Post‐mortem MRI white matter hyperintensities are associated with cortical astrocytopathy and water channel expression in the oldest‐old

Background Cerebrovascular small vessel disease, a common cause of cognitive impairment in older individuals, is characterized by the presence of MRI white matter hyperintensities (WMH). Our previous MRI‐targeted histopathological studies have found WMHs and the surrounding NAWM penumbra to be associated with glial markers. This study aims to assess the relationship between cortical markers of glial dysfunction and post‐mortem (PM) MRI WMH. Method Left brain hemispheres were obtained from 45 Oregon Alzheimer’s Disease Research Center subjects >/ = 80 years of age without significant AD pathology. Intact and sliced hemispheres were scanned at 7T MRI. MRI scans were used to target histological sampling. Slides were stained for glial fibrillary acid protein (GFAP), ionized calcium binding adapter molecule‐1 (IBA1), aquaporin‐1 (AQP1), aquaporin‐4 (AQP4), vimentin and amyloid‐beta (ABETA). Slides were co‐registered to MRI. Tissue masks were hand‐drawn to delineate white matter and cortex. Cortical ROIs were quantified for mean pixel value (AQP1, AQP4, GFAP, and Vimentin) or area coverage (IBA1, ABETA). Cassettes with no local WMH were compared with those in the top third of slide WMH coverage (187 cassettes). Global WMH burden was dichotomized at the mean. Linear mixed‐effects models regressed stain quantification on local WMH (in slide) and global WMH burden, with a random effect for staining batch (levels = 4). The final model controlled for age at death and ABETA expression within a slide. Interactions between local and global WMH were included where appropriate. Result High local WMH burden was associated with greater cortical vimentin (p = 0.036), GFAP (p = 0.066), and AQP4 (p = 0.004), while high global WMH burden was associated with greater IBA1 (p = 0.034) and AQP1 (p = 0.071). An interaction was identified between local and global WMH burden for AQP1 (p