Clearly, the best way to avoid Alzheimer disease is not to grow old. But why? While the correlation between age and AD is clear, it’s not so apparent how the passage of time brings on toxic amyloid-β (Aβ) aggregates.
Aging and the accumulation of neurotoxic Aβ oligomers may be part and parcel of the same process, according to a new study on the roundworm Caenorhabditis elegans. The work shows that slowing aging by suppressing activity of the insulin/insulin-like growth factor signaling pathway also slows the onset of toxicity from constitutively expressed Aβ peptides. Reporting in Science online, Andrew Dillin and colleagues at the Salk Institute and the Scripps Research Institute in La Jolla, California, demonstrate that, just as for longevity, protection from Aβ requires the transcription factor DAF-16 and heat shock factor-1 (HSF-1). Each appears to have distinct activities in preventing the accumulation of toxic Aβ oligomers, however. The results raise the possibility that longevity genes in the insulin/IGF-1 pathway could also fend off late-onset neurodegeneration by preventing the buildup of aggregated proteins.
To look at aging and Aβ, joint first authors Ehud Cohen and Jan Bieschke generated worms constitutively expressing Aβ1-42 in the muscle cells of the worm’s body wall. These worms become paralyzed in early adulthood due to Aβ toxicity. But when the aging program was slowed by suppressing the insulin receptor homolog DAF-2 with RNAi, the worms not only lived longer, but showed a much slower onset of paralysis. This suggests that Aβ toxicity is not due simply to its constant expression over time, but depends in some way on the aging program.
It is well known that DAF-2 knockdown extends lifespan by relieving suppression of both DAF-16 and HSF-1 expression. Likewise, the authors found that reduction of Aβ-elicited paralysis by DAF-2 RNAi required both DAF-16 and HSF-1, because RNAi designed to decrease expression of either of the proteins increased paralysis in DAF-2 RNAi worms.
Both DAF-16 and HSF-1 regulate the expression of genes which protect cells against stress, including many chaperone proteins. To find out if the proteins acted by preventing the formation of a toxic Aβ aggregate, the researchers first looked for Aβ species that correlated with toxicity. By several criteria, including Western blotting and an Aβ aggregation assay, they found that the level of paralysis in response to different RNAi treatments did not correlate with the presence of high-molecular-weight aggregates. Worms treated with HSF-1 RNAi had the highest levels of Aβ fibrils and high-molecular-weight aggregates, while DAF-16 RNAi worms had the lowest, despite both sets of animals having the same time course of paralysis. Instead, paralysis correlated best with the presence of 16 kDa Aβ trimers, which were observed in control, DAF-16 and HSF-1 RNAi mice, but not in DAF-2 RNAi mice. This result is consistent with previous results implicating small soluble Aβ oligomers or trimers in neurotoxicity (see ARF related news story and Townshend et al., 2006).
Based on the abundance of high-molecular-weight Aβ aggregates in HSF-1 RNAi worms, and the paucity of the same in DAF-16 RNAi animals, the researchers hypothesized that HSF-1 controlled disaggregation while DAF-16 controlled aggregation. To support this idea, they tested worm extracts for the ability to disaggregate and degrade Aβ fibrils in vitro. In this assay, extracts from HSF-1 RNAi worms showed a modestly lower disaggregation activity, compared to extracts from DAF-16 RNAi worms.
From these results, the researchers propose a model where small aggregation-prone peptides constitutively form small toxic aggregates. Cells can detoxify the aggregates by either disaggregating them (the HSF-1 pathway) or by further aggregating low-molecular-weight toxic species into higher-molecular-weight aggregates (DAF-16 regulated pathway). Both of these pathways are negatively regulated by the insulin/IGF-1 receptor (DAF-2) signaling pathway.
“According to our model, the aging process actively reduces the cellular ability to detoxify small toxic aggregates by negative regulation of both detoxification mechanisms via the insulin-like signaling pathway,” the authors write. Studies showing that the toxicity of huntingtin protein was also mitigated by DAF-2 mutations in worms (see ARF related news story and see ARF news story and Morley et al., 2002) suggest that the anti-aging effects could be generalizable to other toxic aggregates. This model, while intriguing, will have to be confirmed by other approaches besides RNAi, and in other organisms.—Pat McCaffrey