Intracellular clusters of Aβ oligomers have prion-like seeding capabilities, according to a study published in Neurobiology of Disease on January 29. Researchers led by Gunnar Gouras of Lund University in Sweden reported that adding extract from the brains of AD model mice to neuronal cell lines seeded intracellular inclusions chock-full of Aβ oligomers. The inclusions persisted after multiple cell divisions, and material extracted from these cells induced the same inclusions in other cells. The researchers believe that such prion-like behavior could occur in AD, and that using these cell lines could unlock cellular mechanisms of Aβ’s proteopathic spread in greater detail.

  • When treated with extracts from plaque-ridden mouse brain, cell lines expressing mutant APP developed Aβ oligomer inclusions.
  • The inclusions stayed through cellular generations, and induced similar pathology in naïve cells.
  • Are intracellular inclusions the source of prion-like Aβ seeds in human brain?

Prions, in particular the prion protein PrP, are known to spread from one cell to another via a templating mechanism in which misfolded forms of the protein corrupt healthy versions. Mounting evidence has found a similar modus operandi for other neurodegenerative culprits, including tau, α-synuclein, and Aβ. All three reportedly are able to seed aggregation in animal models (Nov 2010 news; Oct 2011 news; Feb 2012 newsApr 2012 news). Cell culture models exist to study proteopathic transfer of both tau and α-synuclein (Hansen et al., 2011Oct 2014 news). 

Not so with Aβ. For one thing, unlike tau and α-synuclein, Aβ is a mere peptide, produced after multiple cleavages of APP. This makes Aβ difficult to track with fluorescent tags, the researchers pointed out. The other hurdle is conceptual, Gouras claims: Due to the AD field’s focus on extracellular forms of Aβ, researchers are less interested in studying the activity of intracellular pools of the peptide. Gouras proposes that if Aβ does indeed have prion-like capabilities, then concentrated intracellular stores of the peptide would be the most likely origin of infectious seeds.

To test this idea, first author Tomas Olsson and colleagues attempted to seed Aβ aggregates in N2a neuronal cell lines expressing human APP with the Swedish mutation. They doused the cells with saline-soluble cellular extracts prepared from the plaque-riddled brains of 21-month-old APP/PS1 mice, or from wild-type mice. After 12 days in culture, in which fresh extract was added three times, the researchers diluted the culture to generate separate clones, each derived from a single cell. They reasoned that only a fraction of cells would develop inclusions, and indeed, out of 22 clones, seven tested positive for staining with both the 82E1 antibody, specific for the N-terminus of the Aβ peptide, and the OC antibody, which reportedly recognizes Aβ oligomers. The antibodies co-localized in puncta within the cells, indicative of inclusions. The scientists dubbed these cell lines “prion-like clones.” None of 18 clones generated from cells seeded with wild-type extract—dubbed “wild-type clones”—reacted with either antibody.

Persistent Propagation? APP-Swe N2a cells treated with brain extracts from wild-type mice (left), from APP/PS1 mice (middle), or from previously infected cellular clones (right) have inclusions of Aβ oligomers (yellow: merge of 82E1 [green] and OC [red] antibodies). [Courtesy of Olsson et al., Neurobiology of Disease, 2018.]

Strikingly, these induced inclusions of Aβ persisted after more than 10 passages of the cell lines, and storage at -80°C for six months. This so-called “vertical transmission” is a mark of prion proteins. To test whether the inclusions could also pass horizontally, i.e., to other cells, the scientists generated extracts from one of the prion-like clones and dropped them onto naïve N2a cells expressing APP-Swe. Once again, the recipient cells developed Aβ inclusions.

The researchers next attempted to decipher the contents of the inclusions with biochemical experiments. They report that the prion-like clones harbored Aβ oligomers ranging from 250–670 kDa in size, which were resistant to detergent but sensitive to proteinase-K. Compared with wild-type clones, prion-like clones had more APP, Aβ, and C99—hinting at more amyloidogenic β-secretase cleavage of APP. Fourier transform infrared (FTIR) microspectroscopy indicated that the cells contained more disordered structures, suggesting an abundance of disordered oligomers. Extracts prepared from prion-like clones and wild-type clones contained a similar total amount of β sheets; however, the former had a higher proportion of anti-parallel β sheets, a structural feature previously attributed to some species of Aβ oligomer (Cerf et al., 2009; Breydo et al., 2016). 

The data suggest that oligomeric Aβ inside neurons can behave like prions, seeding aggregation of Aβ peptides in other cells, Gouras and Olsson told Alzforum. How might this play out in the human brain, given that this cell culture system relied on cellular extracts? Gouras proposed that an accumulation of Aβ in distal synapses could lead to oligomerization of the peptide and rupture of synaptic membranes, which would release toxic oligomers onto neighboring neurons. Past work from his lab supports the idea that Aβ oligomers burst synapses (Takahashi et al., 2004). 

Work from Mathias Jucker’s lab suggests that soluble oligomers in AD brain extracts, but not from the cerebrospinal fluid, induce fibrillization of Aβ (Langer et al., 2011Sep 2014 news). In particular, Jucker reported that the seeds originated from the membrane fraction of neurons (Marzesco et al., 2016). Jucker told Alzforum that Gouras’ newest findings suggest that intraneuronal Aβ oligomers are part of the prion-like seeding.

Marc Diamond of the University of Texas Southwestern Medical Center in Dallas has generated cellular models of tau propagation. He told Alzforum that the findings raise a host of interesting questions. Diamond noted that like PrP but unlike tau, APP is presumably processed in the secretory pathway, to be displayed on the cell surface. “I am wondering about a clinical correlation: Is there evidence of intracellular Aβ pathology similar to what they have observed in the N2a cell model? At a minimum, this work speaks to conserved cellular mechanisms to take up and propagate aggregates,” he wrote.

Sylvain Lesné of the University of Minnesota in Minneapolis commented that while he found the results interesting, the data appeared preliminary. For example, he said, quantitative measures were lacking in some biochemical analyses, and the number of samples analyzed in some experiments was low.—Jessica Shugart

Comments

  1. Cell models for prion infection, replication, and strain behavior have long been available to prion researchers. These include stable lines producing transmissible prion aggregates and inducible cell lines that are dependent on the addition of exogenous prions. More recently, cell models for prion-like behavior of tau and synuclein aggregates have become available to facilitate studies of seeding, transmission, structural polymorphisms, and the search for drugs to target these mechanisms. All of this has contributed to a growing understanding that the prion-like properties of strain polymorphisms, seeding, transmission, and templated replication are fundamental properties of all amyloids.

    Indeed, even for Aβ amyloid there is evidence from transgenic animals that demonstrates prion-like properties of strain polymorphisms, seeding, and transmission. It isn’t clear why the development of cell models of Aβ prion-like properties has lagged behind, but perhaps bias against the idea of significance of intracellular Aβ may be at least partly responsible. Most of the “action” for Aβ seems to be outside of the cell. Intraneuronal amyloid deposits have been known since the first Aβ and APP antibodies became available, but fell out of favor after the discovery of soluble secreted Aβ. Indeed, intraneuronal amyloid is sometimes viewed as normal because it contains epitopes from APP outside the traditional soluble Aβ regions. However, intraneuronal Aβ is not just APP. It has prion-like properties as well. It is insoluble and protease-resistant. It is aggregated in an amyloid-like structure because it stains with traditional amyloid-binding dyes and reacts with antibodies specific for amyloid aggregates that do not recognize Aβ monomer or soluble APP.

    Now in this paper, Gunnar Gouras and co-workers describe a cellular model of intracellular Aβ amyloid seeding and nucleation. They report the interesting and exciting findings that treating N2a cells expressing human FAD mutant APP with a particulate lysate from transgenic mice that have accumulated amyloid deposits leads to the seeding of amyloid Aβ inclusions of cells that demonstrate prion-like properties. Cells expressing these inclusions were cloned and shown to accumulate amyloid that reacts with 82E1, a monoclonal antibody specific for the amino terminus of Aβ and OC serum that is specific for fibrillar oligomers or fibril aggregates, but does not react with APP or Aβ monomer.

    The clones expressing these amyloid inclusions remain phenotypically stable for at least 10 passages and can be stored frozen at -80°C. Moreover, particulate extracts of these stable cell lines with Aβ amyloid inclusions, but not extracts from wild-type brain extract-treated cells, seeded further formation of intracellular amyloid inclusions in naïve N2A cells, demonstrating the fundamental prion-like property of transmissibility. The authors further demonstrate that the intracellular amyloid inclusions are resistant to proteinase K digestion, contain β-sheet secondary structure, and contain high molecular weight oligomeric Aβ in the range of 250–670 kDa.

    If these cells are like other cellular models of the prion-like properties of amyloids, they will facilitate studies on the strain polymorphisms of Aβ amyloid structures and their nucleation and transmission as well as studies on their pathological significance. They may also be useful in high-throughput screens for drugs that inhibit nucleation, replication, or transmission of Aβ prion-like seeds. Although the results of cell culture studies need to be verified in animal models, it helps to know what to look for first and these clues often come from cell culture models.

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References

News Citations

  1. Insidious Spread of Aβ: More Support for Synaptic Transmission
  2. Seeds of Destruction—Prion-like Transmission of Sporadic AD?
  3. Mice Tell Tale of Tau Transmission, Alzheimer’s Progression
  4. Synthetic Synuclein Corrupts Native Along Mouse Brain Networks
  5. Cellular Biosensor Detects Tau Seeds Long Before They Sprout Pathology
  6. Bad Seeds—Potent Aβ Peptides Instigate Plaques, Won’t Be Fixed

Research Models Citations

  1. APPswe/PSEN1dE9

Paper Citations

  1. . α-Synuclein propagates from mouse brain to grafted dopaminergic neurons and seeds aggregation in cultured human cells. J Clin Invest. 2011 Feb 1;121(2):715-25. PubMed.
  2. . Antiparallel beta-sheet: a signature structure of the oligomeric amyloid beta-peptide. Biochem J. 2009 Aug 1;421(3):415-23. PubMed.
  3. . Structural differences between amyloid beta oligomers. Biochem Biophys Res Commun. 2016 Sep 2;477(4):700-5. Epub 2016 Jun 27 PubMed.
  4. . Oligomerization of Alzheimer's beta-amyloid within processes and synapses of cultured neurons and brain. J Neurosci. 2004 Apr 7;24(14):3592-9. PubMed.
  5. . Soluble Aβ seeds are potent inducers of cerebral β-amyloid deposition. J Neurosci. 2011 Oct 12;31(41):14488-95. PubMed.
  6. . Highly potent intracellular membrane-associated Aβ seeds. Sci Rep. 2016 Jun 17;6:28125. PubMed.

Further Reading

Papers

  1. . Intraneuronal Alzheimer abeta42 accumulates in multivesicular bodies and is associated with synaptic pathology. Am J Pathol. 2002 Nov;161(5):1869-79. PubMed.
  2. . Oligomerization of Alzheimer's beta-amyloid within processes and synapses of cultured neurons and brain. J Neurosci. 2004 Apr 7;24(14):3592-9. PubMed.

Primary Papers

  1. . Prion-like seeding and nucleation of intracellular amyloid-β. Neurobiol Dis. 2018 May;113:1-10. Epub 2018 Feb 4 PubMed.