Involvement of Insulin-Degrading Enzyme in Insulin- and Atrial Natriuretic Peptide-Sensitive Internalization of Amyloid-β Peptide in Mouse Brain Capillary Endothelial Cells

Abstract

Cerebral clearance of amyloid-β peptide (Aβ), which is implicated in Alzheimer's disease, involves elimination across the blood-brain barrier (BBB), and we previously showed that an insulin-sensitive process is involved in the case of Aβ_1-40. The purpose of this study was to clarify the molecular mechanism of the insulin-sensitive Aβ_1-40 elimination across mouse BBB. An in vivo cerebral microinjection study demonstrated that [¹²⁵I]hAβ_1-40 elimination from mouse brain was inhibited by human natriuretic peptide (hANP), and [¹²⁵I]hANP elimination was inhibited by hAβ_1-40, suggesting that hAβ_1-40 and hANP share a common elimination process. Internalization of [¹²⁵I]hAβ_1-40 into cultured mouse brain capillary endothelial cells (TM-BBB4) was significantly inhibited by either insulin, hANP, other natriuretic peptides or insulin-degrading enzyme (IDE) inhibitors, but was not inhibited by phosphoramidon or thiorphan. Although we have reported the involvement of natriuretic peptide receptor C (Npr-C) in hANP internalization, cells stably expressing Npr-C internalized [¹²⁵I]hANP but not [¹²⁵I]hAβ_1-40, suggesting that there is no direct interaction between Npr-C and hAβ_1-40. IDE was detected in plasma membrane of TM-BBB4 cells, and internalization of [¹²⁵I]hAβ_1-40 by TM-BBB4 cells was reduced by IDE-targeted siRNAs. We conclude that elimination of hAβ_1-40 from mouse brain across the BBB involves an insulin- and ANP-sensitive process, mediated by IDE expressed in brain capillary endothelial cells.