Ageing Increases Vulnerability to Aβ42 Toxicity in Drosophila

Abstract

Age is the major risk factor for many neurodegenerative diseases, including Alzheimer's Disease (AD), for reasons that are not clear. The association could indicate that the duration or degree of exposure to toxic proteins is important for pathology, or that age itself increases susceptibility to protein toxicity. Using an inducible Drosophila model of AD, we investigated these possibilities by varying the expression of an Aβ42 transgene in neurons at different adult ages and measuring the effects on Aβ42 levels and associated pathological phenotypes. Acute induction of Arctic Aβ42 in young adult flies resulted in rapid expression and clearance of mRNA and soluble Arctic Aβ42 protein, but in irreversible expression of insoluble Arctic Aβ42 peptide. Arctic Aβ42 peptide levels accumulated with longer durations of induction, and this led to a dose-dependent reduction in negative geotaxis and lifespan. For a standardised level of mRNA expression, older flies had higher levels of Arctic Aβ42 peptide and associated toxicity, and this correlated with an age-dependent reduction in proteasome activity. Equalising Aβ42 protein at different ages shortened lifespan in correlation with the duration of exposure to the peptide, suggesting that Aβ42 expression accumulates damage over time. However, the relative reduction in lifespan compared to controls was greater in flies first exposed to the peptide at older ages, suggesting that ageing itself also increases susceptibility to Aβ42 toxicity. Indeed older flies were more vulnerable to chronic Aβ42 toxicity even with a much lower lifetime exposure to the peptide. Finally, the persistence of insoluble Aβ42 in both young and old induced flies suggests that aggregated forms of the peptide cause toxicity in later life. Our results suggest that reduced protein turnover, increased duration of exposure and increased vulnerability to protein toxicity at later ages in combination could explain the late age-of-onset of neurodegenerative phenotypes.

Lithium suppresses Aβ pathology by inhibiting translation in an adult Drosophila model of Alzheimer's disease

“…However, we have observed both soluble and insoluble Aβ in the Arctic Aβ42 flies [(Rogers et al, 2012), and data not shown]; and the ability of lithium to reduce translation of the Aβ peptide without affecting its clearance may lower the level of soluble Aβ. In a wider context, lithium might be beneficial in ameliorating toxicity of AD by lowering expression of APP and of proteins that are involved in the generation of Aβ from APP.…”

“…Chymotrypsin-like peptidase activity of the proteasome was assayed the using fluorogenic peptide substrate Succinyl-Leu-Leu-Val-Tyr-amidomethylcoumarin (LLVY-AMC), based on a previously published protocol (Bulteau et al, 2002; Rogers et al, 2012). 20 μg of crude fly head homogenate total protein was incubated at 37°C with 25 μM LLVY-AMC in a final volume of 200 μLs.…”

“…AD is a neurodegenerative disorder characterized by the presence of amyloid beta (Aβ) deposits and neurofibrillary, hyperphosphorylated tau tangles in the brain (Spires and Hyman, 2005). Age is the major risk factor for AD, and the fruit fly Drosophila has been used to demonstrate experimentally that the neurons of older adult flies are intrinsically more susceptible to Aβ toxicity (Rogers et al, 2012). The ageing process could contribute to increased vulnerability to protein toxicity through several routes, including reduced protein turnover through inefficient proteasome- and autophagy-mediated clearance mechanisms (Rubinsztein et al, 2011; Rogers et al, 2012).…”

“…Age is the major risk factor for AD, and the fruit fly Drosophila has been used to demonstrate experimentally that the neurons of older adult flies are intrinsically more susceptible to Aβ toxicity (Rogers et al, 2012). The ageing process could contribute to increased vulnerability to protein toxicity through several routes, including reduced protein turnover through inefficient proteasome- and autophagy-mediated clearance mechanisms (Rubinsztein et al, 2011; Rogers et al, 2012). Interestingly, Aβ accumulation has been linked to several processes affected by ageing.…”

Genetics and Pharmacology of Longevity

“…Furthermore, a reduction in proteasome activity and abundance has also been demonstrated in day 20e30 Drosophila heads and is associated with the impaired assembly of the 20S proteasome and the 19S regulatory subunits (Tonoki et al, 2009). A 50% reduction in proteasomal activity has also been detected between days 9 and 27 in Drosophila heads, when measured by a fluorogenic peptide substrate (Rogers et al, 2012). In addition, another study showed reduced proteasomal activity in the spinal cord of rats at 12, 24, and 28 months of age compared with spinal cord from younger animals (Keller, Huang, & Markesbery, 2000), suggesting that a relatively early decline in proteasomal activity is an evolutionary conserved feature of aging.…”