In vivo measurement of plaque burden in a mouse model of Alzheimer's disease

Alzheimer’s disease (AD) is the most common form of dementia in the elderly (1). Over 4 million people are affected by AD in the United States alone, and as the aging population increases, this number is expected to double by 2025 (2). The classic clinical symptoms of AD include memory loss and confusion. The neuropathological features are neurofibrillary tangles (NFTs) formed by paired helical filaments composed of tau protein, senile plaques (SPs) resulting from Aβ deposits, and neuron loss in the limbic and neocortical regions (3,4). Neuronal loss associated with early AD eventually leads to brain dysfunction and atrophy. Volumetric assessment methods are the current state of the art in magnetic resonance imaging (MRI) of AD in humans and in animal models of AD pathologies. These methods measure gross morphological changes and track disease progression, but do not provide information about biochemical changes. Conventional MRI of the brain, which relies on contrast generated by the variation of T1 and T2 or T2* relaxation times of water in tissue, has proven inadequate for observing authentic SPs and NFTs in vivo. This is primarily due to the high resolution and thin section (~10 µm) images required to visualize SPs, which currently are not achievable with MRI. However, interactions between macromolecules and bulk water, and changes in macromolecular content can be indirectly quantified by spatially mapping MR relaxation times. In these relaxometric maps, any change in relaxation times due to the presence of SPs will be reflected over an entire volume-averaged pixel, thereby providing an indirect method of detection without the need for very-high-resolution images. Intriguing data suggesting that plaques can be detected in brain tissue specimens by T2*-weighted MRI have been reported (5); however, another study using similar methods failed to confirm this (6). Both studies were performed on formalin-fixed brain tissue specimens, and the MR characteristics of fixed tissue differ from those in vivo. T2-weighted MRI was employed to visualize plaques in a transgenic (tg) mouse model of AD ex vivo (7,8). Recent work has shown the possibility of detecting Aβ deposits in vivo in the APP/PS1 tg mouse model of AD using T2- and T2*-weighted MRI (9,10). The presence of iron in these plaques also exaggerates the actual size of plaques in T2*-weighted images, which makes the accurate quantification of plaque density particularly challenging (9). Also, although very-high-resolution T2*-weighted MRI (<100 µm) has been used to image plaque formations in human AD-affected tissue specimens (5), other experiments have contradicted such findings by demonstrating that plaques are not visible in T2* images (6). Previous studies have demonstrated the efficacy of gadolinium and magnetically labeled Aβ1–40 for visualizing Aβ deposits in an APP/PS1 tg model ex vivo (11) and in vivo (12). However, given the neurotoxicity of Aβ and the requirement for mannitol injection for the in vivo experiments, these methods may have limited utility in a clinical setting. Further, in vivo relaxometric measurements established differences in T2 on the order of a few milliseconds (13) between these mice and age-matched controls. Although these were meticulous experiments, Campeau et al (14) previously demonstrated that T2 did not significantly differ between AD patients and age-matched normal subjects, which suggests that T2-weighted MRI may have limited clinical utility. An alternate contrast mechanism is T1ρ, or T1rho, the spin lattice relaxation time constant in the rotating frame, which determines the decay of the transverse magnetization in the presence of a “spin-lock” radiofrequency (RF) field. In biological tissues T1ρ is dependent on the macromolecular composition and provides contrast, unlike conventional T1/T2-based methods. While quantifying T2* with MRI can be prone to errors from susceptibility-induced signal losses, and T2-weighted MR signal is degraded by diffusion, T1ρ-weighted MR images are less prone to such losses and have a greater dynamic range of values as recently demonstrated in the human brain (15). Therefore, quantitative T1ρ MRI is an attractive alternative strategy for detecting and quantifying Aβ deposits, especially since it probes slow molecular interactions and the dynamics of water molecules in tissues based on variations in protein content (16). The development of novel diagnostic techniques for AD requires the study of an animal model in vivo before these techniques can be translated to clinical utility. For example, if the disease process can be detected in an in vivo animal model of AD with MRI, and this is validated by histological studies, a paradigm for visualizing the presence of AD pathology can be established. Transgenic (tg) mice (APP/PS1) expressing the pathogenic familial AD-associated form of amyloid precursor protein (APP-Swedish/Tg2576) and presenilin-1 (PS1, P246L/wt) were shown to develop a large number of fibrillary Aβ SP-like deposits in the brain, especially in the cerebral cortex and hippocampus (17,18). In the present work we demonstrate an MRI method for directly visualizing Aβ plaques in the APP/PS1 tg mouse brain in vivo, and show that the T1ρ relaxation rate increases progressively with AD-related pathology in the tg mouse brain.