MAGNETIC RESONANCE IMAGING AND HISTOLOGICAL ANAYLYSIS
OF BETA – AMYLOID PLAQUES IN HUMAN ALZHEIMER’S DISEASE AND APP/PS1 TRANSGENIC MICE

Meadowcroft, Mark David

MAGNETIC RESONANCE IMAGING AND HISTOLOGICAL ANAYLYSIS OF BETA – AMYLOID PLAQUES IN HUMAN ALZHEIMER’S DISEASE AND APP/PS1 TRANSGENIC MICE

Open Access

Author:: Meadowcroft, Mark David
Graduate Program:: Neuroscience
Degree:: Doctor of Philosophy
Document Type:: Dissertation
Date of Defense:: March 09, 2009
Committee Members:: Qing X Yang, Dissertation Advisor/Co-Advisor
Qing X Yang, Committee Chair/Co-Chair
James Robert Connor, Committee Member
Paul Joseph Eslinger, Committee Member
Michael Smith, Committee Member
Robert J Milner, Committee Member
Keywords:: APP
Alzheimer
PS1
Iron
Beta Amyloid
MRI
Plaque
Magnetic Resonance Imaging
Microscopy
Histology
Stain
Abstract:: Alzheimer’s Disease (AD) is a progressive neurological disorder characterized by the hallmark beta-amyloid (AB) plaques and neurofibrillary tangles (NFT) found in the neocortex of brain tissue samples. Other associated pathological changes in the brain consist of degeneration of brain tissue via loss of neurons and neuronal changes associated with loss of synapses and dendritic branch dearborization. The disease consists of a general progression of cognitive and memory impairment leading to an insidious dementia like state followed by eventual patient fatality. Above the age of 65 years old the incidence rate of Alzheimer’s increases exponentially with age. It is believed that one in ten individuals older than the age of 65 and approximately half of those older than 85 years old have been diagnosed with the disease. Approximately 5 million Americans have AD at the present time and with the increased survival age of the world’s inhabitants it is therefore poised to become a greater public health threat as our population ages. Histopathological investigation currently is the only approach that can accurately diagnose an individual with Alzheimer’s disease; however these methods can only be completed post mortem. Methods to noninvasively examine tissue in vivo have been of great interest in the study of AD and other neurological disorders. Magnetic resonance imaging offers the unique ability to perform detailed in vivo evaluation of neural tissue to aid in this endeavor. Imaging of beta-amyloid plaques in human Alzheimer’s disease and transgenic models that mimic plaque production has been challenging for AD research. The development of MR imaging technology capable of visualizing and quantifying AB plaques in the Alzheimer’s brain is critically important for translational and clinical investigations. Understanding the transverse MR relaxation associated with the neurodegenerative processes in AD would provide insight into future technologies that will aid in clinical studies. Currently, the histo-pathological basis of image contrast and the relaxation mechanism associated with Aâ plaques is not well understood. Analysis and comparison between histology of a tissue sample and MRI in a one-to-one fashion is of importance for validating new imaging methods and contrasts, especially in molecular imaging research. To help in this endeavor, our development of a histological RF coil has been realized and implemented. Formaldehyde fixed tissue samples are cut on a cryostat, placed between two glass coverslips for positioning inside of the histological coil for MR imaging, then following the scanning protocols are carefully removed and histologically stained. The experimental data collected with the histological coil demonstrate the feasibility of imaging thin slices of tissue samples and unambiguously co-registering these MR images to various histological staining results. With the aid of the coil, T2*-weighted images and parametric maps were directly compared to histology stains acquired form a set of Alzheimer’s disease and transgenic APP/PS1 tissue slices. For AD tissue samples, transverse T2* relaxation due to Aâ plaques is shown to be directly associated with the gradation of iron concentration and to a minor degree with plaque size. For the transgenic APP/PS1 model, which generates Aâ plaques and aims to mimic pathology, plaque morphology and size were associated with image contrast and transverse relaxation. It has been assumed within the MR community that MR contrast associated with beta-amyloid plaques is due to iron concentration alone. However, the contrast due to transgenic plaques with considerably less iron load are equally conspicuous as the human AD plaques in the MR images. These data suggest a duality in the relaxation mechanism were both high focal iron concentration and highly compact fibrillar beta-amyloid masses cause rapid proton transverse magnetization decay. For human AD tissue, the former mechanism is likely the dominant source of T2* relaxation; for APP/PS1 animals, the latter is likely the major cause of increased transverse proton relaxation rate in beta-amyloid plaques. Histological evaluation on the APP plaques and AD plaques demonstrate important differences in iron load and plaque morphology that are hypothesized to be related to differences in transverse relaxation mechanisms. This insight is essential for understanding the histo-pathological underpinning of MRI measurements associated with Aâ plaques. Transgenic mouse models that reproduce neuropathology are important in understanding disease etiology and testing new clinical procedures. Utilization of the transgenic APP/PS1 model brought into the light important dissimilarities between the naturally occurring Aâ plaques found in the AD brain and those engineered to amass in the transgenic model. Understanding the benefits and disadvantages of the APP/PS1 mouse model is important in considering the outcome from studies involving their usage and eventually transitioning these treatments to the in vivo AD brain. Using various histological stains and microscopy methods aimed at viewing plaques morphology, iron management and inflammatory response we show that there are distinct differences between the naturally occurring human AD plaques and those found in the APP/PS1 model. Clear morphological differences in plaque composition and ultra-structure allude to differences in amyloidogenesis and component fibril formation. Transgenic animal plaques are morphologically distinct from Alzheimer’s plaques as seen in both low magnification traditional microscopy and ultra-high magnification transmission electron microscopy. Microglial and astrocyte staining shows a differing pattern of inflammatory response being much more pronounced in the AD neural tissue compared to the transgenic tissue. Iron mismanagement and focal iron deposition is found ubiquitously throughout the AD tissue cortex, with plaques showing a close relationship to iron accumulation within them, and is not seen in the transgenic model. The data presented here emphasizes pronounced histological grounds for MR relaxation differences between naturally occurring Aâ plagues in the AD brain and those instituted to produce in the APP/PS1 transgenic mouse model. The data suggest that utilization of these transgenic models for various magnetic resonance, histological and pharmacological studies of Alzheimer’s disease should be done with the consideration of these observations. The evolution and translation of the MR approaches outlined in this work into the clinical domain to accurately diagnose AD and quantify in vivo beta-amyloid plaques would be advantageous for future medical diagnosis. The data presented here represents a foundation for understanding the transverse relaxation mechanisms associated with beta-amyloid plaques in Alzheimer’s disease. Translating these results to an in vivo clinical setting presents a challenge to the MR community. Currently the methods presented here are beyond the resolution limitations and time constraints of low field clinical scanners. However, with advancements in future MR technology there is potential for expanding this research into a valid system.

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