Regulation of Steady-state β-Amyloid Levels in the Brain by Neprilysin and Endothelin-converting Enzyme but Not Angiotensin-converting Enzyme

Abstract

The deposition of ␤-amyloid in the brain is a pathological hallmark of Alzheimer disease (AD). Normally, the accumulation of ␤-amyloid is prevented in part by the activities of several degradative enzymes, including the endothelin-converting enzymes, neprilysin, insulin-degrading enzyme, and plasmin. Recent reports indicate that another metalloprotease, angiotensin-converting enzyme (ACE), can degrade ␤-amyloid in vitro and in cellular overexpression experiments. In addition, ACE gene variants are linked to AD risk in several populations. Angiotensin-converting enzyme, neprilysin and endothelin-converting enzyme function as vasopeptidases and are the targets of drugs designed to treat cardiovascular disorders, and ACE inhibitors are commonly prescribed. We investigated the potential physiological role of ACE in regulating endogenous brain ␤-amyloid levels for two reasons: first, to determine whether ␤-amyloid degradation might be the mechanism by which ACE is associated with AD, and second, to determine whether ACE inhibitor drugs might block ␤-amyloid degradation in the brain and potentially increase the risk for AD. We analyzed ␤-amyloid accumulation in brains from ACE-deficient mice and in mice treated with ACE inhibitors and found that ACE deficiency did not alter steady-state ␤-amyloid concentration. In contrast, ␤-amyloid levels are significantly elevated in endothelin-converting enzyme and neprilysin knock-out mice, and inhibitors of these enzymes cause a rapid increase in ␤-amyloid concentration in the brain. The results of these studies do not support a physiological role for ACE in the degradation of ␤-amyloid in the brain but confirm roles for endothelin-converting enzyme and neprilysin and indicate that reductions in these enzymes result in additive increases in brain amyloid ␤-peptide levels.

Angiotensin-Converting Enzyme Converts Amyloid β-Protein 1–42 (Aβ1–42) to Aβ1–40, and Its Inhibition Enhances Brain Aβ Deposition

“…An acute, one-shot oral administration of captopril to the young Tg2576 mice resulted in a significant decrease in serum and brain ACE activities; however, the level of brain A␤ 1-42 or A␤ 1-40 was not significantly affected. A recent in vivo study also shows that ACE inhibitors at doses similar to those used clinically do not increase the levels of brain A␤s (Eckman et al, 2006). Previous studies using purified human seminal plasma ACE and cultured cells showed that ACE degrades A␤s and ACE inhibition increases A␤ levels in APP-and ACE-transfected cells (Hu et al, 2001;Hemming and Selkoe, 2005;Oba et al, 2005).…”

BRI2 (ITM2b) Inhibits A Deposition In Vivo

“…For brain A␤ ELISAs from 2-month-old Tg2576 mice, hemi-forebrains were homogenized in radioimmunoprecipitation assay buffer (0.1% SDS, 0.5% deoxycholate, 1% Triton X-100, 150 mM NaCl, and 50 mM Tris-HCl) and then ultracentrifuged at 100,000 ϫ g for 1 h. To measure the endogenous mouse A␤ levels, hemi-forebrains of nontransgenic littermates of the TgCRND8 mice expressing BRI2 were homogenized in 0.2% diethylamine buffer containing 50 mM NaCl and 1ϫ protease inhibitor mixture (Roche). Endogenous mouse A␤ levels were measured using the previously validated rodent-specific A␤ ELISA system as previously reported (Eckman et al, 2006). For plasma A␤ analysis, blood was collected in EDTA-coated tubes after cardiac puncture.…”

In vitro and in vivo degradation of Aβ peptide by peptidases coupled to erythrocytes

“…The ability to hydrolyze insulin is not shared by NEP which may makes a better candidate. NEP has been considered a promising candidate for AD treatment, since it appears to be the major, or one of the major enzymes involved in Aβ clearance in the brain [11,19,20,28]. NEP has other advantages over IDE and MP-1.…”