Cerebral deposition of the amyloid ␤ protein (A␤), an invariant feature of Alzheimer's disease, reflects an imbalance between the rates of A␤ production and clearance. The causes of A␤ elevation in the common late-onset form of Alzheimer's disease (LOAD) are largely unknown. There is evidence that the A␤-degrading protease neprilysin (NEP) is down-regulated in normal aging and LOAD. We asked whether a decrease in endogenous NEP levels can prolong the halflife of A␤ in vivo and promote development of the classic amyloid neuropathology of Alzheimer's disease. We examined the brains and plasma of young and old mice expressing relatively low levels of human amyloid precursor protein and having one or both NEP genes silenced. NEP loss of function 1) elevated whole-brain and plasma levels of human A␤ 40 and A␤ 42 , 2) prolonged the half-life of soluble A␤ in brain interstitial fluid of awake animals, 3) raised the concentration of A␤ dimers, 4) markedly increased hippocampal amyloid plaque burden, and 5) led to the development of amyloid angiopathy. A ϳ50% reduction in NEP levels, similar to that reported in some LOAD brains, was sufficient to increase amyloid neuropathology. These findings demonstrate an important role for proteolysis in determining the levels of A␤ and A␤-associated neuropathology in vivo and support the hypothesis that primary defects in A␤ clearance can cause or contribute to LOAD pathogenesis. (Am J Pathol

Insulin degrading enzyme (IDE) is expressed in the brain and may play an important role there in the degradation of the amyloid beta peptide (Abeta). Our results show that cultured human cerebrovascular endothelial cells (HCECs), a primary component of the blood-brain barrier, express IDE and may respond to exposure to low levels of Abeta by upregulating its expression. When radiolabeled Abeta is introduced to the medium of cultured HCECs, it is rapidly degraded to smaller fragments. We believe that this degradation is largely the result of the action of IDE, as it can be substantially blocked by the presence of insulin in the medium, a competitive substrate of IDE. No inhibition is seen when an inhibitor of neprilysin, another protease that may degrade Abeta, is present in the medium. Our evidence suggests that the action of IDE occurs outside the cell, as inhibitors of internalization fail to affect the rate of the observed degradation. Further, our evidence suggests that degradation by IDE occurs on the plasma membrane, as much of the IDE present in HCECs was biotin-labeled by a plasma membrane impermeable reagent. This activity seems to be polarity dependent, as measurement of Abeta degradation by each surface of differentiated HCECs shows greater degradation on the basolateral (brain-facing) surface. Thus, IDE could be an important therapeutic target to decrease the amount of Abeta in the cerebrovasculature.

No abstract

No abstract