Overexpression of Neprilysin Reduces Alzheimer Amyloid-β42 (Aβ42)-induced Neuron Loss and Intraneuronal Aβ42 Deposits but Causes a Reduction in cAMP-responsive Element-binding Protein-mediated Transcription, Age-dependent Axon Pathology, and Premature Death in Drosophila

Abstract

The amyloid-␤42 (A␤42) peptide has been suggested to play a causative role in Alzheimer disease (AD). Neprilysin (NEP) is one of the rate-limiting A␤-degrading enzymes, and its enhancement ameliorates extracellular amyloid pathology, synaptic dysfunction, and memory defects in mouse models of A␤ amyloidosis. In addition to the extracellular A␤, intraneuronal A␤42 may contribute to AD pathogenesis. However, the protective effects of neuronal NEP expression on intraneuronal A␤42 accumulation and neurodegeneration remain elusive. In contrast, sustained NEP activation may be detrimental because NEP can degrade many physiological peptides, but its consequences in the brain are not fully understood. Using transgenic Drosophila expressing human NEP and A␤42, we demonstrated that NEP efficiently suppressed the formation of intraneuronal A␤42 deposits and A␤42-induced neuron loss. However, neuronal NEP overexpression reduced cAMP-responsive element-binding protein-mediated transcription, caused age-dependent axon degeneration, and shortened the life span of the flies. Interestingly, the mRNA levels of endogenous fly NEP genes and phosphoramidon-sensitive NEP activity declined during aging in fly brains, as observed in mammals. Taken together, these data suggest both the protective and detrimental effects of chronically high NEP activity in the brain. Down-regulation of NEP activity in aging brains may be an evolutionarily conserved phenomenon, which could predispose humans to developing late-onset AD. Alzheimer disease (AD)3 is a progressive neurodegenerative disease defined by two protein deposits in autopsied brains: extracellular amyloid plaques composed of the 40-or 42-amino acid ␤-amyloid peptides (A␤40 or A␤42), and intracellular neurofibrillary tangles composed of abnormally hyperphosphorylated microtubule-associated protein Tau (1). A␤ peptides are physiological metabolites of the amyloid precursor protein (APP) resulting from sequential cleavage by ␤-secretase and ␥-secretase complex, whose catalytic subunits are presenilin 1 (PS1) and presenilin 2 (PS2) (2). Molecular genetic studies of early onset familial AD patients have identified causative mutations in APP, PS1, and PS2 genes, and these mutations promote A␤42 production, aggregation, and stability against clearance (3). Thus, the increased A␤42 levels in the brain are believed to initiate AD pathogenesis.The mechanisms by which A␤42 reaches pathological levels in the brains of late-onset AD (LOAD) patients are not well understood. The steady state level of A␤42 in the brain reflects the balance between production and clearance, and a reduction in clearance activity would raise A␤42 levels. A deficiency in neprilysin (NEP), a major rate-limiting A␤-degrading enzyme (4, 5), accelerates formation of extracellular amyloid deposits (6), amyloid angiopathy (6), synaptic dysfunctions (7), and memory defects (7) caused by human A␤ in transgenic mice. In LOAD patients, NEP mRNA and pro-

Intraneuronal Aβ accumulation and neurodegeneration: Lessons from transgenic models

“…Interestingly, the severity of the neurodegenerative phenotype correlated with the propensity of the expressed Aβ peptide to form oligomers (Crowther et al, 2005). Overexpression of neprilysin, a protease known to degrade Aβ peptides, efficiently suppressed the formation of intraneuronal Aβ 42 deposits and led to a significant reduction in Aβ 42 -induced neuron loss (Iijima-Ando et al, 2008). In a further related Drosophila model, overexpression of wild-type or Aβ 42 with the Arctic mutation in a small group of neurons in adult flies led to intracellular Aβ accumulation and the induction of a variety of age-dependent changes, including depletion of presynaptic mitochondria, slowdown of bi-directional transport of axonal mitochondria and decreased numbers of synaptic vesicles (Zhao et al, 2010).…”

Integration of Taste and Calorie Sensing inDrosophila

“…Mixed-sex populations of w 1118 flies were kept for all assays unless noted otherwise. Other fly stocks used were Gr66a-Gal4 (Wang et al, 2004); UAS-shi ts (Kitamoto, 2001); UAS-InR DN (UAS-InR.K1409A; Bloomington); tsh-gal80 (Clyne and Miesenböck, 2008); UAS-PKIF and UAS-PKIG 19,20 F (Kiger et al, 1999); UASdCREB2b (Iijima-Ando et al, 2008); and tubGal80 ts (McGuire et al, 2004); dnc 1 (Dudai et al, 1976); Ilp2-Gal4 (Rulifson et al, 2002);and Ilp2, Ilp3, and Ilp5 (Grönke et al, 2010).…”

Expression of β-Amyloid Induced Age-Dependent Presynaptic and Axonal Changes in Drosophila

“…Drosophila AD models were generated by expression of human A␤ or APP/ presenilin/BACE (␤-secretase activity of the ␤-site APP-cleaving enzyme). These AD flies exhibited age-dependent AD-like pathology and behavioral changes (Finelli et al, 2004;Greeve et al, 2004;Crowther et al, 2005;Iijima-Ando et al, 2008).…”