Tau seeding and spreading in vivo is supported by both AD-derived fibrillar and oligomeric tau

Abstract

Insoluble fibrillar tau, the primary constituent of neurofibrillary tangles, has traditionally been thought to be the biologically active, toxic form of tau mediating neurodegeneration in Alzheimer’s disease. More recent studies have implicated soluble oligomeric tau species, referred to as high molecular weight (HMW), due to their properties on size-exclusion chromatography, in tau propagation across neural systems. These two forms of tau have never been directly compared. We prepared sarkosyl-insoluble and HMW tau from the frontal cortex of Alzheimer patients and compared their properties using a variety of biophysical and bioactivity assays. Sarkosyl-insoluble fibrillar tau comprises abundant paired-helical filaments (PHF) as quantified by electron microscopy (EM) and is more resistant to proteinase K, compared to HMW tau, which is mostly in an oligomeric form. Sarkosyl-insoluble and HMW tau are nearly equivalent in potency in HEK cell bioactivity assay for seeding aggregates, and their injection reveals similar local uptake into hippocampal neurons in PS19 Tau transgenic mice. However, the HMW preparation appears to be far more potent in inducing a glial response including Clec7a-positive rod microglia in the absence of neurodegeneration or synapse loss and promotes more rapid propagation of misfolded tau to distal, anatomically connected regions, such as entorhinal and perirhinal cortices. These data suggest that soluble HMW tau has similar properties to fibrillar sarkosyl-insoluble tau with regard to tau seeding potential, but may be equal or even more bioactive with respect to propagation across neural systems and activation of glial responses, both relevant to tau-related Alzheimer phenotypes.

Reply: Soluble oligomers or insoluble fibrils?

“…Equal amounts of total tau—quantified by WB—were loaded onto the EM grids (3 µl at 0.1 µg/µl tau). 5) Stern and Selkoe note that we demonstrate that the PBS soluble and sarkosyl insoluble preparations are not 100% “clean” and that some fibrillar tau is present even in the PBS fraction (Fig 1 in [ 23 ]). Not unexpectedly, some short fibrils can be observed in the minimally processed PBS fraction, in agreement with the supplemental data shown in Stern et al 2023 after a soak and spin protocol, using cryoEM [ 30 ].…”

“…Because of the widely different extraction conditions, aqueous phosphate-buffered saline (PBS) homogenates and sarkosyl detergent insoluble fractions clearly enrich for different forms of tau. Beyond the long-standing use of these preparations in the tau field, and the marked differences between extraction conditions, we present substantial data in this [ 23 ] and other manuscripts [ 3 , 5 , 14 , 28 , 31 – 33 ] which demonstrate that tau present in the PBS fraction (referred to as HMW tau in our manuscript) is different than the fibrillar, classical paired helical filament (PHF) tau in the sarkosyl insoluble fraction, yet they are both bioactive (and, as shown in the current work, similarly bioactive). Multiple lines of evidence support this: 1) In our original 2015 description of the bioactive HMW fraction, we showed that it was detected by the oligomer-specific antibody T22, disappeared after urea incubation, could be detected in the interstitial fluid from mouse brain, and we demonstrated (by atomic force microscopy) that tau immunoprecipitated from the HMW fraction contained small globular structures, all consistent with the behavior of oligomeric species of tau [ 32 ].…”

Longitudinal characterization of behavioral, morphological and transcriptomic changes in a tauopathy mouse model

“…In the study of autophagy homeostasis of misfolded protein diseases [ 44 ], P301S mice (2–3 months old) were stained with MC1 (an antibody raised to paired helical filaments that are the predominant component of tangles). In a recent study of tau seeding and spreading mechanisms [ 45 ], tau oligomers and tau fibrils (two main precursors of NFTs) were examined in 6-month-old P301S mice.…”

The Role of Tau Pathology in Alzheimer’s Disease and Down Syndrome

“…Recent studies have implicated fibrillar, truncated and oligomeric Tau in the seeding, aggregation, and propagation of Tau pathology in the brain [ 23 , 32 , 33 , 79 , 80 , 81 ]. Interestingly, oligomeric and fibrillar Tau appear to be equivalent in potency, in terms of seeding competency, and are both known to be taken up by local neurons after intracranial injection [ 82 ]. However, oligomeric Tau appears to drive a more potent glial response in the brain and a more rapid propagation of misfolded Tau to other brain regions [ 82 ] (see Figure 1 below).…”