) to investigate whether glymphatic system is enhanced by delayed normobaric hypoxia and eventually results in amyloid clearance and improves cognitive function after stroke (Chapter 4), 4) to investigate whether the thalamic SND site also benefits from delayed normobaric hypoxic exposure (Chapter 5). Methods: The photothrombotic (PT) model of focal cerebral ischemia was selected in the project, due to its advantages of minimal invasion, easy operation and consistent infarct size and control over location. In the stress studies (Chapters 2 and 3), repeated restraint stress was applied as it is one of the most widely used models for inducing chronic stress in rodents’ studies. Three experimental groups have been chosen to run in the stress studies: shams, occlusion alone, and occlusion with stress. As in delayed normobaric hypoxia studies (Chapter 4 and 5), four experimental groups have been chosen to run in those studies: shams, stroke alone, stroke with 8 h hypoxia/day (11% oxygen) and stroke with 24 h hypoxia/day (11% oxygen) for two weeks. Cylinder task was used to assess the motor function of forelimb and paired associates learning testing was used to assess the cognitive function of learning and memory in stroke animals. Immunohistochemistry was used to investigate the cellular morphology and interested protein distribution. Western blotting and ELISA were applied to assess the interested protein levels and real-time PCR was used to examine the interested gene transcription levels. In vivo imaging by confocal microscope was used to observe the clearance rate of paravascular pathway. Results: 1) Chronic stress exposure following PT stroke affected motor function recovery and suppressed the key components of the NVU, including blood vessels, astrocytes, microglia and neurons remodelling in peri-infarct regions (Chapter 2). 2) Chronic stress exposure following PT stroke was associated with increased levels of Amyloid beta accumulation and neuronal loss at sites of thalamic secondary neurodegeneration (Chapter 3). 3) Delayed normobaric hypoxic exposure exhibited significantly enhanced cognitive function, enhanced vasculogenesis, and reduced Aβ levels and aggregation, potentially through the promoted glymphatic system (Chapter 4). 4) Delayed normobaric hypoxic exposure enhanced vasculogenesis, rescued neuron loss, reduced Aβ accumulation at thalamic secondary neurodegeneration post-stroke (Chapter 5). Conclusions: Overall, the experiments in this thesis were directed towards understanding how recovery following stroke can be altered by chronic stress and high altitude training. Certainly, these two topics in the first instance appear strange companions. However, they are conceptually related through the ability of stress increase neurodegenerative features and for delayed normobaric hypoxia to mitigate stroke induced neurodegeneration. All the important findings in this thesis provide potential targets for translational medical research in the future. First, the results of the stress studies suggest that monitoring the stress levels in stroke patients and any intervention that can reduce stress, such as anti-depressant drug and environmental enrichment; may potentially improve function outcomes in stroke patients. Second, high altitude training has the potential to be an effective intervention in stroke patients as it could increase vessel density, promote glymphatic system, reduce Aβ oligomerization.
- Subject
- stroke; recovery; neurovascular remodelling; strategies
- Identifier
- http://hdl.handle.net/1959.13/1353461
- Identifier
- uon:31100
- Rights
- Copyright 2017 Zhao Zidan
- Language
- eng
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