Memory Circuitry in MCI and Early Alzheimerâs Disease Prodrome: Molecular Drivers

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Abstract The renewal of this research program addresses two critical bottlenecks in Alzheimer's disease (AD) research: earlier detection of those at risk and identification of important biological processes during preclinical and early stages of disease. These challenges are the key to development of diagnostic and therapeutic approaches, as well as prevention strategies that will need to be implemented at least 5-10 years before dementia onset. During the prior funding period, this project contributed novel information on individuals with amnestic mild cognitive impairment (MCI) and helped drive the field toward a focus on pre-MCI stages. A major innovation has been the investigation of euthymic older adults with cognitive complaints that score within the normal range on cognitive testing. In this group we have reported alterations in brain structure, functional activation, and connectivity in a network of memory-related regions that are typically intermediate between the pattern seen in cognitively normal controls (CN) and individuals with MCI. This phenotype, now referred to as subjective cognitive decline (SCD) and defined by an international consensus panel in 2013, is influencing large-scale studies including the Alzheimer's Disease Neuroimaging Initiative (ADNI), which adopted the Cognitive Change Index (CCI) developed in this project to recruit a similar group (SMC). We piloted a dual- tracer PET approach to study molecular signatures as a function of stage of disease, initially examining the relationship of amyloid burden and immune activation (microglia). With the new availability of PET tracers for in vivo measurement of tau burden, we will measure both tau and amyloid in preclinical and prodromal stages of AD, focusing on targeted regions based on neuropathological studies. The role of amyloid and tau deposition as molecular drivers of early functional disruption of brain activity and connectivity, as well as neurodegeneration, will be investigated using an integrated ensemble of advanced MRI approaches including structural MRI, memory task and resting state fMRI, arterial spin labeled perfusion (3D pCASL), and diffusion imaging (DTI and NODDI) on the Prisma 3T platform with 64 channel RF coil and new multiband acquisition sequences. Banking and analysis of fluid biomarkers (blood, CSF) and analysis of APOE and other candidate genes will provide a biological context and new opportunities to understand very early changes from a systems biology perspective. The overall objectives of this research are to validate early prognostic biomarkers and enhance the understanding of biological mechanisms in early stages through accomplishing three specific aims: (1) Determine the stage-specific profile of functional disruption of brain activity and connectivity, tau and amyloid burden, microvascular pathology, neurodegeneration, and inflammation in preclinical and prodromal AD; (2) Determine the temporal relationships and interactions among pathophysiological domains; and (3) Determine which combination of baseline markers is most predictive of clinical and MRI progression on follow- up. The ultimate impact of this research is to facilitate the development of effective precision medicine for AD.