Characterisation of the axonal response to passive and environmentally induced neuroplasticity in an APP/PS1 model of Alzheimer's disease

Synapses are points of communication between neural cells capable of neuroplastic adaptation, and their dysfunction is a common feature in Alzheimer's disease (AD). Axonal boutons are at the output sites of a synapse, and reorganisation of these presynaptic boutons can lead to large-scale connectivity changes. The aim of the current project is to use animal models to study axonal plasticity in a healthy nervous system, and nervous system marked by AD pathology, and to investigate if there are any potential therapeutic benefits to modulating synaptic plasticity in these models. To this end a series of experiments were developed using the amyloidosis APP/PS1 mice and their wild type (WT) counterparts (crossed with fluorescent reporters linked to the Thy 1 promotor for synaptic analysis studies) to characterise synaptic plasticity across the lifespan, following mid-life environmental enrichment (EE), and in response to non-invasive low-intensity intermittent theta burst brain stimulation (LI-iTBS). First, using histological analysis, fibrillar plaque pathology was quantified in 6, 12, and 18-22-month-old APP/PS1 mice in cortical regions functionally associated with environmental input provided by the EE paradigm (cognitive ‚Äö- prefrontal cortex [PFC]; sensory ‚Äö- somatosensory cortex [SS2]; motor ‚Äö- primary motor cortex [M1]). Next, 6-month-old APP/PS1 mice and age matched controls were subjected to 6 months of EE. The effects of midlife EE were assessed on learning and memory (Barnes maze), plaque pathology deposition, and axonal connectivity in the three cortical regions of interest. Findings from these experiments suggest that the PFC region is selectively susceptible to amyloid deposition and less responsive to the attenuating effects of EE, while maintaining overall higher density of pre-synaptic outputs compared to SS2 and M1. Midlife EE improved learning and memory in both APP/PS1 and age matched WT control groups. To further investigate not only density, but dynamic turnover of axonal boutons, an in vivo live imaging through cranial windows was used to assess the plasticity of axonal synapses in the M1/SS1 cortex of both APP/PS1 and WT control group (10-13 months of age). Although there were no changes in density of axonal boutons observed, the turnover rates were significantly lower in APP/PS1 group compared to the WT controls, suggesting reduced synaptic plasticity in the APP/PS1 model of AD. To study the effects of LI-iTBS, a novel rodent specific transcranial magnetic stimulation (TMS) coil was first assessed for its potential to induce plasticity in laboratory mice. In a series of motor learning experiments, it was demonstrated that using the LI- iTBS protocol delivered with the novel rodent specific TMS coil produced behavioural changes suggestive of network level neuroplastic effect. However, when probing the post-mortem brain tissue with biochemical assays, no changes were observed in any of the global neurophysiological correlates of these behavioural effects, suggesting a more targeted approach was needed to assess synaptic changes in response to LI-iTBS. Using in vivo imaging, axonal bouton density and turnover were quantified before and after LI-iTBS in APP/PS1 and WT controls (10-13 months of age). These experiments demonstrated that both WT controls and APP/PS1 animals exhibited increased turnover of synaptic axonal boutons for up to 6 days following a single stimulation event. These results suggest that although the imaged axons maintained the overall number of their synaptic outputs, the targets of these outputs varied post-stimulation. In addition, APP/PS1 mice showed a lower baseline turnover prior to any intervention, which after LI-iTBS increased to the baseline levels observed in WT controls, pointing to possible clinical applications of LI-iTBS to enhance neuroplasticity in the AD brain. In this thesis the structural changes of axonal boutons were explored across the lifespan, and following environmentally and passively induced neuroplasticity. The evidence suggests that overall, some features of bouton dynamics are remarkably stable, while others are highly dynamic and sensitive to change in neurodegeneration and following LI-iTBS. In addition, to the best of my knowledge this is the first study that looked at the effect of LI-iTBS on synaptic plasticity in an animal model of AD. The change in bouton dynamics following LI-iTBS points to possible clinical applications of non-invasive brain stimulation to promote and/or to restore some aspects of cortical plasticity in AD.