Basic Science and Pathogenesis.

Background: Impaired interstitial fluid drainage in the brain is indicated by the presence of perivascular β-amyloid (Aβ) deposits and is attributed to alterations in contractility and relaxation of vascular smooth muscle cells (SMCs). The brain microvasculature in Alzheimer disease (AD) accumulates amyloid-forming amylin secreted from the pancreas. Here, we tested the hypothesis that cerebrovascular amylin deposits perturbs cerebral Aβ efflux by impairing cerebral vasodilation.

Methods: Using transgenic rats expressing amyloid-forming human amylin in the pancreas (HIP rats) (aged 16-months) and wild-type (WT) littermates that express non-amyloidogenic rat amylin, we conducted comparative analyses of cerebral blood flow (CBF), pressure myography in isolated pial arteries and vascular SMC oxidative stress experiments.

Results: Longitudinal brain MRI measurements revealed consistent structural alterations that progressed more rapidly with aging in HIP vs. WT rats, leading to 14.9% reduction in CBF in HIP rats. Plasma nitrite and nitrate, stable nitric oxide (NO) end products, were increased in HIP vs. WT by 84.7% and 24.87%, respectively. Pressure myography experiments using pial arteries showed that both WT and HIP arteries developed arterial tone (e.g. pressure-induced constriction); however, arteries from HIP rats show significant elevations (56.9-142.3%) in arterial tone compared to WT rats at physiologically-relevant intravascular pressures (e.g. 60-100 mmHg). Consistent with these results, vascular SMCs from HIP rats showed elevated (12.6% increase) lipid peroxidation, which was replicated in SMCs incubated with exogenous human amylin (29.6%). Increased lipid peroxidation contributes to oxidative stress in the vascular wall and reduces NO bioavailability, altering vasodilatory function. Both arginase activity and expression (of Arginase 1 and 2) were increased in brain microvascular lysates from HIP rats compared to those from WT by 17.6%, 63.9%, and 57.8%, respectively, suggesting arginase-NO dysregulation. A possible impact of increased blood amylin concentration on cerebrovascular arginase-NO regulation was further tested in brain microvascular lysates from rats intravenously injected with amyloid-forming human amylin (55.0% reduction in arginase activity).

Conclusion: Our results indicate perivascular Aβ deposits in the setting of AD are potentially linked to amylin vasculopathy and altered spontaneous contraction/relaxation of cerebrovascular SMCs. Future experiments will focus on delineating molecular markers of amylin-induced alterations of SMC contractile phenotype.