Positron emission tomography imaging of the cellular phase of Alzheimer’s disease

Abstract

Alzheimer’s Disease (AD) is the commonest fatal neurodegenerative disease causing dementia. It has a devastating effect on individuals and their families and causes progressive cognitive, social and functional decline with no current cure. AD is characterised neuropathologically by amyloid-β (Aβ) and tau protein deposition, but this does not fully explain how the brain is affected and why neurodegeneration occurs. Key molecular drivers of disease could include cell stress and impaired mitochondrial oxidative phosphorylation with synaptic loss. Elucidating key mechanisms of this disease process in vivo has the potential for better diagnostic classification, and a better understanding of the neurobiology in order to prioritise targets for earliest interventions to delay or reverse disease. In this study I set out to determine whether multimodal imaging using PET (amyloid-beta, the sigma-1 receptor, mitochondrial complex I, synaptic vesicle 2A) complemented by MR (structural, arterial spin labelling and neurite orientation diffusion density imaging) can determine key clinically relevant cellular mechanisms in AD patients. Chapter 1 provides an introduction to AD and imaging. Chapter 2 outlines the material and methods used in this study.4 In Chapter 3 I aimed to boost the diagnostic power of one key trigger of AD - amyloid-β, using [18F]Florbetapir PET, with machine learning applied to big data. I created an algorithm that utilised spatial information by k-means clustering then an optimised quadratic support vector machine based on the progression of regional brain vulnerabilities to AD across 758 volunteers from the Alzheimer’s Disease Neuroimaging Initiative (ADNI). I was able to develop an automated diagnostic decision support and show that my algorithm outperforms classifiers proposed previously for amyloid-β PET. In Chapter 4 I aimed to understand the underlying mechanisms of AD using multiple imaging modalities in my highly phenotyped cohort of early AD and controls. I quantified the in vivo density of the endoplasmic reticulum cell stress marker, the sigma 1 receptor (S1R) using [11C]SA4503 PET, as well as that of mitochondrial complex I (MC1) with [18F]BCPP-EF and the pre-synaptic vesicular protein SV2A with [11C]UCB-J. I also integrated these assessments with regional brain volumes and brain perfusion (CBF) measured with MRI arterial spin labelling and followed the AD patients longitudinally to estimate rates of change with disease progression over 12-18 months. I generated new evidence for widespread cellular stress and bioenergetic abnormalities in early AD and showed how they may be clinically meaningful. In Chapter 5 I sought to understand synaptic microstructure from my early AD cohort in further detail. I quantified in vivo measures of microarchitecture5 evaluating extracellular free water (FISO), neurite density (NDI) and orientation dispersion (ODI) using neurite orientation dispersion imaging (NODDI), as well as more conventional DTI measures of fractional anisotropy (FA), mean/axial/radial diffusivity (MD, AD, RD, respectively), and the pre-synaptic vesicular protein SV2A with [11C]UCB-J PET in my early AD cohort compared to controls. My results showed increased extracellular free water (FISO) using NODDI MRI could be more sensitive to differences in pathology related to synapse loss that could be used to support early-stage evaluations of novel therapeutics for AD Overall, my study provides a systematic investigation of the cellular phase of AD using PET and MR imaging. My main finding was that widespread cell stress in AD leads to regionally vulnerable expression of pathology. If these findings were applied to both clinical practice and drug discovery they could lead to better diagnostic classification and prioritise targets to modulate the disease.