14 December 2009. It is not just material goods that are best when they come in small packages, but amyloid-β (Aβ) as well, according to a paper in the December 10 issue of Cell. Researchers led by Howard Hughes Investigator Andrew Dillin at The Salk Institute, La Jolla, California, report that mice overproducing Aβ survive longer without disease symptoms when they tightly bundle the rogue peptide into dense plaques. The work supports the idea that small, oligomeric forms of the peptide are the most toxic, and that keeping them under wraps is good for the brain. In this case, the ability to bundle and detox Aβ arises from knocking out one copy of the insulin-like growth factor 1 (IGF-1) receptor, a component of the insulin/IGF-1 signaling pathway. The work suggests that reducing insulin/IGF-1 signaling (IIS), for example, by eating drastically fewer calories, could be one way for humans to stave off AD—worth keeping in mind as some of us contemplate stuffing ourselves, as well as the holiday fowl, in a little over two weeks.
“The work also represents a celebration for the aging field,” said Dillin. The IIS pathway has long been linked to longevity. Toning down IIS extends the lifespan of worms (see ARF related news story), flies (see Tatar et al., 2001), and mice (see ARF related news story), and the presence of genetic variations in the IGF-1 pathway in the oldest old hints that the same might be true in humans (see ARF related news story on longevity in humans). While tweaking this pathway might, in the future, extend our lives, more importantly, it might grant a reprieve from age-related diseases such as Alzheimer, Parkinson, Huntington, and other neurodegenerative diseases that emerge as people grow older. “The hypothesis that we can learn enough about the aging process to manipulate it and change age-onset diseases was put out several decades ago, and this is the first demonstration that that may be possible,” said Dillin. He also hopes the work will help dispel what he considers a popular misconception of aging research, namely that researchers want to extend life expectancy to 250 years old. “It’s really about allowing a person who’s 60 years old and diagnosed with Alzheimer disease to live to be 85 and get to know their grandkids,” he told ARF.
Mice may not worry about their grandpups, but they do exhibit some of the symptoms and pathology of AD when they overproduce Aβ. To see if IGF-1 contributes to mouse Aβ toxicity as it does in worms (see ARF related news story and ARF meeting report), first author Ehud Cohen and colleagues crossed double-transgenic AD mice (APPSwe/PS1ΔE9) with animals that lack one genetic copy of the IGF-1 receptor (Igf1R+/-). They then tested the offspring (AD/Igf1R+/-) for a variety of AD-related symptoms.
“What was really surprising was that cognitive impairments were fully restored,” said Dillin. APP/PS mice normally show impaired learning and memory by 12 months of age, but Cohen and colleagues found that the 11- to 15-month-old AD/Igf1R+/- mice perform just as well as wild-type animals in the Morris water maze test of spatial memory. Unlike age-matched APP/PS mice, the IIS-impaired animals also performed as well as normal mice on the rotarod, a device that tests motor skills. Furthermore, AD/Igf1R+/- animals outlived APP/PS1 animals, which begin to die starting at 16 months of age. (Typically, laboratory mice live for about two years.)
What explains the protection? The researchers found that reactive astrocytosis, an aspect of inflammation, was halved in AD/Igf1R+/- animals compared to AD mice. Cortical/hippocampal levels of neuronal (NeuN) and synaptic (synaptophysin) markers, which are reduced in APP/PS1 animals, were normal, suggesting that reduced IGF signaling protects against neuron loss, according to the authors. (Most AD mice do not show frank neuronal loss, and to the extent that they do, it tends to be in non-cholinergic systems. See ARF related news story.)
When the researchers assessed Aβ deposits, they found that IGF-1 signaling seems to have no effect on when plaques emerge, at around eight or nine months, in both APP/PS1 and AD/Igf1R+/- mice, but that it does appear to change their morphology. Plaques in AD/Igf1R+/- mice were smaller and more condensed than in their APP/PS1 counterparts, as judged by immunoreactivity to the Aβ monoclonal antibody 82E1. Electron microscopy confirmed that the plaques from the Igf1R heterozygotes were much denser than those from animals with two copies of the gene. Plaques in the IIS-compromised animals were also more resistant to proteinase K, another indication that the deposits are tightly packed.
Packing more Aβ per plaque could prove beneficial if it reduced the amount of oligomeric Aβ species floating around the brain, since those soluble oligomers are now widely believed to be the most toxic form of the peptide (see ARF related news story). In fact, that is what the researchers discovered. They found that 12- to 13-month-old AD/Igf1R+/- had a higher Aβ load but less than half the amount of soluble Aβ40/42 compared to age-matched APP/PS1 mice. Also, using size exclusion chromatography to separate large aggregates, then SDS electrophoresis for analysis, the researchers showed that in the IGF-1R-deprived animals, Aβ existed in bigger aggregates than in the APP/PS1 animals. It is in those larger aggregates that the scientists found Aβ dimers, which may be a particularly toxic form of the peptide for humans, and potentially mice (see ARF related news story).
The data suggest that toning down the IGF-1 pathway might be one way to delay the onset of symptoms in AD. IGF-1 has, curiously enough, been touted as a potential treatment for AD, but Dillin says that if that approach is successful, it is likely because it overactivates the pathway, leading to a compensatory repression of the IGF receptor. So far, clinical trials of IGF-1 have proven disappointing (see ARF related news story).
Dillin suggested that other targets in the pathway might prove more productive. Forkhead transcription factors lie downstream, for example, and have been linked to human longevity (see, e.g., Willcox et al., 2008).
One question this work raises is whether putting the brakes on this pathway in already aged animals, or eventually humans, would have a similar effect to knocking out the gene from the embryonic stage of development, as in this AD/Igf1R+/- model. Dillin thinks it might. His lab has knocked down the IGF-1 receptor in aged worms that produce Aβ and found this strategy to protect against toxicity.—Tom Fagan.
Cohen E, Paulsson JF, Blinder P, Burstyn-Cohen T, Du D, Estepa G, Adame A, Pham HM, Holzenberger M, Kelly JW, Masliah E, Dillin A. Reduced IGF-1 signaling delays age-associated proteotoxicity in mice. Cell 2009 December 10; 139:1-13. Abstract