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Templated seeding induces the formation of Alzheimer paired helical tau filaments in human brain organoids

Mehtap BaciogluiD, Panagiotis KatsinelosiD, Manuel SchweighauseriD, Filomena GalloiD, Karin MuelleriD, Rafailia Christou, Janine BrandesiD, Sofia LövestamiD, Sally A. CowleyiD, William A. McEwaniD, Sjors H. W. ScheresiD, Michel GoedertiD, Maria Grazia SpillantiniiD

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Abstract

The prion concept postulates that seeded aggregation results in the recruitment of soluble protein monomers and their assembly into amyloid filaments with the same structures as those of the seeds. Here we show that this concept applies to tau filaments formed in cortical organoids obtained from human iPSCs of control subjects. Tau seeds were extracted from the frontal cortex of individuals with Alzheimer disease. When added to human cortical organoids expressing endogenous levels of three-repeat tau, they gave rise to filaments of three-repeat tau whose formation required endogenous tau expression. By cryo-EM, these seeded tau filaments were identical to the paired helical filaments of the seeds. This novel finding demonstrates the faithful templated seeding of tau in human nerve cells expressing endogenous levels of tau and opens the way to studying the mechanisms of paired helical filament formation and their downstream effects in a human system amenable to genetic modification.

In-depth summary

Claude Opus 5.5 · xhigh effort · 7 Nov 2026 · from the full text of version 1 (5 Oct 2026) · CC BY 4.0

Cortical organoids from healthy human iPSCs, seeded with Alzheimer brain tau, form three-repeat tau filaments that cryo-EM shows are identical to Alzheimer paired helical filaments and need endogenous tau.

The paper describes earlier faithful seeding in vitro with phosphomimetic recombinant tau and in mice with 4-repeat mouse tau, while earlier organoid studies reported no amyloid-beta or tau filaments. Here human organoids with endogenous tau levels give paired helical filaments (PHFs) with the seed structure.

Key results

  1. ShownTau assembly in organoids grows with time after seeding and with seed dose, and the dose response levels off at about 500-1,000 ng.

    MC-1 (conformational anti-tau antibody) positive area was small at 28 days post-seeding (dps) and stronger and more diffuse at 92 and 129 dps (n=3-8 organoids per time point). Doses of 4-2,000 ng were tested at 129 dps (n=5-8). · Templated seeding by tau seeds from AD brains in…; Figure legends

  2. ShownSeeded assembly requires endogenous tau. It occurred in MAPT+/+ organoids but not in isogenic MAPT+/- or MAPT-/- organoids.

    MC-1 staining at 129 dps with 500 ng seed (n=7-12 organoids per group) and western blots of sarkosyl-insoluble tau with BR133 agreed. Heterozygous organoids had lower tau levels than MAPT+/+, and no tau bands were seen in MAPT-/-. · Templated seeding in human brain cortical…; Figure legends

  3. ShownThe recovered filaments are made of 3R tau only, so the AD seeds are not what was extracted from the organoids.

    AD seeds showed bands at 60, 64 and 68 kDa plus a weak 72 kDa band. Seeded organoids at 129 dps showed a single 55 kDa band that was negative with the 4R-specific antibody RD4. · Tau seeds; DISCUSSION

  4. ShownCryo-EM shows that most seeded filaments are PHFs identical to those from AD brain, with a minority of single protofilaments.

    More than 70% of filaments had two identical protofilaments with the Alzheimer tau fold; 24% were single protofilaments. The map resolved to 4.5 Å and PDB 6HRE fitted as a rigid body. Organoids from three iPSC clones (CRL1-3) were used. · Seeded tau filaments in human brain cortical…; Model building and refinement; Figure legends

  5. ShownOrganoids at 129 dps contain abundant filaments decorated by MC-1, and MC-1 signal co-localises with the amyloid dye pFTAA.

    Immunogold electron microscopy of resin-embedded organoids seeded with 500 ng AD-tau showed MC-1-decorated filaments, some in parallel bundles. Extensive MC-1 and pFTAA co-localisation was reported (Figure S1). · Templated seeding by tau seeds from AD brains in…; Seeded tau filaments in human brain cortical…; DISCUSSION

Methods, data and reproducibility

Human iPSC-derived cortical organoids, including isogenic MAPT knockout lines, were exposed once to sarkosyl-insoluble tau from AD frontal cortex. Assembly was followed by MC-1 immunostaining, western blotting, immunogold electron microscopy and cryo-EM helical reconstruction.

  • Cortical organoids from three healthy-donor iPSC clones (SIGi001-A, CRL-2429, NN0003920; CRL1-3) seeded with AD tau · human · n = 3-12 organoids per group in quantifications
  • Isogenic iPSC line SFC856-03-04 as MAPT+/+, MAPT+/- and MAPT-/- organoids · human · n = 7-12 organoids per group
  • Sarkosyl-insoluble tau seeds from frontal cortex of two sporadic AD cases · human · n = 2 cases
  • Reference Alzheimer tau fold model used for rigid-body fitting into the organoid PHF map · human · PDB: 6HRE
Methods & tools
MC-1 antibody, pFTAA, RELION, CTFFIND 4.1, relion_helix_inimodel2d, ISOLDE, ChimeraX, Fiji, QuPath, Titan Krios, Falcon-4i detector, Gatan K3 detector
Reagents & environment
RELION and CTFFIND 4.1 for cryo-EM processing; atomic model PDB 6HRE; further acquisition and refinement details are in Table S1.
Reproducibility gaps
No code or data deposition (for example a map accession) is given in the text; cryo-EM details are only in supplementary Table S1.
Limitations (4)
  • Microglia are absent from the organoids, so their role in tau aggregation and spreading was not tested. · stated by the authors
  • Whether Pick's disease seeds can seed the Pick tau fold in these organoids is untested, as is seeding of 4R tauopathies. · stated by the authors
  • The organoid PHF map reaches only 4.5 Å, so the existing atomic model was fitted as a rigid body without refinement, which limits how finely identity with the seed fold is established. · noted in this summary
  • Seeds came from only two AD cases, and the quantification groups are small (n=3-12 organoids). · noted in this summary

Read this if you want a human iPSC organoid system that reproduces Alzheimer PHF structures by seeding at endogenous tau levels, for example to test tau-lowering or genetic manipulation.

Citations
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