The Aβ fibrils that form amyloid plaques come in many shapes and sizes. According to a study published in Nature on January 4, unique assortments of Aβ fibril structures may underlie different clinical variants of Alzheimer’s disease. Structural analyses conducted on postmortem brain samples revealed that a single Aβ40 fibril structure predominated in people with typical AD or with the posterior cortical atrophy (PCA) variant of the disease, while Aβ40 fibrils in people with a rapidly progressive form of AD (rAD) were more of a mixed bag. The structure of Aβ42 fibrils, on the other hand, varied widely within and among people from all AD subtypes, reported researchers led by Robert Tycko of the National Institutes of Health in Bethesda, Maryland, and John Collinge of University College London. While it is unclear whether these distinctive fibrillar palettes are a cause or consequence of differing rates of disease progression, the findings caution against painting Aβ fibrils—or efforts to target them—with a broad brush.

“This work supports the idea that different Aβ conformers could be responsible for different clinical subtypes of the disease,” commented Mathias Jucker of the University of Tübingen. However, he and other commentators also pointed out that a technique used by the researchers—amplification of fibrils from brain extracts in vitro—was vulnerable to certain biases, evoking the early days of PCR, where it turned out later that transcripts were not always amplified in proportion to their relative abundance.

Findings in animal models have provided the lion’s share of the evidence that Aβ fibrils—much like prion proteins—come in strains. Researchers have reported that when plaque-laden tissue is transferred from one animal brain to another, the plaques that sprout in the recipient bear an uncanny resemblance to those from the donor (see Meyer-Luehmann et al., 2006Sep 2013 news on Heilbronner et al., 2013; and Jul 2014 news). This could indicate that the unique structural characteristics of donor fibrils seed recipient Aβ monomers to fibrillize in their image.

To take a closer look at the complement of Aβ structures in the AD brain, Tycko and colleagues previously developed a technique to examine fibril-enriched postmortem brain extracts via nuclear magnetic resonance (NMR) spectroscopy. While exquisitely sensitive to differences in secondary structure, the technique requires copious amounts of protein. To generate enough fibrils, Tycko and colleagues amplified the extracted fibrils by using them as seeds—mixing them with Aβ40 monomers to coax the fibrils to propagate. Using this technique on brain extracts from two different AD patients—one who had been originally diagnosed with Lewy body disease, and the other with typical AD—the researchers reported that each patient harbored one predominant Aβ40 fibril structure (see Sep 2013 news on Lu et al., 2013). 

For the current study, first author Wei Qiang, now at Binghamton University in New York State, and colleagues expanded the number of brain samples to determine whether the structure of the amplified fibrils would correlate with the clinical manifestations of the donor’s AD. They prepared amyloid-enriched extracts from the frontal cortices of six patients with typical AD (tAD), three with PCA (a form of AD associated with problems in visual processing), three people who died without dementia but nevertheless had amyloid plaques in their brains, and six with rAD, a swiftly progressing form that can be confused with the prion disorder Creutzfeldt-Jakob disease (see Feb 2013 news). In addition to the frontal cortex, the researchers also prepared extracts from the occipital and parietal cortices from a subset of eight patients (three with tAD, three with PCA, and two with rAD). 

Aβ40: Fibril of Conformity? A fibril structure with regularly spaced knobs (blue arrows) predominates in mixtures amplified from typical AD (left) and PCA (middle) patients, while other species (red arrows) abound in a sample from a patient with rapid-progression AD (right). [Image courtesy of Qiang et al., Nature, 2017.]

After seeding either Aβ40 or Aβ42 monomers with the patient brain extracts, the researchers examined the fibrillar products via transmission electron microscopy before they turned to NMR. While electron microscopy does not resolve every structure present in the mixture, the researchers wanted to first get a sense of the qualitative differences between fibrils generated from different clinical subtypes. The Aβ40 fibrils amplified from most samples, especially those from tAD and PCA patients, appeared to contain a predominant flavor of fibril with “knobs” spaced about every 107 nm. This Aβ40 fibril amplified from rAD patient samples as well, but so did many others. On the other hand, Aβ42 fibrils amplified from all of the samples appeared as a mish-mash of different shapes (see image below). 

Aβ42: No Pattern in Fibril Chaos? Aβ42 fibrils amplified from typical AD (left), PCA (middle), or rapidly progressing AD (right) appeared to have no predominant fibril structure. [Image courtesy of Qiang et al., Nature 2017.]

To compare fibril structures quantitatively, the researchers next used NMR. They found little variation in Aβ40 structures among and within fibril samples amplified from people with PCA and tAD, with one structure accounting for 80 percent of the fibrils. While that structure accounted for 65 percent of fibrils amplified from rAD patients, the remaining fibrils differed from the other two AD subtypes, and varied more within individual rAD samples as well. Fibril samples prepared from patients who died without dementia did not produce enough high-quality spectra for significant comparison to the other clinical subtypes. Qiang would not speculate on why that would be the case, but said that using more samples in future studies should allow for accurate comparisons. Interestingly, the fibril composition from frontal, parietal, and occipital regions of the brain did not differ significantly in any patient, suggesting little brain regional variation in Aβ fibril structure. This could imply that a patient with PCA gets visual symptoms first because fibrils deposit in their posterior cortex first, not because they have a different type of fibril than a person who gets memory symptoms first.

In keeping with the electron microscopy findings, NMR analysis indicated that Aβ42 fibrils amplified from each sample contained a wide variety of structures, with no dominant species. This heterogeneity held across the clinical subtypes. This finding jibes with those from a previous study, in which Aβ42 aggregates came in a wider variety of sizes and conformations than do Aβ40 aggregates (see Cohen et al., 2015). Qiang speculated that Aβ42 produces more heterogeneous fibrils because of its high propensity to aggregate.

Are the Aβ40 fibril structures in rAD important in its diagnosis or progression? Qiang said that it is difficult to know if they are a cause or consequence of disease. For example, in rAD patients—who die months, rather than years, after diagnosis—perhaps the fibrils predominantly found in tAD do not have enough time to form. On the other hand, perhaps some of the fibril strains present in rAD patients is extremely toxic, accelerating progression. Researchers generally believe that Aβ42 is more amyloidgenic than Aβ40.

Other scientists praised the study for its experimental rigor and provocative findings. However, all pointed out that “amplification bias” plagues studies that rely upon seeding to grow fibrils. “If there is a form of Aβ fibril that happens to amplify well in vitro, it will be the ‘dominant’ form detected,” commented Marc Diamond of University of Texas Southwestern Medical Center in Dallas. Diamond, who developed biosensor cell lines to detect tau prions without amplifying them, said he did so to reduce such bias (see Oct 2014 news). However, he added that as long as Tycko’s technique is reproducible, it may still prove useful despite potentially missing some strains. “If it were possible to use these methods to reliably amplify specific Aβ conformers from peripheral fluids (e.g. CSF) in a disease-specific way, it would be amazing,” he added. Jucker agreed the findings were interesting despite the amplification bias.

David Brody and Thomas Esparza of Washington University in St. Louis have developed a method to purify soluble Aβ aggregates from postmortem AD brain samples (see May 2016 conference news; Esparza et al., 2016). They wondered if different oligomer structures begat different fibrils, or vice versa. Brody said that it would be fascinating and “right at the edge of feasibility” to purify soluble aggregates and amplify fibrils from the same brain samples to examine how they might be related.

Brody and Esparza also wondered whether differences in fibril structures underlie some people’s resistance to dementia despite having rampant amyloid deposition. “The structural determinants of dementia versus those that allow resilience, given a similar level of Aβ pathology, are likely to be of great clinical relevance,” they added.

Finally, Diamond said the findings caused him to wonder whether different Aβ fibril conformers might promote distinct conformations of tau, or trigger regional vulnerability to tau pathology in the brain.—Jessica Shugart

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  1. "Amplification bias" is certainly an issue that needs to be considered when fibril structures from brain tissue are amplified by seeded growth. Certain structures may self-propagate more efficiently than others, depending on the precise details of the experimental conditions for seeded growth. When many successive rounds of seeded growth are used, it is even possible to "purify" a single structure from an initially heterogeneous mixture, as we have demonstrated in earlier in vitro studies of fibril growth and fibril structure.

    To minimize this potential problem in the experiments with brain tissue discussed above, we used a single amplification step, first sonicating our brain extract vigorously so that all Aβ fibrils in the extract were broken into short fragments, and then adding a single aliquot of solubilized Aβ peptide to the sonicated extract. TEM images were recorded after four hours, showing long fibrils that grew from sonicated seeds when AD brain tissue was used (but not when non-AD control tissue was used).

    Despite the possible remaining effects of amplification bias, we did observe a statistically significant difference in ssNMR data between rAD and tAD tissue samples, and real differences in ssNMR spectra of fibrils derived from different tissue samples. If anything, one would expect such differences to be reduced by amplification bias.

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References

News Citations

  1. Does Aβ Come In Strains? Glimpse Into Human Brain Suggests Yes
  2. More Evidence Ties Aβ Strains to Distinct Pathologies
  3. Mistaken Identity—Prion Disease or Alzheimer’s on Fast Forward?
  4. Cellular Biosensor Detects Tau Seeds Long Before They Sprout Pathology
  5. Aβ Oligomers Purified from Human Brain

Paper Citations

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Further Reading

Papers

  1. . Distinct prion-like strains of amyloid beta implicated in phenotypic diversity of Alzheimer's disease. Prion. 2016 Jan 2;10(1):9-17. PubMed.

Primary Papers

  1. . Structural variation in amyloid-β fibrils from Alzheimer's disease clinical subtypes. Nature. 2017 Jan 12;541(7636):217-221. Epub 2017 Jan 4 PubMed.