SUMMARY This project applies advanced in vivo imaging to determine the pathological mechanisms of white matter hyperintensities (WMHs) and their contribution to cognitive decline in older adults at risk of Alzheimer's disease (AD). WMHs are bright patches on T2- MRI but do not inform the pathology underlying their appearance. Our work is innovative because it utilizes, for the first time, non-invasive methods to quantify axon and myelin rarefaction, fluid retention, WMHs-associated cortical atrophy, and blood-brain barrier dysfunction as in vivo drivers of WMHs formation. We will also test typical pathological markers of WMHs, i.e., hypoperfusion and compromised vascular reactivity. All imaging measurements will be validated against well-established CSF markers. Our scientific premise is that different locations of WMHs (deep vs. periventricular) correspond to distinct mechanisms (vascular vs. neurodegenerative) and cognitive profiles. Thus, our work will allow the use of pathological drivers of WMHs to develop targeted strategies to stop their growth and ameliorate associated cognitive decline in the future. In Aim 1, we will test the hypothesis that vascular pathology predominates in deep WMHs by measuring cortical cerebral blood flow, vascular reactivity, and blood-brain barrier dysfunction. We will use specialized arterial spin labeling and a hypercapnic functional MRI approach for these measurements. We will validate measurements with CSF markers of vascular injury (e-selectin), inflammation (adhesion molecules, VCAM/ICAM), and blood-brain barrier permeability (albumin extravasation). In Aim 2, we will test the hypothesis that neurodegenerative pathology predominates in periventricular WMHs by measuring axon rarefaction, demyelination, and fluid retention using advanced diffusion imaging and myelin water fraction imaging. Validation markers will be CSF levels of tau and myelin basic protein. In independent ex vivo samples of donor brain tissue, we will quantify vascular pathology using smooth muscle actin and albumin immunohistochemical stains in the two WMHs. We will also quantify axon and myelin density in the WMHs and normal-appearing white matter and perform neuron counting in gray matter using an array of stains. Postmortem MRI will be used to identify WMHs on donor brains. The quantitative neuropathology will serve as an independent validation of our hypotheses regarding WMHs location and mechanisms. In Aim 3, we will test the hypothesis that deep WMHs interrupt discrete short-range association fibers and striatal fibers, causing specific cognitive deficits, especially those related to processing speed. Periventricular WMHs, on the other hand, interrupt long-range association tracts causing global cognitive impairment. We will use mediation analysis to explain whether the two types of WMHs influence the relationship between the different imaging markers and distinct cognitive symptoms. The proposal takes advantage of the Cores and affi...