Misfolded proteins that cause neurodegenerative diseases act an awful lot like prions, even if they do not fit the exact definition. For example, Aβ and tau are not considered infectious from person to person, and no one has shown that distinct conformations stably pass between animals while keeping their shape as prions do. That is, until now. In the May 22 Neuron online, scientists led by Marc Diamond, Washington University in St. Louis, report that misfolded forms, or “strains,” of tau can propagate from one mouse to the next by injection, suggesting the protein behaves more like a prion than previously realized. “This is a carefully done and exciting study that convincingly demonstrates the existence of tau strains in living systems,” Mathias Jucker, University of Tübingen, Germany, wrote in an email to Alzforum (see full comment below). 

Tau can misfold in many different ways. At last year’s Alzheimer’s Association International Conference, held in Boston, Diamond revealed that different conformations, or strains, of the protein manifest unique physical properties and toxicities (see Aug 2013 news story). When inoculated into successive cultures of naïve cells, each strain seeded its own particular brand of inclusion over and over. Other researchers wondered whether this faithful tau transmission would occur in a more natural system. 

To demonstrate this, co-first authors David Sanders and Sarah Kaufman and colleagues made tau fibrils in vitro and used them to corrupt a tau fragment expressed in HEK293 cells. These cells expressed the core, aggregation-prone portion of tau, called the repeat domain (tau RD). Sanders isolated single colonies and picked two to characterize more fully—clone 9 and clone 10. Clone 10 produced larger aggregates that lay outside the nucleus. Clone 9 seeded more efficiently and produced many small intranuclear deposits. 

The researchers then injected these two strains into transgenic mice expressing full-length human mutant tau (P301S), which causes inherited tauopathies. After three weeks, they looked at pathology in the brain. Clone 9 caused tangle-like aggregates in the CA1 and CA3 areas of the hippocampus, while clone 10 led to puncta in the mossy fiber tracts of CA3. When Sanders and colleagues injected brain homogenates from these animals into a second group of mice, and four weeks later transferred brain material from those to a third group, each new recipient developed similar clone-specific pathology. When they extracted the tau from the third round of mice and put it back into tau RD-expressing HEK293 cells, inclusions formed that were identical to those in the initial tau RD cells. 

Tau aggregates in disease-specific ways: speckled inclusions in AD (bright green, left), disordered ones in CBD (middle), and mosaic ones in Pick’s (right). Neuron, Sanders et al., Figure 2J.

How does the stable passage of a specific strain of misfolded tau relate to human disease? As Diamond presented at AAIC, different human tauopathies seem to come with their own unique tau conformations. Cell cultures seeded with brain extracts from 29 patients revealed that Alzheimer’s disease tissue induces patterns of tau inclusions that almost exclusively appeared speckled, while those generated by corticobasal degeneration extracts were mostly disordered, and Pick’s tissue yielded mosaics (see image). These results echoed a recent study showing patient brain extracts seeded disease-specific tau pathology in mice (see Clavaguera et al., 2013). “We think the molecular structure of the aggregate will allow us to predict the disease it came from,” said Diamond.

Up to now, only bona fide prions have shown the ability to stably hold their shape between inoculations from one animal to the next, Diamond said. His data support the idea that tau should be considered a prion, he argues. Lary Walker, Emory University, Atlanta, told Alzforum that this new study strongly supports the prion concept in neurodegenerative disease pathogenesis. Uniting the neurodegenerative and prion fields would be helpful because people who study neurodegeneration could learn from the extensive literature already available for prions, he said.

However, Walker and several other scientists noted that in the public imagination, the word “prion” connotes infectivity. Spreading the erroneous idea that tauopathies are contagious in the way of prion diseases such as bovine spongiform encephalopathy could cause unnecessary anxiety. “As a field, we have to agree on a less frightening definition of prion, or come up with another word that encapsulates all of these diseases,” Walker said (see additional email comment below).

Diamond said that while spontaneous transmission does not occur between people, under the right circumstances—tissue transplantation, for instance—pathogenic proteins such as tau could conceivably be passed among individuals. The field should be aware of that possibility, he said. A survey counters that idea, reporting that when people received human growth hormone prepared from the pooled pituitary glands of cadavers that likely contained traces of Aβ, tau, or α-synuclein, the incidence of AD and PD did not rise (see Feb 2013 news story). In the future, Diamond and colleagues plan to examine patients with tauopathies to see if their diseases can be defined by aggregate structures and tau conformations. They will also examine whether tau strains dictate how pathology spreads, and look for genes that promote it, with an eye toward new drug targets.

Walker pointed out that the study has implications for therapeutics. It suggests that a molecule could bind and disrupt tau aggregation in mice but prove ineffective in humans because of the differences in tau’s conformation. “That means you may want to be as close as possible to the ultimate target when developing a drug,” he told Alzforum. On the plus side, Luc Buée of INSERM in Lille, France, added that researchers may be able to tailor therapies to target specific conformations of tau and avoid the normal protein.

In related news, Walker and Jucker on May 15 won, along with one other scientist, the 2014 MetLife Foundation Award in Medical Research for their pioneering work on the prion-like nature of Aβ, on which Diamond based his current research (see May 2014 news story).—Gwyneth Dickey Zakaib

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  1. This is a very carefully done and exciting study that convincingly demonstrates the existence of tau conformers ("strains") in living systems. In vivo strains have been suggested previously for tau (e.g., Clavaguera et al., 2013), synuclein (e.g., Guo et al., 2013), and Aβ (e.g., Lu et al., 2013Heilbronner et al., 2013; see also LeVine and Walker, 2010, for review), but the previous studies have been largely suggestive rather than providing proof of the strain concept.  I am not sure whether this most recent study from the Diamond lab will settle the debate about whether one should call pathogenic tau seeds "prions" or not (and in my view, this debate is no longer fruitful); rather, we should build on these exciting new tau results and the overall recent insights that not only prions, but also Aβ, tau, and synuclein can form self-propagating pathogenic aggregates that in turn suggest therapeutic and diagnostic targets (for review see Jucker and Walker, 2013).

    References:

    . Brain homogenates from human tauopathies induce tau inclusions in mouse brain. Proc Natl Acad Sci U S A. 2013 Jun 4;110(23):9535-40. PubMed.

    . Distinct α-synuclein strains differentially promote tau inclusions in neurons. Cell. 2013 Jul 3;154(1):103-17. PubMed.

    . Molecular Structure of β-Amyloid Fibrils in Alzheimer's Disease Brain Tissue. Cell. 2013 Sep 12;154(6):1257-68. PubMed.

    . Seeded strain-like transmission of β-amyloid morphotypes in APP transgenic mice. EMBO Rep. 2013 Oct 30;14(11):1017-22. PubMed.

    . Molecular polymorphism of Abeta in Alzheimer's disease. Neurobiol Aging. 2010 Apr;31(4):542-8. PubMed.

    . Self-propagation of pathogenic protein aggregates in neurodegenerative diseases. Nature. 2013 Sep 5;501(7465):45-51. PubMed.

  2. This study is an elegant demonstration of tau strains, which extends earlier work that demonstrated strains of tau using human brain lysates (Claveguera et al., 2013). Indeed, Sanders et al. add to the growing body of literature demonstrating that other non-prion neurodegenerative disease proteins, such as α-synuclein and Aβ, can exist as different strains of disease proteins (Guo et al., 2013; Heilbronner et al., 2013; Lu et al., 2013). However, contrary to the authors’ assertions, the existence of pathological strains of tau does not establish that these pathological tau proteins are prions. Indeed, neither pathological tau nor pathological species of α-synuclein and Aβ can be termed prions because they are neither infectious nor zoonoses in the way prion diseases are. The very name prion was coined by S. Prusiner in 1982 when he renamed the scrapie agent a prion (Prusiner, 19821). Notably, the scrapie agent is the extensively studied infectious entity responsible for a highly infectious neurodegenerative disease in sheep. By renaming the scrapie agent a prion, Prusiner defined the prion as a "proteinacious infectious particle.” (“Because the novel properties of the scrapie agent distinguish it from viruses, plasmids, and viroids, a new term ‘prion’ is proposed to denote a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids.”)

    Accordingly, infectivity is one of the essential components of the definition of prions, and this is the definition that is found repeatedly in online dictionaries and in Google with no mention of strains being essential to the definition of prions. Infectious spread of prions is very real and not just through iatrogenic means, but through the food supply, which justifies categorizing infectious prion diseases as zoonoses. This was dramatically evidenced by the spread of prion disease from cattle to humans, which was termed “Mad Cow” disease (Sikorska et al., 2012). Moreover, in the original population of the Foré linguistic group of Papua New Guinea in whom person-to-person spread of the infectious prion disease kuru occurred through recurrent exposure to the CNS tissues of afflicted individuals, there were never any reports that AD, PD, or other non-prion neurodegenerative tauopathies, synucleinopathies, or Aβopathies were spread by ritual cannibalism, which is thought to be the basis for this person-to-person spread of kuru. This is despite the fact that pathological tau begins to deposit in the CNS as early as the second decade of life, followed soon thereafter by Aβ. Moreover there are no epidemiological data to support the notion that there is infectious spread of AD, PD, or other tauopathies, Aβopathies, and synucleinopathies through blood transfusions, organ transplants or other means. Indeed, we found no evidence of the spread of any of these diseases in middle-aged and older individuals who, as children, were treated between 1965 to 1985 with daily injections of human growth hormone extracted from postmortem human pituitaries, and who were longitudinally followed, despite the fact that 24 cases of prion disease occurred in this cohort of more than 7,000 individuals (Irwin et al., 2013). 

    Thus, despite the fact that infectious prion diseases are clearly zoonoses and there is no evidence that AD, PD, and related synucleinopathies, Aβopathies, and tauopathies are infections or zoonotic diseases, the starker contrast between prions and non-prion neurodegenererative disease proteins with respect to infectivity comes not from human data, but from data on the very high prevalence of readily transmissible or infectious prion diseases in sheep, cattle, moose, elk, and other animals for which there is just no counterpart for tauopathies, Aβopathies, or synucleinopathies. Indeed, the estimated economic costs incurred by responding to bovine spongiform encephalopathy in the EU between November 2000 and December 2010 ranged between €1,847 million and €2,094 million (Probst et al., 2013). Nothing comparable has occurred for tauopathies, Aβopathies, or synucleinopathies in livestock, nor is anything likely to occur, because there is no reservoir of infectious tauopathies, Aβopathies, or synucleinopathies in livestock or other mammals. 

    Therefore, for those who agree with Stan Prusiner that a prion is a “proteinacious infectious particle,” the burden of proof is on them—if they wish to call tau, α-synuclein, and Aβ prions—to demonstrate the infectivity of these neurodegenerative disease proteins in humans, and, more importantly, I think, to demonstrate the existence of reservoirs of infectious tauopathies, synucleinopathies, and Aβopathies affecting thousands or perhaps hundreds of thousands of sheep, deer, elk or other animals in the wild as is clearly the case for prion diseases, which in part contributed to Prusiner’s definition of prions as proteinaceous infectious particles. 

    1 For clarity, the abstract of the paper by Prusiner introducing the term prion is here in this footnote. Abstract: After infection and a prolonged incubation period, the scrapie agent causes a degenerative disease of the central nervous system in sheep and goats. Six lines of evidence including sensitivity to proteases demonstrate that this agent contains a protein that is required for infectivity. Although the scrapie agent is irreversibly inactivated by alkali, five procedures with more specificity for modifying nucleic acids failed to cause inactivation. The agent shows heterogeneity with respect to size, apparently a result of its hydrophobicity; the smallest form may have a molecular weight of 50,000 or less. Because the novel properties of the scrapie agent distinguish it from viruses, plasmids, and viroids, a new term "prion" is proposed to denote a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids. Knowledge of the scrapie agent structure may have significance for understanding the causes of several degenerative diseases.

    View all comments by John Trojanowski
  3. These thorough and elegant studies are the strongest evidence yet that seeds of tau protein behave like prions. They compellingly reinforce the prion concept as one of the most important pathogenic principles of our time. But since a prion is defined as an infectious particle, I am uncomfortable with the use of the word to describe agents of non-infectious diseases (*see definition below). On the other hand, the word "prion" is useful, concise, and undoubtedly here to stay. Perhaps, in the broader context of prionology, we should keep the word and agree on a more comprehensive, accurate, and less disquieting definition (proteinaceous inductive particle?).

    * infectious (adjective): 1. Of a disease or disease-causing organism) liable to be transmitted to people, organisms, etc. through the environment.

    (From the Oxford Dictionaries online)

  4. I agree with Mathias Jucker that the current debate about whether we should refer to protein amyloids that cause disease as “prions” or “prion-like” or some other word is not fruitful. One could theoretically use “infectious” as a critical criterion to define a bacterium, but that is obviously not very useful, or even accurate. For example, consider non-pathogenic E. coli that reside in the gut. If such bacteria were inoculated artificially into an inappropriate location, e.g., the brain or blood, they would produce disease that is “transmissible” through transfer of biological fluids. Yet nobody would consider this important, and it would not engender any interest from a public health standpoint. Similarly, work by our lab and others involve inoculation of pure protein into animal hosts that are primed to develop pathology, and this pathology is “transmissible” from animal to animal. Unless human tau were delivered in a similar fashion into patients, it seems highly unlikely that it would cause any problems in the general population. Indeed, were it not for relatively extreme human activities—cannibalism, brain surgery, tissue transplantation, feeding offal to livestock—human prion disease would not be known to be infectious, and instead would look like every other amyloidosis, with sporadic and genetic causes (and this is of course how virtually all prion cases present).

    With that said, I think the field would be remiss in dismissing the possible infectious nature of protein amyloids simply because there is no known evidence of interperson transmission. In theory this could happen, and we should be aware of it as a possibility. We do a disservice by drawing an artificial line between PrP prions and other types of prions based simply on known infectivity, when there are so many other critical biological similarities.

References

News Citations

  1. Are Protein Strains The Cause of Different Tauopathies?
  2. In Case You Wondered: Neurodegenerative Diseases Are Not Contagious
  3. Jucker, Walker, Yan, and Shen Win 2014 MetLife Awards

Research Models Citations

  1. Tau P301S (line PS19)

Paper Citations

  1. . Brain homogenates from human tauopathies induce tau inclusions in mouse brain. Proc Natl Acad Sci U S A. 2013 Jun 4;110(23):9535-40. PubMed.

Further Reading

Papers

  1. . Biology and genetics of prions causing neurodegeneration. Annu Rev Genet. 2013;47:601-23. PubMed.
  2. . Prion, prionoids and infectious amyloid. Parkinsonism Relat Disord. 2014 Jan;20 Suppl 1:S80-4. PubMed.
  3. . Neurodegenerative lesions: Seeding and spreading. Rev Neurol (Paris). 2013 Oct;169(10):825-33. PubMed.
  4. . Self-propagation of pathogenic protein aggregates in neurodegenerative diseases. Nature. 2013 Sep 5;501(7465):45-51. PubMed.

Primary Papers

  1. . Distinct tau prion strains propagate in cells and mice and define different tauopathies. Neuron. 2014 Jun 18;82(6):1271-88. Epub 2014 May 22 PubMed.