A nontransgenic mouse model shows inducible amyloid-β (Aβ) peptide deposition and elucidates the role of apolipoprotein E in the amyloid cascade

Abstract

The amyloid-␤ (A␤) peptide, a major pathological hallmark of Alzheimer's disease (AD), undergoes a cascade of interactions resulting in the formation of soluble aggregates and their conversion in the brain to insoluble deposits and mature senile plaques. Furthermore, the apoE4 isoform of apolipoprotein E (apoE), which is the major genetic risk factor of AD, is associated with increased A␤ deposition. It is not known how the different A␤ aggregates in the amyloid cascade are formed, contribute to the pathogenesis of AD, or are affected by apoE4. To investigate the initial aggregation stages underlying the amyloid cascade in vivo and how apoE affects them, we examined the effects of prolonged inhibition and subsequent reactivation of the A␤-degrading protease neprilysin on deposition, disaggregation, and fibrillization of A␤ in apoEtransgenic and control mice. In control mice, intracerebroventricular infusion of thiorphan, which inhibits neprilysin, induced A␤42 and A␤40 deposition and fibrillization. On termination of thiorphan treatment, the number of A␤ deposits decreased, whereas the fibrillar A␤ deposits were unaffected. Similar treatments in apoE-deficient mice and mice transgenic for human apoE4 or apoE3 revealed that apoE4 enhances specifically the nucleation and aggregation of immunopositive A␤ deposits and that reversible disaggregation of these deposits and their irreversible conversion to fibrillar deposits are stimulated similarly by the different apoE isoforms. Deposition of A␤ and its enhancement by apoE4 were accompanied by increased astrogliosis both far from and near the A␤ deposits, suggesting that astrogliosis might be triggered by both insoluble and soluble A␤ aggregates.C onverging genetic and histopathological observations led to the formulation of the amyloid hypothesis, which proposes that accumulation of amyloid-␤ (A␤) peptide, a major constituent of the brain plaques characteristic of Alzheimer's disease (AD), is the primary event in AD pathogenesis (1, 2). Recent findings suggest that brains of individuals with AD also contain soluble and neurotoxic A␤ oligomers, which seem to be an intermediate state in the A␤-aggregation cascade, whose downstream product is the senile plaque (3-10). There is poor correlation between the severity of dementia and the density of amyloid plaques in AD (11-13); furthermore, in mice transgenic for the human A␤ precursor protein (APP), synaptic degeneration and cognitive decline are observed even before amyloid is deposited (14-16). This finding suggests that the pathological effects of A␤ in AD are mediated by A␤ aggregates that precede the formation of the senile plaque. The relative contributions of the different soluble and insoluble A␤ aggregates to the pathology of AD and the mechanisms underlying their formation in vivo are not yet known.Apolipoprotein E (apoE), the major brain lipid-binding protein, is expressed in humans as three isoforms (apoE2, apoE3, and apoE4), which differ in one or two amino acids (17). The apoE4 genotype is the major genetic ...

Characterization of Oligomer Formation of Amyloid-β Peptide Using a Split-luciferase Complementation Assay

“…Recently endogenous murine A␤ was reported to have a crucial role in synaptic activity (35), and endogenous murine A␤ oligomers suggested a role regulating synaptic activity. Moreover, by the prolonged inhibition of A␤-degrading enzyme neprilysin in the brain of wild-type mice, it has been reported that murine A␤ is deposited in the brain (36), suggesting that murine A␤ may undergo a process of oligomerization and fibrillization similar to human A␤.…”

Amino-terminal Domain Stability Mediates Apolipoprotein E Aggregation into Neurotoxic Fibrils

“…In postmortem AD brain sections, apoE4 is associated with greater A deposition, amyloid dye reactivity, and density of neuritic plaques than apoE3 12 . Transgenic mouse models of AD also show that apoE is central to influencing A amyloid plaque pathology and deposition 6,11 . Older transgenic mice expressing the V717F mutant of amyloid precursor protein (APP) develop significantly greater quantities of thioflavin-S-reactive neuritic plaques in the hippocampus when also expressing human apoE in astrocytes under the glial fibrillary acidic protein (GFAP) promoter 6 .…”

“…A and mediation of fibrillization and plaque formation, the binding and disruption of membranes by apoE, apoE-mediated lipid transport, and apoE-mediated neuronal sensitivity to injury and recovery [3][4][5][6][7][8][9][10][11] .…”

Neprilysin-2 Is an Important β-Amyloid Degrading Enzyme

“…4 However, when treated with thiorphan, an NEP endopeptidase inhibitor, mice and rats demonstrate pathological accumulations of A␤ after only 1 month. 2,5 This was also found in mice treated with phosphoramidon, another NEP inhibitor. 6 These results indicate that there may exist additional NEPlike endopeptidases in the metalloprotease 13 (M13) family that are central to the A␤ clearance pathway.…”

“…Previous studies have demonstrated that treating rodents with inhibitors of NEP/NEP2 (ie, phosphoramidon and thiorphan) produces more dramatic elevations in A␤ (30-to 50-fold). 2,5,6 However, despite the lack of both NEP and NEP2 genes, only modest elevations in A␤ peptides were observed in our double-knockout studies-well below known plaque-inducing levels. As expected, the immunohistochemical analysis of 14-month-old DKO mice did not show evidence of plaque deposition (data not shown).…”