title: C-Terminally Truncated Forms of Tau, But Not Full-Length Tau or Its C-Terminal Fragments, Are Released from Neurons Independently of Cell Death.
abstract: Recent evidence suggests that tau aggregation may spread via extracellular release and subsequent uptake by synaptically connected neurons, but little is known about the processes by which tau is released or the molecular forms of extracellular tau. To gain insight into the nature of extracellular tau, we used highly sensitive ELISAs, which, when used in tandem, are capable of differentiating between full-length (FL) tau, mid-region-bearing fragments, and C-terminal (CT) fragments. We applied these assays to the systematic study of the conditioned media of N2a cells, induced pluripotent stem cell-derived human cortical neurons, and primary rat cortical neurons, each of which was carefully assessed for viability. In all three neuronal models, the bulk of extracellular tau was free-floating and unaggregated and <0.2% was encapsulated in exosomes. Although most intracellular tau was FL, the majority of extracellular tau was CT truncated and appeared to be released both actively by living neurons and passively by dead cells. In contrast, only a small amount of extracellular tau was aggregation-competent tau (i.e., contained the microtubule-binding regions) and this material appears to be released solely due to a low level of cell death that occurs in all cell culture systems. Importantly, amyloid β-protein (Aβ)-induced neuronal compromise significantly increased the quantity of all forms of extracellular tau, but the presence of Aβ before detectable cell compromise did not increase extracellular tau. Collectively, these results suggest that factors that induce neuronal death are likely to be necessary to initiate the extracellular spread of tau aggregation.
  
  
  CONCLUSIONS
  Recent studies suggest that the transfer of tau between neurons underlies the characteristic spatiotemporal progression of neurofibrillary pathology. We searched for tau in the conditioned medium of N2a cells, induced pluripotent stem cell-derived human cortical neurons, and primary rat cortical neurons and analyzed the material present using four different tau ELISAs. We demonstrate that the majority of tau released from healthy neurons is C-terminally truncated and lacks the microtubule-binding region (MTBR) thought necessary for self-aggregation. A small amount of MTBR-containing tau is present outside of cells, but this appears to be solely due to cell death. Therefore, if propagation of tau aggregation is mediated by extracellular tau, our findings suggest that neuronal compromise is required to facilitate this process.
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115.
authors.forename: Daniel
authors.initials: D
authors.lastname: Kanmert
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115, School of Bimolecular and Biomedical Science, University College Dublin, Dublin 4, Republic of Ireland, and.
authors.forename: Adam
authors.initials: A
authors.lastname: Cantlon
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115.
authors.forename: Christina R
authors.initials: CR
authors.lastname: Muratore
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115.
authors.forename: Ming
authors.initials: M
authors.lastname: Jin
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115.
authors.forename: Tiernan T
authors.identifier: http://orcid.org/0000-0002-3538-0731
authors.initials: TT
authors.lastname: O'Malley
authors.affiliation: Department of Internal Medicine, University of Iowa Carver College of Medicine, Iowa City, Iowa 52242.
authors.forename: Gloria
authors.initials: G
authors.lastname: Lee
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115.
authors.forename: Tracy L
authors.identifier: http://orcid.org/0000-0001-7846-1542
authors.initials: TL
authors.lastname: Young-Pearse
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115.
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Selkoe
authors.affiliation: Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115, dwalsh3@partners.org.
authors.forename: Dominic M
authors.initials: DM
authors.lastname: Walsh
mesh_terms: D000818:Animals; D016923:Cell Death / Q000502:physiology; D002460:Cell Line; D017077:Culture Media, Conditioned / Q000737:chemistry; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D057026:Induced Pluripotent Stem Cells / Q000378:metabolism; D058953:Neural Stem Cells / Q000378:metabolism; D009474:Neurons / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism*; D051381:Rats; D016875:tau Proteins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D017077:Culture Media, Conditioned; D010446:Peptide Fragments; D016875:tau Proteins
keywords: ELISA; exosomes; neurons; secretion; tau