Aquaporin-4 and Caveolin-1 as Mediators of Fibrinogen-Driven Cerebrovascular Pathology in Cerebral Amyloid Angiopathy

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Cerebral Amyloid Angiopathy (CAA), characterized by amyloid-β (Aβ) accumulation within perivascular spaces (PVS), contributes to vascular damage and inflammation in Alzheimer's disease (AD). Despite its significance, the mechanisms driving Aβ deposition in PVS and the resulting vascular pathology remain poorly understood. Growing evidence suggests that fibrinogen, the main component in blood clots, interacts with Aβ and exacerbates inflammation in AD. Fibrinogen also co-deposits with Aβ in the PVS of CAA-positive vessels in the brains of hereditary CAA patients. However, the mechanisms by which fibrinogen contributes to cerebrovascular impairment remain poorly understood. To investigate this, we used TgSwDI transgenic mice, which develop robust CAA pathology, and observed a significant increase in fibrin(ogen) extravasation and colocalization with Aβ in the PVS. Moreover, we observed a significant aquaporin-4 (AQP4) depolarization in CAA-laden blood vessels of TgSwDI mice, which correlated with fibrin(ogen)-Aβ colocalization. Given AQP4 crucial role in Aβ clearance through glymphatic pathway, its depolarization may disrupt critical Aβ clearance, thereby exacerbating CAA pathology. Additionally, Caveolin-1, a protein involved in non-specific transcytosis across the endothelium, significantly increased with age in TgSwDI mice and correlated with fibrin(ogen) extravasation. To further explore the relationship between fibrin(ogen) and these cerebrovascular alterations, we depleted fibrinogen in TgSwDI mice using siRNA approach. This intervention resulted in decreased CAA, restored polarized expression of AQP4, reduced caveolin-1 levels, attenuated microglial activation, and improved spatial memory in fibrinogen-depleted TgSwDI mice. These findings suggest that targeting fibrinogen could be a promising strategy for mitigating CAA pathology and its associated cerebrovascular pathology. Significance Statement: Our study uncovers the mechanism by which fibrin(ogen)-Aβ colocalization exacerbates CAA pathology. Our findings highlight the potential link between fibrinogen/ fibrin(ogen)-Aβ colocalization and AQP4 depolarization thereby exacerbating CAA pathology. The age-dependent increase of endothelial caveolin-1 could facilitate fibrin(ogen) extravasation, assisting the later to binds to Aβ in the perivascular space which ultimately induce microglial neuroinflammation and AQP4 depolarization, thus exacerbating CAA pathology. Furthermore, fibrinogen depletion could mitigate CAA severity, reduce microglial activation, restore AQP4 polarization and memory impairment. These results suggest that targeting fibrinogen and caveolin-1-mediated transcytosis may offer new strategies to address CAA-associated cerebrovascular pathology.

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