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.

Reference:
Cohen E, Bieschke J, Perciavalle RM, Kelly JW, Dillin A. Opposing activities protect against age onset proteotoxicity. 2006 August 10. Sciencexpress online publication. Abstract

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  1. This paper attempts to address two important questions that stem from the “amyloid cascade” model of Alzheimer pathology: why is AD age-dependent, and which specific forms of the β amyloid peptide (Aβ) are responsible for the toxicity? The experimental model employed in this study are transgenic Caenorhabditis elegans worms engineered to constitutively express human Aβ1-42 in muscle cells. These transgenic worms accumulate intracellular Aβ and show a progressive paralysis that begins in adulthood. Cohen et al. manipulated two (likely interacting) stress response pathways in C. elegans and examined the effect on Aβ toxicity and the accumulation of Aβ species. One response pathway, which is controlled by the HSF-1 transcription factor, regulates the response to heat shock, and is the major pathway by which cells detoxify misfolded proteins (which are a primary result of heat shock). The other stress response pathway, which is controlled by DAF-2 (a homolog of insulin/IGF receptor) and the downstream transcription factor DAF-16, predominantly controls the worm response to starvation, although activation of this pathway broadly contributes to stress resistance. A byproduct of activating DAF-16 (or inactivating DAF-2, a negative regulator of DAF-16), is a significant increase in worm lifespan.

    The results from inactivating HSF-1 by RNA interference (RNAi) are reasonably straightforward. HSF-1 inactivation increases the rate of paralysis of the Aβ worms, and leads to the accumulation of higher-molecular-weight species of Aβ. Extracts of Aβ worms with decreased HSF-1 levels are better at seeding formation of amyloid fibrils and worse at disaggregating amyloid fibrils in vitro. These results are consistent with a model in which activation of HSF-1 in Aβ worms leads to the expression of protective chaperone proteins, which counter both the formation of higher molecular forms of Aβ and also protect the worms from Aβ toxicity. Chaperone proteins are known to be induced by and co-localize with Aβ in this worm model (Fonte et al., 2002), and unpublished experiments from my lab have demonstrated that forced expression of some of these chaperone proteins can suppress Aβ toxicity in C. elegans. The results of the HSF-1 RNAi experiments are all consistent with the idea that chaperone proteins play an important role in modulating the accumulation of toxic intracellular Aβ species.

    The results of experiments using DAF-2 and DAF-16 RNAi are more complicated and harder to interpret. Knockdown of DAF-2 by RNAi (activation of the DAF-2/DAF-16 stress response pathway) protects from Aβ-induced paralysis, and knockdown of DAF-16 by RNAi (inactivation of this stress response pathway) increases paralysis, consistent with expectations. However, DAF-2 RNAi suppression of toxicity is associated with an increase in higher-molecular-weight Aβ species and in vitro fibril seeding, while DAF-16 RNAi decreases levels of higher-molecular-weight Aβ and in vitro fibril seeding capacity. These results strongly argue that the higher-molecular-weight Aβ species are not the toxic component, which is perhaps not surprising, given that these AD model worms are born with fibrillar amyloid deposits (Link et al., 2001) but do not show paralysis until they are adults. Conversely, Aβ can induce paralysis and protein oxidation in C. elegans independent of the formation of detectable amyloid (Drake et al., 2003). So, if the higher-molecular-weight Aβ species are not relevant, what are the HSF-1 and DAF-2/DAF-16 pathways doing to suppress Aβ toxicity? By altering their extraction protocol, these researchers were able to identify a ~16 kD Aβ species that correlated with levels of paralysis in the RNAi experiments, implying that this could be the key toxic species regulated by the HSF-1 and DAF-2/DAF-16 pathways.

    What does this study tell us about the age-dependence of Alzheimer disease? Perhaps that it is the decay of protective cellular systems with age that “unleash” toxic Aβ species. If this speculation is true, then perhaps AD risk alleles will be identified that function in stress response pathways analogous to those investigated in this study. What is the toxic Aβ species in this worm model? This is trickier to answer, because there are some unproven assumptions underlying this study. One assumption is that these stress response pathways operate at the level of Aβ accumulation/multimerization, not somewhere downstream in the toxic process. Accumulation of polyglutamine repeat proteins in a similar C. elegans model (Gidalevitz et al., 2006) have been shown to generally perturb protein homeostasis, so it is possible that these pathways actually act on non-native forms of endogenous worm proteins whose folding has been secondarily perturbed by Aβ accumulation. In this case, the forms of Aβ that accumulate when these response pathways are inhibited may be irrelevant. It is also important to point out that correlation of any specific form of Aβ with toxicity does not prove causality. Although many current studies have pointed to oligomeric Aβ species as being directly involved in Aβ toxicity, it is actually quite difficult to show that a specific oligomeric form of Aβ visualized on a denaturing gel is really the toxic form in vivo. Nevertheless, this study does add to the growing body of evidence that high-molecular-weight forms of Aβ (e.g., Aβ fibrils) are not the key toxic species in numerous AD model systems.

    References:

    . Oxidative stress precedes fibrillar deposition of Alzheimer's disease amyloid beta-peptide (1-42) in a transgenic Caenorhabditis elegans model. Neurobiol Aging. 2003 May-Jun;24(3):415-20. PubMed.

    . Interaction of intracellular beta amyloid peptide with chaperone proteins. Proc Natl Acad Sci U S A. 2002 Jul 9;99(14):9439-44. PubMed.

    . Progressive disruption of cellular protein folding in models of polyglutamine diseases. Science. 2006 Mar 10;311(5766):1471-4. PubMed.

    . Visualization of fibrillar amyloid deposits in living, transgenic Caenorhabditis elegans animals using the sensitive amyloid dye, X-34. Neurobiol Aging. 2001 Mar-Apr;22(2):217-26. PubMed.

  2. Is there any evidence of pancreatitis prior to the development of AD which may explain the large weight loss which is reported to occur several years prior to AD? It's of interest that HSF-1 is activated in acute pancreatitis (1) and β-APP is activated by HSF-1 (2). Perhaps AD occurs as a result of a prolonged stress response to pancreatitis. The subsequent increased ornithine decarboxylase activity in response to increased APP expression (3) may be expected to reduce the arginine load; however, the trade-off is the development of AD if this response continues. The study by He and colleagues (4) finding that GSK-3β and ERK MAPK facilitate the inactivation of activated HSF-1 has me wondering whether we may exacerbate pancreatitis as a consequence of GSK-3β inhibition. It's interesting that lithostathine and pancreatitis-associated protein are involved in the early stages of AD (5). Also of interest is that pancreatitis-associated protein (PAP)-like protein is elevated in the early stages of scrapie infection (6).

    The study by Wolozin and colleagues, soon to be the subject of an ARF live discussion, reporting a decreased risk of emphysema for those using statin drugs might implicate α1 antitrypsin. There are conflicting reports of α1 antitrypsin deficiency associated with pancreatitis. Are those using statins less likely to develop pancreatitis?

    References:

    . Heat shock factor-1 and nuclear factor-kappaB are systemically activated in human acute pancreatitis. JOP. 2006;7(2):174-84. PubMed.

    . Heat shock factor-1 mediates the transcriptional activation of Alzheimer's beta-amyloid precursor protein gene in response to stress. Brain Res Mol Brain Res. 1996 Jan;35(1-2):325-8. PubMed.

    . Altered subcellular localization of ornithine decarboxylase in Alzheimer's disease brain. Biochem Biophys Res Commun. 2006 Jun 2;344(2):640-6. PubMed.

    . Glycogen synthase kinase 3beta and extracellular signal-regulated kinase inactivate heat shock transcription factor 1 by facilitating the disappearance of transcriptionally active granules after heat shock. Mol Cell Biol. 1998 Nov;18(11):6624-33. PubMed.

    . Lithostathine and pancreatitis-associated protein are involved in the very early stages of Alzheimer's disease. Neurobiol Aging. 2001 Jan-Feb;22(1):79-88. PubMed.

    . Cloning and expression analysis of an ovine PAP-like protein cDNA, a gene differentially expressed in scrapie. Gene. 2006 Jul 5;376(1):116-22. PubMed.

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References

News Citations

  1. Aβ Star is Born? Memory Loss in APP Mice Blamed on Oligomer
  2. Insulin/Heat Shock Responses Compete to Control Aging and Polyglutamine Aggregation
  3. Huntington Disease: Three Ways to Tackle Triplet Disorder

Paper Citations

  1. . Effects of secreted oligomers of amyloid beta-protein on hippocampal synaptic plasticity: a potent role for trimers. J Physiol. 2006 Apr 15;572(Pt 2):477-92. PubMed.
  2. . The threshold for polyglutamine-expansion protein aggregation and cellular toxicity is dynamic and influenced by aging in Caenorhabditis elegans. Proc Natl Acad Sci U S A. 2002 Aug 6;99(16):10417-22. PubMed.
  3. . Opposing activities protect against age-onset proteotoxicity. Science. 2006 Sep 15;313(5793):1604-10. PubMed.

Further Reading

Papers

  1. . Opposing activities protect against age-onset proteotoxicity. Science. 2006 Sep 15;313(5793):1604-10. PubMed.

News

  1. A Matter of Life and DAF: Pathways to Longevity Revealed in <I>C. Elegans</I>
  2. The Long and Short of It: Insulin Trumps Telomeres in Organismal Aging

Primary Papers

  1. . Opposing activities protect against age-onset proteotoxicity. Science. 2006 Sep 15;313(5793):1604-10. PubMed.