Title: The Seeds of Neurodegeneration: Prion-like Spreading in ALS
Abstract: Misfolded proteins accumulating in several neurodegenerative diseases (including Alzheimer, Parkinson, and Huntington diseases) can cause aggregation of their native counterparts through a mechanism similar to the infectious prion protein's induction of a pathogenic conformation onto its cellular isoform. Evidence for such a prion-like mechanism has now spread to the main misfolded proteins, SOD1 and TDP-43, implicated in amyotrophic lateral sclerosis (ALS). The major neurodegenerative diseases may therefore have mechanistic parallels for non-cell-autonomous spread of disease within the nervous system. Misfolded proteins accumulating in several neurodegenerative diseases (including Alzheimer, Parkinson, and Huntington diseases) can cause aggregation of their native counterparts through a mechanism similar to the infectious prion protein's induction of a pathogenic conformation onto its cellular isoform. Evidence for such a prion-like mechanism has now spread to the main misfolded proteins, SOD1 and TDP-43, implicated in amyotrophic lateral sclerosis (ALS). The major neurodegenerative diseases may therefore have mechanistic parallels for non-cell-autonomous spread of disease within the nervous system. Neurodegenerative disorders, such as Parkinson, Huntington, and Alzheimer diseases, frontotemporal lobar degeneration (FTLD), and amyotrophic lateral sclerosis (ALS) are associated with the accumulation of misfolded proteins both inside and outside of neuronal and glial cells in the central nervous system. Although the major protein component of the pathological aggregations is characteristic for each neurodegenerative disease (such as α-synuclein in Parkinson, huntingtin in Huntington, or Aβ in Alzheimer disease), several proteins misfold and accumulate in multiple diseases (especially TDP-43, which misaccumulates in ALS, FTLD, and many other conditions). Conversely, neurodegenerative conditions can be associated with the presence of more than one accumulated protein (such as Aβ and tau in Alzheimer disease). These misfolded protein aggregates are pathological hallmarks of each disease. One widely held view is that these aggregates play a vital role in disease initiation and progression, with the misfolded versions of endogenous proteins likely to acquire toxic properties, potentially through increased hydrophobicity and/or sequestration of essential cellular components within the aggregates, generation of oxidative species, proteasome inhibition, and other pathways (reviewed in Ilieva et al., 2009Ilieva H. Polymenidou M. Cleveland D.W. Non-cell autonomous toxicity in neurodegenerative disorders: ALS and beyond.J. Cell Biol. 2009; 187: 761-772Crossref PubMed Scopus (280) Google Scholar). An alternative view is that the large aggregates detected immunohistochemically represent not the toxic species but the final product of a defensive cell response aimed at protecting cells from more toxic oligomeric species that remain undetectable by most techniques. Prion diseases or transmissible spongiform encephalopathies are a class of neurodegenerative diseases that, as their name suggests, can be transmitted from individual to individual through ingestion or internalization of contaminated material (reviewed in Aguzzi et al., 2008Aguzzi A. Sigurdson C. Heikenwaelder M. Molecular mechanisms of prion pathogenesis.Annu. Rev. Pathol. 2008; 3: 11-40Crossref PubMed Scopus (144) Google Scholar). The nature of the infectious agent and the transmission mechanism in prion diseases have been the subject of intense interest since the demonstration, 45 years ago, of transmissibility of human prions (Gajdusek et al., 1966Gajdusek D.C. Gibbs C.J. Alpers M. Experimental transmission of a Kuru-like syndrome to chimpanzees.Nature. 1966; 209: 794-796Crossref PubMed Scopus (289) Google Scholar). It is now widely accepted that the prion, the infectious agent of prion diseases, consists of misfolded form(s)—designated PrPSc—of a normal protein, the cellular prion protein or PrPC, as was proposed by Prusiner in the early 1980s (Prusiner, 1982Prusiner S.B. Novel proteinaceous infectious particles cause scrapie.Science. 1982; 216: 136-144Crossref PubMed Google Scholar). With the amplification of infectious prions in vitro (Castilla et al., 2005Castilla J. Saá P. Hetz C. Soto C. In vitro generation of infectious scrapie prions.Cell. 2005; 121: 195-206Abstract Full Text Full Text PDF PubMed Scopus (443) Google Scholar) and most recently their production from purified recombinant protein (Wang et al., 2010Wang F. Wang X. Yuan C.G. Ma J. Generating a prion with bacterially expressed recombinant prion protein.Science. 2010; 327: 1132-1135Crossref PubMed Scopus (244) Google Scholar), the evidence for the protein-only prion model is now overwhelming. Prions replicate by recruiting PrPC in the ordered PrPSc-containing aggregates and by inducing a pathological conformation on the native endogenous prion protein. Although such replication/transmission mechanisms were long thought to be uniquely associated with transmissible prion diseases, in the past decade an increasing list of neurodegenerative (and other) diseases have been shown to include “prion-like” phenomena (Table 1). We use the term “prion-like” to describe molecular events that share similarities with the infectious cycle of the mammalian prion protein's self-perpetuating seeded aggregation and spreading. Beyond infectious prions, the notion of prion-like spreading of misfolded conformations of proteins linked to human neurodegenerative diseases arose from demonstration of Aβ plaque formation in the brains of primates after injection of brain extracts of human Alzheimer patients (Baker et al., 1994Baker H.F. Ridley R.M. Duchen L.W. Crow T.J. Bruton C.J. Induction of beta (A4)-amyloid in primates by injection of Alzheimer's disease brain homogenate. Comparison with transmission of spongiform encephalopathy.Mol. Neurobiol. 1994; 8: 25-39Crossref PubMed Scopus (57) Google Scholar). Prion-like spread was then established by the work of Jucker and Walker, whose groups showed that Aβ aggregation is hastened by the presence of preformed Aβ aggregates or “seeds” in vivo. In particular, they showed that intracerebral injection of brain extracts from autopsy material of human Alzheimer disease patients or from aged Alzheimer disease model mice—both containing ordered aggregates of human Aβ—into transgenic mice expressing human amyloid precursor protein (APP) accelerated the aggregation of human Aβ produced as a proteolytic fragment of transgene-encoded APP (Kane et al., 2000Kane M.D. Lipinski W.J. Callahan M.J. Bian F. Durham R.A. Schwarz R.D. Roher A.E. Walker L.C. Evidence for seeding of beta -amyloid by intracerebral infusion of Alzheimer brain extracts in beta -amyloid precursor protein-transgenic mice.J. Neurosci. 2000; 20: 3606-3611Crossref PubMed Google Scholar, Meyer-Luehmann et al., 2006Meyer-Luehmann M. Coomaraswamy J. Bolmont T. Kaeser S. Schaefer C. Kilger E. Neuenschwander A. Abramowski D. Frey P. 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Peripherally applied Abeta-containing inoculates induce cerebral beta-amyloidosis.Science. 2010; 330: 980-982Crossref PubMed Scopus (124) Google Scholar) that is reminiscent of cerebral β-amyloid angiopathy associated with Alzheimer disease in humans (Thal et al., 2008Thal D.R. Griffin W.S. de Vos R.A. Ghebremedhin E. Cerebral amyloid angiopathy and its relationship to Alzheimer's disease.Acta Neuropathol. 2008; 115: 599-609Crossref PubMed Scopus (86) Google Scholar).Table 1Prion-like Phenomena in Neurodegenerative DiseasesNative ProteinAggregated Protein or PeptideMain Associated Diseases in humansAcquired by Infection in HumansSubcellular LocalizationSeeded AggregationCell-to-Cell SpreadingInducible Clinical Disease in MiceNativeAggregatesIn Vitroin Cell CultureIn Vivo in Micein Cell CultureIn Vivo in MiceSynthetic SeedBrain ExtractPrPCPrPSc(variant, introgenic) Creutzfeldt-Jacob, Kuruyesplasma membrane anchoredmostly extracellularyesyesyesyesyesyesyes(sporadic, familial) Creutzfeldt-Jacob, Fatal familial insomnia, Gerstmann-Straussler ScheinkernoTauTaufrontotemporal lobar dementia, Alzheimernocytoplasmiccytoplasmicyesyesyesyesyesn.d.noα-synucleinα-synucleinParkinson, lewy body dementianonuclear and synapticcytoplasmicyesyesyesyesyesn.d.yes (acceleration in mutant mice)APPβ-amyloidAlzheimernotransmembranemostly extracellularyesyesyesnoyesnonoHuntingtinPolyQHuntingtonnonuclearnuclearyesyesn.d.yesn.d.n.d.n.d.Ataxinsspinocerebellar ataxiasnoSOD1SOD1amyotrophic lateral sclerosisnocytoplasmiccytoplasmicyesyesn.d.yesn.d.n.d.n.d.TDP-43TDP-43amyotrophic lateral sclerosis, frontotemporal lobar degenerationnonuclearmostly cytoplasmicyesyesn.d.n.d.n.d.n.d.n.d.FUS/TLSFUS/TLSamyotrophic lateral sclerosis, frontotemporal lobar degenerationnonuclearmostly cytoplasmicn.d.n.d.n.d.n.d.n.d.n.d.n.d.Summary of current evidence on seeded aggregation, spreading, and clinical disease induced in experimental animals by injection of aggregated proteins associated with human neurodegenerative diseases. We note that “no” signifies that there is currently no evidence supporting the respective events.PrPC: cellular prion protein, PrPSc: pathologic prion protein, Tau: microtubule-associated protein, PolyQ: polyglutamine, APP: amyloid precursor protein, SOD1: superoxide dismutase 1, TDP-43: TAR DNA-binding protein 43, FUS/TLS: fused in sarcoma, translocated in liposarcoma, n.d.: not done. Open table in a new tab Summary of current evidence on seeded aggregation, spreading, and clinical disease induced in experimental animals by injection of aggregated proteins associated with human neurodegenerative diseases. We note that “no” signifies that there is currently no evidence supporting the respective events. PrPC: cellular prion protein, PrPSc: pathologic prion protein, Tau: microtubule-associated protein, PolyQ: polyglutamine, APP: amyloid precursor protein, SOD1: superoxide dismutase 1, TDP-43: TAR DNA-binding protein 43, FUS/TLS: fused in sarcoma, translocated in liposarcoma, n.d.: not done. Following a similar paradigm, intracerebral injection of mutant tau aggregate-containing brain extracts seed widespread aggregation of normal human tau in transgenic mice that do not otherwise develop aggregates (Clavaguera et al., 2009Clavaguera F. Bolmont T. Crowther R.A. Abramowski D. Frank S. Probst A. Fraser G. Stalder A.K. Beibel M. Staufenbiel M. et al.Transmission and spreading of tauopathy in transgenic mouse brain.Nat. Cell Biol. 2009; 11: 909-913Crossref PubMed Scopus (348) Google Scholar). 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The propagation of the above misfolded proteins in cultured cells once again resembles PrPSc, which was initially shown to replicate in cells over 40 years ago, a feature that has been exploited to establish a quantitative cell-based assay for determination of infectious prion titers (Klöhn et al., 2003Klöhn P.C. Stoltze L. Flechsig E. Enari M. Weissmann C. A quantitative, highly sensitive cell-based infectivity assay for mouse scrapie prions.Proc. Natl. Acad. Sci. USA. 2003; 100: 11666-11671Crossref PubMed Scopus (170) Google Scholar, Mahal et al., 2007Mahal S.P. Baker C.A. Demczyk C.A. Smith E.W. Julius C. Weissmann C. Prion strain discrimination in cell culture: the cell panel assay.Proc. Natl. Acad. Sci. USA. 2007; 104: 20908-20913Crossref PubMed Scopus (81) Google Scholar). Except for PrPSc in prion disease, there is currently no evidence that the other induced aggregates can spread between individuals to cause acquired disease, either in humans or in experimental animals. 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This is an important distinction, with the difference between prions and prionoids probably lying in the potency of induced aggregation and spreading, rather than any difference in underlying molecular events. This augmented potency of prions to induce transmissible disease may be dependent on (but not restricted to) their remarkable resistance to endogenous proteases and other routes of pathogen elimination (Shorter and Lindquist, 2005Shorter J. Lindquist S. Prions as adaptive conduits of memory and inheritance.Nat. Rev. Genet. 2005; 6: 435-450Crossref PubMed Scopus (225) Google Scholar). Regardless, the prion-like replication that occurs within affected cells followed by transfer from cell to cell provides a molecular pathway for disease spread within the nervous system following focal generation of an initiating misfolding event. 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ALS is a neurodegenerative condition that targets primarily motor neurons, resulting in progressive paralysis and death within a few years from onset. Just like Alzheimer, Parkinson, and other neurodegenerative diseases, a proportion (∼10%) of ALS is dominantly inherited, with the remaining 90% (referred to as sporadic) of unknown origin. The identification in 1993 of mutation in the gene encoding superoxide dismutase 1 (SOD1) as the first or second most common form of inherited ALS (Rosen et al., 1993Rosen D.R. Siddique T. Patterson D. Figlewicz D.A. Sapp P. Hentati A. Donaldson D. Goto J. O'Regan J.P. Deng H.X. et al.Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis.Nature. 1993; 362: 59-62Crossref PubMed Scopus (3049) Google Scholar), and subsequent generation of transgenic mice expressing ALS-causing mutants in SOD1, initiated the molecular era of deciphering disease mechanism. A flurry of approaches established that non-cell-autonomous disease depends on one or more toxic properties of mutant SOD1. The latter drives disease initiation when synthesized within motor neurons, whereas its synthesis by glial neighbors provokes rapid disease advance (reviewed in Ilieva et al., 2009Ilieva H. Polymenidou M. Cleveland D.W. Non-cell autonomous toxicity in neurodegenerative disorders: ALS and beyond.J. Cell Biol. 2009; 187: 761-772Crossref PubMed Scopus (280) Google Scholar). Along with prion-infected mice, the ALS-linked mutant SOD1 mice are among the most faithful models of neurodegeneration, recapitulating the selective progressive loss of motor neurons that leads to the paralysis characteristic of human ALS. In both inherited and sporadic ALS, affected neurons and glial cells contain abnormal proteinaceous accumulations, often labeled by anti-ubiquitin antibodies. The major protein component of these accumulations in familial cases with SOD1 mutations—and in mutant SOD1 mice—is SOD1 itself. An initial view that SOD1 inclusions were not found in sporadic disease, e.g., Kerman et al., 2010Kerman A. Liu H.N. Croul S. Bilbao J. Rogaeva E. Zinman L. Robertson J. Chakrabartty A. Amyotrophic lateral sclerosis is a non-amyloid disease in which extensive misfolding of SOD1 is unique to the familial form.Acta Neuropathol. 2010; 119: 335-344Crossref PubMed Scopus (49) Google Scholar, has recently been challenged (Bosco et al., 2010Bosco D.A. Morfini G. Karabacak N.M. Song Y. Gros-Louis F. Pasinelli P. Goolsby H. Fontaine B.A. Lemay N. McKenna-Yasek D. et al.Wild-type and mutant SOD1 share an aberrant conformation and a common pathogenic pathway in ALS.Nat. Neurosci. 2010; 13: 1396-1403Crossref PubMed Scopus (156) Google Scholar, Forsberg et al., 2010Forsberg K. Jonsson P.A. Andersen P.M. Bergemalm D. Graffmo K.S. Hultdin M. Jacobsson J. Rosquist R. 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