Deposition of aggregated cystatin C-amyloid-β complexes and reduced cathepsin B activity modulate cerebral amyloid angiopathy pathogenesis

Cystatin C (CST3) colocalizes with amyloid-β (Aβ) around vessels in cerebral amyloid angiopathy (CAA). This study aimed to characterize CST3-Aβ interactions in CAA. Brain tissues from 12 Alzheimer disease (AD) and 5 non-AD control subjects, along with AD model mice (J20), were examined. In AD brains, immunohistochemistry revealed Aβ positivity around cortical and leptomeningeal vessels as well as in intra- and extracellular areas. Congo red-positive vessels were also present. Conversely, control brains showed only few Aβ-positive cells. CST3 was primarily intracellular in controls, but in AD, it was found both to be intracellular and perivascular, colocalizing with Aβ and Congo red in affected vessels. Vessel quantification showed a positive correlation between CST3-positive and Aβ- or Congo red-positive vessels. Moreover, most CST3-positive vessels were Cathepsin B (CatB)-negative. CatB was significantly decreased in AD and inversely correlated with CAA severity. Immunoprecipitation followed by Western blotting, dot blot, and TEM revealed oligomeric aggregates and short fibrillar CST3 bound to Aβ in both AD and J20 mice brains. In J20 mice, CST3 was only neuron-positive at 2 months, and vessel-positive, colocalizing with Aβ from 3 months. These findings suggest that aggregated CST3 binds to Aβ and accumulates around vessels, potentially contributing to CAA pathogenesis.