Tau pathology-dependent remodelling of cerebral arteries precedes Alzheimer's disease-related microvascular cerebral amyloid angiopathy

Alzheimer's disease (AD) is characterised by pathologic cerebrovascular remodelling. Whether this occurs already before disease onset, as may be indicated by early Braak tau-related cerebral hypoperfusion and blood-brain barrier (BBB) impairment found in previous studies, remains unknown. Therefore, we systematically quantified Braak tau stage- and cerebral amyloid angiopathy (CAA)-dependent alterations in the alpha-smooth muscle actin (alpha-SMA), collagen, and elastin content of leptomeningeal arterioles, small arteries, and medium-sized arteries surrounding the gyrus frontalis medialis (GFM) and hippocampus (HIPP), including the sulci, of 17 clinically and pathologically diagnosed AD subjects (Braak stage IV-VI) and 28 non-demented control subjects (Braak stage I-IV). GFM and HIPP paraffin sections were stained for general collagen and elastin with the Verhoeff-van Gieson stain; alpha-SMA and CAA/amyloid beta (A beta) were detected using immunohistochemistry. Significant arterial elastin degradation was observed from Braak stage III onward and correlated with Braak tau pathology (rho = 0.909, 95 % CI 0.370 to 0.990, p < 0.05). This was accompanied by an increase in neutrophil elastase expression by alpha-SMA-positive cells in the vessel wall. Small and medium-sized arteries exhibited significant CAA-independent alpha-SMA loss starting between Braak stage I and II-III, along with accumulation of phosphorylated paired helical filament (PHF) tau in the perivascular space of intraparenchymal vessels. alpha-SMA remained at the decreased level throughout the later Braak stages. In contrast, arterioles exhibited significant alpha-SMA loss only at Braak stage V and VI/in AD subjects, which was CAA-dependent/correlated with CAA burden (rho = -0.422, 95 % CI -0.557 to -0.265, p < 0.0001). Collagen content was only significantly changed in small arteries. Our data indicate that vessel wall remodelling of leptomeningeal arteries is an early-onset, Braak tau pathology-dependent process unrelated to CAA and AD, which potentially may contribute to downstream CAA-dependent microvascular pathology in AD.